General Chemistry MCAT Flashcards

Question

What is the general trend of the electronegativity in the periodic table?

Answer 1

Electronegativity is a measure of the attractive force that an atom will exert on an electron in a chemical bond. The greater the electronegativity of an atom, the greater its attraction for bonding electrons. Electronegativity values are related to ionization energies: the lower the ionization energy, the lower the electronegativity; Electronegativity increases across a period from left to right and decreases in a group from top to bottom. Electronegativity can also be called “ nuclear positivity.” It is a result of the nucleus' attraction for electrons; that is, the Zeff perceived by the electrons in a bond.

Answer 2

* Cs = largest, least electronegative, lowest ionization energy, most endothermic electron affinity * F = smallest, most electronegative, highest ionization energy, most exothermic electron affinity * Left → Right * Atomic radius ↓ * Ionization energy ↑ * Electron affinity ↑ * Electronegativity ↑ * Top → Bottom * Atomic radius ↑ * Ionization energy ↓ * Electron affinity ↓ * Electronegativity ↓

Answer 3

shiny, conduct electricity well, malleable, ductile, solids, except for mercury liquid. They generally have high melting points and densities, exceptions, lithium, which has a density that is about half that of water. Metals have the ability to be deformed without breaking; the ability of metal to be hammered into shapes is called malleability, and its ability to be pulled into wires is called ductility. At the atomic level, a metal is defined by a low Zeff, low electronegativity (high electropositivity), large atomic radius, and low ionization energy. These combined characteristics make it fairly easy for metals to give up one or more electrons. Because the valence electrons of all metals are only loosely held to their atoms, they are free to move, which makes metals good conductors of heat and electricity. Multiple oxidation states. * MeTals lose electrons to become caTions = posiTive (+) ions * Nonmetals gain electrons to become aNions = Negative (-) ions

Answer 4

• dull, poor conductors of electricity, brittle,upper right side of table, brittle with no luster, high ionization energies, electron affinities, electronegativities, small atomic radii, poor conductors

Answer 5

share both charactersitics, physical properties, densities melting points and boiling points vary widely. Include boron, silicon, germanium, arsenic, antimony, tellurium, and polonium,

Answer 6

lower densites that mothers, alkali metals have only one loosely bound electron in their outermost shells. Their Zeff values are very low, giving them the largest atomic radii of all the elements in their respective periods. This low Zeff value also explains the other trends: low ionization energies, low electron affinities, and low electronegativity’s. Alkali metals easily lose one electron to form univalent cations, and they react very readily with nonmetals, especially the halogens, as in NaCl.

Answer 7

characteristic metal properties most similar to alkali, therefore are called active metals not naturally found in elemental(neutral) state slightly higher effective nuclear charges and thus slightly smaller atomic radii

Answer 8

highly reactive nonmetals, seven valence electrons all different states, chemical reactivity is more uniform, and due to their very high electronegativities and electron affinities, they are especially reactive toward the alkali and alkaline earth metals. In this group, fluorine has the highest electronegativity, and it has the highest electronegativity of all the elements. The halogens are so reactive that they are not naturally found in their elemental state but rather as ions (called halides)

Answer 9

inert, low chemical reactivities, high ionization energies, no tendency to gain or lose electrons no electronegativities, low boiling points and gases at room temp

Answer 10

: low electron affinities, low ionization energies, low electronegativities, very hard high melting and boiling points, malleable and good conductors loosely held electrons filling the d subshell, different oxidation states, and losing diff. numbers of electrons. Form different complex ions forming colored solutions with nonmetals low solubility

Answer 11

strong attractive forces

Answer 12

* • Hydrogen is excused from the octet rule because it doesn’ t have enough “ space” for eight electrons, it only has the one s-subshell, which can hold a maximum of two electrons. * • Lithium, beryllium, and boron are just lazy— they have enough room because they have both s- and p-orbitals to hold a total of eight electrons, but they’ d rather not put in all the hard work to getting all eight. * • All the elements in period 3 and greater have extra storage space, so they can hold more than eight electrons if necessary.

Answer 13

one or more electrons from an atom with lower ionization energy, typically a metal, transferred to an atom with greater electron affinity toward nonmetal. Positively charged cation is electrostatically attracted to the negatively charged anion. Electronegativity difference greater than 1.7. high melting and boiling points, dissolve readily, when solids from crystalline, dissociate in water and polar solvents.

Answer 14

electron pair shared between two atoms, two nonmetals, if the electron pair is shared equally, the covalent bond is nonpolar; and if the pair is shared unequally, the bond is polar. If the shared electrons are contributed by only one of the two atoms, the bond is called coordinate covalent. Covalent compounds contain discrete molecular units with relatively weak intermolecular interactions. As a result, these compounds tend to have lower melting and boiling points. In addition, since they do not break down into constituent ions, they are poor conductors of electricity in the liquid state or in aqueous solutions.

Answer 15

* Bond length is the average distance between the two nuclei of the atoms involved in a bond. As the number of shared electron pairs increases, the two atoms are pulled closer together, resulting in a decrease in bond lengths. * Bond energy is the energy required to break a bond by separating its components into their isolated, gaseous atomic states. the greater the number of pairs of electrons shared between the atomic nuclei, the more energy is required to “ break” the bond(s) holding the atoms together. Thus, triple bonds have the greatest bond energy, and single bonds have the lowest bond energy. the greater the bond energy is, the “ stronger” the bond.

Answer 16

the atom with the higher electronegativity gets the larger “ share” of the electron pair(s). A polar bond is a dipole, with the positive end of the dipole at the less electronegative atom and the negative end at the more electronegative atom. Nonpolar covalent bond- atoms that have identical or nearly identical electronegativities share electron pair(s), they do so with equal distribution of the electron, no separation of charge across the bond, only same elements have purely equal distribution. H2,N2,O2,F2,Cl2,Br2,I2 Polar covalent bonds- .4-1.7 separation of charge across bond. results in the more electronegative element acquiring a greater portion of the electron pair(s) and taking on a partial negative charge, δ − , and the less electronegative element acquiring a smaller portion of the electron pair(s) and taking on a partial positive charge, δ +. dipole movement. μ =vector quantity. q=charge magnitude r=distance between two partial charges In a coordinate covalent bond, the shared electron pair comes from the lone pair of one of the atoms in the molecule, while the other atom involved in the bond contributes nothing. The same bond connectivity and differ only in the arrangement of the electron pairs, then these structures represent different resonance forms for a single compound.

Answer 17

* V is the normal number of electrons in the atom’ s valence shell, Nnonbonding is the number of nonbonding electrons, and Nbonding is the number of bonding electrons. The charge of an ion or compound is equal to the sum of the formal charges of the individual atoms comprising the ion or compound. * Valence shell electron pair repulsion theory (VSEPR theory)- three-dimensional arrangement of atoms surrounding a central atom is determined by the repulsions between the bonding and the nonbonding electron pairs in the valence shell of the central atom. These electron pairs arrange themselves as far apart as possible, thereby minimizing the repulsive forces * Electronic geometry describes the spatial arrangement of all pairs of electrons around the central atom, including the bonding and the lone pairs. In contrast, the molecular geometry describes the spatial arrangement of only the bonding pairs of electrons.

Answer 18

• London Dispersion forces: In a given instantaneous moment, the electron density may be unequally distributed between the two atoms. This results in a rapid polarization and counterpolarization of the electron cloud and the formation of short-lived dipole moments. these dipoles interact with the electron clouds of neighboring compounds, inducing the formation of more dipoles. The attractive interactions of these short-lived and rapidly shifting dipoles are known as dispersion forces, or London forces. Large molecules with electrons that are far from the nucleus are easily polarizable and thus possess greater dispersion forces.

Answer 19

• Dipole-Dipole Interactions: polar molecules form The positive region of one molecule is close to the negative region of another molecule. Dipole— dipole interactions are present in the solid and liquid phases but become negligible in the gas phase because of the significantly increased distance between gas particles. Polar species also tend to have higher melting and boiling points than those of nonpolar species of comparable molecular weight.

Answer 20

• A hydrogen bond is a specific, unusually strong form of dipole– dipole interaction, which may be intra- or intermolecular. Not bonds, no sharing of electrons. When hydrogen is bound to one of three highly electronegative atoms— fluorine, oxygen, or nitrogen— the hydrogen atom carries only a small amount of the electron density in the covalent bond.

Answer 21

• Sigma bonds allow for free rotation about their axes because the electron density of the bonding orbital is a single linear accumulation between the atomic nuclei. When the orbitals overlap in such a way that there are two parallel electron cloud densities, a pi (π ) bond is formed. Pi bonds do not allow for free rotation because the electron densities of the orbital are parallel.

Answer 22

* The atoms or molecules in a gaseous sample move rapidly and are far apart from each other. In addition, only very weak intermolecular forces exist between gas particles; this results in certain characteristic physical properties, such as the ability to expand to fill any volume and to take on the shape of a container * standard temperature and pressure, or STP, which refers to conditions of 273.13 K (0° C) and 1 atm. standard state conditions- STP (273 K and 1 atm) is generally used for gas law calculations; standard state conditions (298 K and 1 atm)

Answer 23

* ideal gases: An ideal gas represents a hypothetical gas whose molecules have no intermolecular forces and occupy no volume. Although real gases deviate from this ideal behavior at high pressures and low temperatures * kinetic molecular theory: * Gases are made up of particles whose volumes are negligible compared to the container volume. * Gas atoms or molecules exhibit no intermolecular attractions or repulsions. * Gas particles are in continuous, random motion, undergoing collisions with other particles and the container walls. * Collisions between any two gas particles are elastic, meaning that there is conservation of both momentum and kinetic energy.

Answer 24

* The average kinetic energy of gas particles is proportional to the absolute temperature (in Kelvin) of the gas, and it is the same for all gases at a given temperature, irrespective of chemical identity or atomic mass. * k=Boltzmann constant. * Urms=resultant quantity * R is the ideal gas constant * M is the molecular mass * bell-shaped curve flattens and shifts to the right as the temperature increases, indicating that at higher temperatures, more molecules are moving at higher speeds. * The higher the temperature, the faster the molecules move. The larger the molecules, they slower they move.

Answer 25

* r=diffusion M=Molar masses * This equation is used to determine relative rates of diffusion or effusion. * Diffusion: When gases mix with one another. The kinetic molecular theory of gases predicts that heavier gases diffuse more slowly than lighter ones because of their differing average speeds. All gas particles have the same average kinetic energy at the same temperature, it must be true that particles with greater mass travel at a slower average velocity. * Effusion: When a gas moves through a small hole under pressure. the flow of gas particles under pressure from one compartment to another through a small opening

Answer 26

•that all gases at a constant temperature and pressure occupy volumes that are directly proportional to the number of moles of gas present. Equal amounts of all gases at the same temperature and pressure will occupy equal volumes. One mole of any gas, irrespective of its chemical identity, will occupy 22.4 liters at STP * where n1 and n2 are the number of moles of gas 1 and gas 2, and V1 and V2 are the volumes of the gases * PV = nRT pressure (P), volume (V), temperature (T), and number of moles (n), R=8.21 × 10-2 (L· atm)/(mol· K) or 8.314 J/(K· mol) * We define density as the ratio of the mass per unit volume of a substance and, for gases, express it in units of grams per liter (g/L) V2 is then used to find the density of the gas under nonstandard conditions:

Answer 27

• Boyle's law is a derivation of the ideal gas law and states that pressure and volume are inversely related: When one increases, the other decreases. for a given gaseous sample held at constant temperature conditions) the volume of the gas is inversely proportional to its pressure:

Answer 28

* Charles' law is also a derivation of the ideal gas law and states that volume and temperature are directly proportional: When one increases, the other does too. The law states that at constant pressure, the volume of a gas is proportional to its absolute temperature, * if one extrapolates the V versus T plot for a gas back to where V = 0 (as it should for an ideal gas), we find that T→ 0 K!

Answer 29

•when the gas atoms or molecules are forced into close proximity under high pressure and at low temperature. Under these “ nonideal” conditions, the molecular volume and intermolecular forces become significant. As the pressure of a gas increases, the particles are pushed closer and closer together. As the condensation pressure for a given temperature is approached, intermolecular attraction forces become more and more significant, until the gas condenses into the liquid state. At extremely high pressure, however, the size of the particles becomes relatively large compared to the distance between them, and this causes the gas to take up a larger volume than would be predicted by the ideal gas law. As the temperature of a gas is decreased, the average velocity of the gas molecules decreases, and the attractive intermolecular forces become increasingly significant. As the condensation temperature is approached for a given pressure, intermolecular attractions eventually cause the gas to condense to a liquid state. As the temperature of a gas is reduced toward its condensation point (which is the same as its boiling point), intermolecular attraction causes the gas to have a smaller volume than that which would be predicted by the ideal gas law. The closer the temperature of a gas is to its boiling point, the less ideal is its behavior. where a and b are physical constants experimentally determined for each gas. The a term corrects for the attractive forces between molecules (a for attractive) and as such will be smaller for gases that are small and less polarizable He, larger for gases that are larger and more polarizable Xe, and largest for polar molecules. The b term corrects for the volume of the molecules themselves. Larger values of b are thus found for larger molecules. Numerical values for a are generally much larger than those for b.

Answer 30

- (pure substances composed of two or more elements in a fixed proportion

Answer 31

combination of two or more atoms held together by covalent bonds. Smallest unit of a compound that displays the identifying properties of that compound. Can be composed of two or more atoms of the same element (N2 and O2) may be composed of two or more atoms of different elements CO2 or SOCl2.

Answer 32

do not form true molecules with opposite charges arranged in the solid state, form from combinations of elements with large electronegativity differences (that sit far apart on the periodic table), such as sodium and chlorine. Molecular compounds form from the combination of elements of similar electronegativity (that sit close to each other on the periodic table), such as carbon with oxygen

Answer 33

measurement of mass, sum of atomic weights of all the atoms in a molecule, units are atomic mass units (amu), formal weight of ionic compound by added up atomic weights of constituent ions by empirical formula in grams

Answer 34

quantity of any thing (molecules, atoms, dollar bills, chairs, etc.) equal to the number of particles found in 12 grams of carbon-12.

Answer 35

One mole of a compound has a mass in grams equal to the molecular weight of the compound expressed in amu and contains 6.022 × 1023 molecules of that compound.

Answer 36

• acid-base reactions, redox reactions, and precipitation reactions. one mole of HCl has the ability to donate one mole of hydrogen ions (H+) in solution, but one mole of H2SO4 has the ability to donate two moles of hydrogen ions, and one mole of H3PO4 has the ability to donate three moles of hydrogen ions. One mole of sodium has the ability to donate one mole of electrons, while one mole of magnesium has the ability to donate two moles of electrons. To find one mole of hydrogen ions for a particular acid-base reaction, we could “ source” those protons from one mole of HCl, or we could instead use a half-mole of H2SO4. If we're using H3PO4, we'd only need one-third of a mole. This is what we mean by “ different capacities to act in certain ways.” How many things we are interested in. One mole of hydrogen ions (one equivalent) will be donated by one mole of HCl, but two moles of hydrogen ions (two equivalents) will be donated by one mole of H2SO4, and three moles of hydrogen ions (three equivalents) will be donated by one mole of H3PO4. Simply put, an equivalent is a mole of charge. one mole of HCl will donate one mole (one equivalent) of hydrogen ions, a certain mass amount of HCl will donate one equivalent of hydrogen ions. This amount of compound, measured in grams, that produces one equivalent of the monovalent particle of interest (protons, hydroxide ions, electrons, or ions) is called the gram equivalent weight. With the gram equivalent weight= molar mass/n (number of protons, hydroxide ions, electrons or monovalent ions. “ produced” or “ consumed” per molecule of the compound in the reaction. For example, you would need 49 grams of H2SO4 (molar mass = 98 g/mol) to produce one equivalent of hydrogen ions, because each molecule of H2SO4 can donate two hydrogen ions (n = 2). Simply put, an equivalent weight of a compound is the mass that provides one mole of charge. If the amount of a compound in a reaction is known and you need to determine how many equivalents are present, equivalents=mass of compound/gram equivalent weight.

Answer 37

measure of concentration, units are equivalents/liters. 1 N solution of acid contains a concentration of hydrogen ions equal to 1 mole/liter; a 2 N solution of acid contains a concentration of hydrogen ions equal to 2 moles/liter. The actual concentration of the acidic compound may be the same or different from the normality, because different compounds have different capacities to donate hydrogen ions. In a 1 N acid solution consisting of dissolved HCl, the molarity of HCl is 1 M because HCl is a monoprotic acid, but if the dissolved acid is H2SO4, then the molarity of H2SO4 in a 1 N acid solution is 0.5 M, because H2SO4 is a diprotic acid. Molarity=normality/n. where n is the number of protons, hydroxide ions, electrons, or monovalent ions “ produced” or “ consumed” per molecule of the compound in the reaction. There is a real benefit to working with equivalents and normality because it allows a direct comparison of quantities of the “ thing” you are most interested in. So it is very convenient to be able to say that one equivalent of acid (hydrogen ion) will neutralize one equivalent of base (hydroxide ion). The same could not necessarily be said to be true if we were dealing with moles of acidic compound and moles of basic compound. For example, one mole of HCl will not completely neutralize one mole of Ca(OH)2, because one mole of HCl will donate one equivalent of acid but Ca(OH)2 will donate two equivalents of base.

Answer 38

states that any pure sample of a given compound will contain the same elements in an identical mass ratio.

Answer 39

The empirical formula gives the simplest whole number ratio of the elements in the compound. The molecular formula gives the exact number of atoms of each element in the compound and is usually a multiple of the empirical formula. For example, the empirical formula for benzene is CH, while the molecular formula is C6H6

Answer 40

• percent composition by mass of an element is the weight percent of a given element in a specific compound. You can calculate the percent composition of an element by using either the empirical or the molecular formula; just be sure to use to the appropriate mass measurement: formula weight for empirical formula or molar mass for molecular formula. Formula weight is simply the mass of the atoms in the empirical formula of a compound. The molecular formula is either the same as the empirical formula or a multiple of it. To calculate the molecular formula, you need to know the mole ratio (this will give you the empirical formula) and the molecular weight (molecular weight ÷ empirical formula weight will give you the multiplier for the empirical formula to molecular formula conversion).

Answer 41

Combination reactions- having two or more reactants forming one product and generally have more reactants than products. A + B → C.

Answer 42

generally have more products than reactants. C → A + B and is the opposite of a combination reaction: A single compound reactant breaks down into two or more products, usually as a result of heating or electrolysis.

Answer 43

an atom (or ion) of one compound is replaced by an atom of another element. Redox reactions.

Answer 44

elements from two different compounds swap places with each other to form two new compounds. This type of reaction occurs when one of the products is removed from the solution as a precipitate or gas or when two of the original species combine to form a weak electrolyte that remains undissociated in solution

Answer 45

are a specific type of double-displacement reaction in which an acid reacts with a base to produce a salt.

Answer 46

usually written such as displacements, the ionic constituents of the compounds are in solution, so we can write the chemical reaction in ionic form. t's not taking part in the overall reaction but simply remains in the solution unchanged. We call such species spectator ions. With net ionic being equations list only the elements important for demonstrating the actual reaction that occurs during a displacement reaction.

Answer 47

look at Charge on each side, Number of atoms of each element. mass of the reactants consumed must equal the mass of products generated. number of atoms on the reactant side equals the number of atoms on the product side. Stoichiometric coefficients, which are placed in front of each compound, are used to indicate the number of moles of a given species involved in the reaction. When balancing equations, focus on the least represented elements first and work your way to the most represented element of the reaction (usually oxygen or hydrogen).

Answer 48

- most useful bit of information to glean from a balanced reaction is the mole ratio of reactants consumed to products generated. Furthermore, you can generate the mole ratio of one reactant to another or one product to another. All these ratios can be generated by a comparison of the stoichiometric coefficients.\

Answer 49

• reactant is known as the limiting reactant (or reagent) because it limits the amount of product that can be formed in the reaction. The reactant that remains after all the limiting reactant is used up is called the excess reactant (or reagent). 1. All comparisons of reactants must be done in units of moles. Gram-to-gram comparisons will be useless and maybe even misleading. 2. It is not the absolute mole quantities of the reactants that determine which reactant is the limiting reactant. Rather, the rate at which the reactants are consumed (the stoichiometric ratios of the reactants) combined with the absolute mole quantities determines which reactant is the limiting reactant.

Answer 50

• of a reaction is either the amount of product predicted (theoretical yield) or obtained (raw or actual yield) when the reaction is carried out. Theoretical yield is the maximum amount of product that can be generated as predicted from the balanced equation, assuming that all of the limiting reactant is consumed, no side reactions have occurred, and the entire product has been collected. Actual yield is the amount of product that you are actually able to obtain. The ratio of the actual yield to the theoretical yield, multiplied by 100 percent, gives you the percent yield,

Answer 51

Reactions can be spontaneous or nonspontaneous; the change in Gibbs free energy determines whether or not a reaction will occur, by itself, without outside assistance. even if a reaction is spontaneous, this does not necessarily mean that it will run quickly. In fact, nearly every reaction that our very lives depend upon, while perhaps spontaneous, proceeds so slowly that without the aid of enzymes and other catalysts,

Answer 52

Many reactions proceed by more than one step, the series of which is known as the mechanism of a reaction and the sum of which gives the overall reaction. Mechanisms are proposed pathways for a reaction that must coincide with rate data information from experimental observation. The molecule A2B, which does not appear in the overall reaction, is called an intermediate. Reaction intermediates are often difficult to detect because they may be consumed almost immediately after they are formed, but a proposed mechanism that includes intermediates can be supported through kinetic experiments. One of the most important points for you to remember is that the slowest step in any proposed mechanism is called the rate-determining step, because it acts like a kinetic “ bottleneck,” preventing the overall reaction from proceeding any faster than the slowest step.

Answer 53

•Reaction Rates- take measurements of the concentrations of reactants and products and their change over time. Definition of rate- 2A + B → C. one mole of C is produced from every two moles of A and one mole of B, we can describe the rate of this reaction in terms of either the disappearance of reactants over time or the appearance of products over time. Because the reactants, are being consumed in the process of formation of the products, we place a minus sign in front of the rate expression in terms of reactants. For the above reaction, the rate of the reaction with respect to A is − Δ [A]/Δ t, with respect to B is − Δ [B]/Δ t, and with respect to C is Δ [C]/Δ t. with the stoichiometric coefficient for the reactions unequal, with the rates of change of concentrations unequal. two moles of A are consumed for every mole of B consumed, rate− Δ [A] = 2 rate− Δ [B]. for every two moles of A consumed, only one mole of C is produced; thus, we can say that the rate− Δ [A] = 2 rateΔ [C]. rate of consumption of B is equal to the rate of production of C. standard rate of reaction in which the rates with respect to all reaction species are equal, the rate of concentration change of each species should be divided by the species' stoichiometric coefficient: aA + bB → cC + dD: Rate is expressed in the units of moles per liter per second (mol/L· s) or molarity per second (M/s).

Answer 54

* nearly all forward, irreversible reactions, the rate is proportional to the product of the concentrations of the reactants, each raised to some power with the rate is proportional to [A]x[B]y. called the rate law. k is the reaction rate coefficient or rate constant. Rate is always measured in units of concentration over time; that is, molarity/second. The exponents x and y (or x, y, and z, if there are three reactants, etc.) are called the orders of the reaction: x is the order with respect to reactant A, and y is order with respect to reactant B. The overall order of the reaction is the sum of x + y (+ z). These exponents may be integers, fractions, or zero and must be determined experimentally. * The units of k vary depending on the order of the reaction .This formula is used for the experimental determination of the rate law or to calculate a rate given reaction data. The values of x and y usually aren't the same as the stoichiometric coefficients. The orders of a reaction must be determined experimentally. There are only two cases in which you can take stoichiometric coefficients as the orders of reaction. The first is when the reaction mechanism is a single step and the balanced “ overall” reaction is reflective of the entire chemical process. The second is when the complete reaction mechanism is given and the rate-determining step is indicated. The stoichiometric coefficients on the reactant side of the rate-determining step are the orders of the reaction. The second trap to be wary of is mistaking the equilibrium aspect of the law of mass action for the kinetic aspect. The expression for equilibrium includes the concentrations of all the species in the reaction, both reactants and products. The expression for chemical kinetics, the rate law expression, includes only the reactants. Keq tells you where the reaction's equilibrium position lies. The rate tells you how quickly the reaction will get there. rate constant, k. Technically speaking, it's not a constant, because its particular value for any specific chemical reaction will depend on the activation energy for that reaction and the temperature at which the reaction takes place. for a specific reaction, at a specific temperature, the rate coefficient is constant. For a reversible reaction, the Keq is equal to the ratio of the rate constant, k, for the forward reaction, divided by the rate constant, k− 1, for the reverse reaction. notion and principles of equilibrium apply to the system only at the end of the reaction; that is, after the system has reached equilibrium. The reaction rate, while it theoretically can be measured at any time, is usually measured at or near the beginning of the reaction to minimize the effects of the reverse reaction.

Answer 55

•Values of k, x, and y in the rate law equation (rate = k[A]x>[B]y) must be determined experimentally for a given reaction at a given temperature. 1st step) write out generic rate law look for necessary data. 2) identify a pair of trials in which the concentration of one of the reactants is changed while the concentration of all other reactants remains constant. Under these conditions, any change in rate of product formation (if there is any) from one trial to the other is solely due to the change in concentration of one reactant. Let's imagine that compound A's concentration is constant, while the concentration of B doubled. If the rate of the formation of product C has subsequently quadrupled, then you can say to yourself, “ Doubling the concentration of B has resulted in a quadrupling of the production rate of C, so to determine the order of the reaction, y, with respect to reactant B, I need to calculate the power by which the number 2 must be raised to equal 4. Because 2>y = 4, y = 2.” And repeat for other reactant using different data from a different pair of trials, always making sure that the concentration of only the reactant whose order you are trying to determine is changed from one trial to the other while the concentration of any other reactant remains the same. replacing the x and the y (and sometimes z) with actual numbers. To determine the value of the rate constant k, you will need to plug in actual values for the reactant concentrations and the product formation rate, once you know the values for the exponent for each reactant. You can use the data from any one of the trials; pick whichever trial has the most arithmetically convenient numbers.

Answer 56

* chemical reactions on the basis of kinetics into classes of reactions called zero-order, first-order, second-order, mixed-order, or higher-order reactions. * Zero-order reactions- rate of formation of product C is independent of changes in concentrations of any of the reactants, A and B. These reactions have a constant reaction rate equal to the rate coefficient (rate constant) k. The rate law for a zero-order reaction is where k has units of M s− 1 dependent upon temperature; thus, it is possible to change the rate for a zero-order reaction by changing the temperature. The only other way to change the rate of a zero-order reaction is by the addition of a catalyst, which lowers the energy of activation, thereby increasing the value of k. * First-Order Reactions- A first-order reaction (order = 1) has a rate that is directly proportional to only one reactant, such that doubling the concentration of, say, reactant A results in a doubling of the rate of formation of product C. where k has units of s− 1. A classic example of a first-order reaction is the process of radioactive decay. From the rate law, in which the rate of decrease of the amount of a radioactive isotope A is proportional to the amount of A, with the concentration of radioactive substance A at any time t can expressed in the third equation. where [Ao] is the initial concentration of A, [At] is the concentration of A at time t, k is the rate constant, and t is time. It is important to recognize that a first-order rate law with a single reactant suggests that the reaction begins when the molecule undergoes a chemical change all by itself, without a chemical interaction, and, usually, without a physical interaction with any other molecule. * Second-Order Reactions- A second-order reaction (order = 2) has a rate that is proportional either to the product of the concentrations of two reactants or to the square of the concentration of a single reactant (and zero-order with respect to any other reactant). where k has units of M− 1s− 1. It is important to recognize that a second-order rate law often suggests a physical collision between two reactant molecules, especially if the rate law is first-order with respect to each of the two reactants. * Higher-Order Reactions- very few— almost zero— reactions in which a single-reaction step involves a termolecular process; in other words, there are almost no elementary processes whose rate is third-order with respect to a single reactant. This is because it is almost impossible to get three particles to collide simultaneously. * Mixed-Order Reactions- Mixed-order reactions sometimes refer to noninteger orders (fractions) and in other cases to reactions whose order varies over the course of the reaction. Fractions are more specifically described as broken-order, and in recent times, the term mixed-order has come to refer solely to reactions whose order changes over time where A is the single reactant and E is the catalyst. The result of the large value for [A] at the beginning of the reaction is that k3[A] >> k2, and the reaction will appear to be first-order; at the end of the reaction, k2 >> k3[A] because [A] will have a low value, making the reaction appear to be second-order.

Answer 57

•Collision Theory of Chemical Kinetics- states that the rate of a reaction is proportional to the number of collisions per second between the reacting molecules. ot all collisions result in a chemical reaction. An effective collision (one that leads to the formation of products) occurs only if the molecules collide with each other in the correct orientation and with sufficient energy to break the existing bonds and form new ones. The minimum energy of collision necessary for a reaction to take place is called the activation energy, Ea, or the energy barrier. Only a fraction of colliding particles have enough kinetic energy to exceed the activation energy. This means that only a fraction of all collisions are effective. where Z is the total number of collisions occurring per second and f is the fraction of collisions that are effective.

Answer 58

When molecules collide with sufficient energy at least equal to the activation energy, they form a transition state in which the old bonds are weakened and the new bonds begin to form. The transition state then dissociates into products, and the new bonds are fully formed. For the reaction A2 + B2→ 2AB, the change along the reaction coordinate, which is a measure of the extent to which the reaction has progressed from reactants to products. transition state, also called the activated complex, has greater energy than either the reactants or the products and is denoted by the symbol ‡ . An amount of energy at least equal to the activation energy is required to bring the reactants to this energy level. Once an activated complex is formed, it can either dissociate into the products or revert to reactants without any additional energy input. Transition states are distinguished from reaction intermediates in that, existing as they do at energy maxima, transition states exist on a continuum rather than having distinct identities and finite lifetimes. A potential energy diagram illustrates the relationship between the activation energy, the heats of reaction, and the potential energy of the system. The most important features to recognize in such diagrams are the relative energies of all of the products and reactants. The enthalpy change of the reaction (Δ H) is the difference between the potential energy of the products and the potential energy of the reactants. A negative enthalpy change indicates an exothermic reaction (heat is given off), and a positive enthalpy indicates an endothermic reaction (heat is absorbed). The activated complex, the transition state, exists at the top of the energy barrier. The difference in potential energies between the activated complex and the reactants is the activation energy of the forward reaction; the difference in potential energies between the activated complex and the products is the activation energy of the reverse reaction. – Δ H = exothermic = heat given off. +Δ H = endothermic = heat absorbed. •Factors affecting reaction rates with. Kinetics and thermodynamics should be considered separately. Note that the potential energy of the product can be raised or lowered, thereby changing the value of Δ H without affecting the value of forward Ea.

Answer 59

the greater the concentrations of the reactants, the greater the number of effective collisions per unit time. the reaction rate will increase for all but zero-order reactions. For reactions occurring in the gaseous state, the partial pressures of the gas reactants serve as a measure of concentration

Answer 60

For nearly all reactions, the reaction rate will increase as the temperature increases. Because the temperature of a substance is a measure of the particles' average kinetic energy, increasing the temperature increases the average kinetic energy of the molecules. Consequently, the proportion of molecules having energies greater than Ea (and thus capable of undergoing reaction) increases with higher temperature. You'll often hear that raising the temperature of a system by 10° C will result in an approximate doubling of the reaction rate

Answer 61

rate at which a reaction takes place may also be affected by the medium in which it takes place. Some molecules are more likely to react with each other in aqueous environments, while others are more likely to react in a nonaqueous solvent, such as DMSO (dimethylsulfoxide) or ethanol. Furthermore, the physical state of the medium (liquid, solid, or gas) can also have a significant effect. Generally, polar solvents are preferred because their molecular dipole tends to polarize the bonds of the reactants, thereby lengthening and weakening them, which permits the reaction to occur faster.

Answer 62

substances that increase reaction rate without themselves being consumed in the reaction. Catalysts interact with the reactants, either by adsorption or through the formation of intermediates, and stabilize them so as to reduce the energy of activation necessary for the reaction to proceed. While many catalysts, including all enzymes, chemically interact with the reactants, upon formation of the products, they return to their original chemical state. They may increase the frequency of collisions between the reactants; change the relative orientation of the reactants, making a higher percentage of the collisions effective; donate electron density to the reactants; or reduce intramolecular bonding within reactant molecules. In homogeneous catalysis, the catalyst is in the same phase (solid, liquid, gas) as the reactants. In heterogeneous catalysis, the catalyst is in a distinct phase. only effect of the catalyst is the decrease in the energies of activation, Ea, for both the forward and reverse reactions. The presence of the catalyst has no impact on the potential energies of the reactants or the products or the difference between them. This means that catalysts change only the rate of reactions, and in fact, they change the forward rate and the reverse rate by the same factor. Consequently, they have no impact whatsoever on the equilibrium position or the measure of Keq. Remember that as useful as catalysts are in biological and non-biological systems, catalysts are not miracle workers: They will not transform a nonspontaneous reaction into a spontaneous one; they only make spontaneous reactions go more quickly toward equilibrium.

Answer 63

•irreversible; that is, the reaction proceeds in one direction only, the reaction goes to completion, and the amount of product formed is the maximum as determined by the amount of limiting reactant present. Reversible reactions are those in which the reaction can proceed in one of two ways: forward and reverse. (From the perspective of the direction in which the overall reaction is written, the forward reaction is the one that goes from “ reactants” on the left to “ products” on the right.) Reversible reactions usually do not proceed to completion because by definition the products can react together to re-form the reactants. When the reaction system is closed and no products or reactants are removed or added, the system will eventually “ settle” into a state in which the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of the products and reactants are constant. In this dynamic equilibrium state, the forward and reverse reactions are occurring— they haven't stopped, as in a static equilibrium— but they are going at the same rate; thus, there is no net change in the concentrations of the products or reactants. At equilibrium, the concentrations of A and B are constant (though not necessarily equal), and the reactions A → B and B → A continue to occur at equal rates. Equilibrium can be thought of as a balance between the two reactions (forward and reverse). Better still, equilibrium should be understood on the basis of entropy, which is the measure of the distribution of energy throughout a system or between a system and its environment. For a reversible reaction at a given temperature, the reaction will reach equilibrium when the system's entropy— or energy distribution— is at a maximum and the Gibbs free energy of the system is at a minimum. At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, entropy is at a maximum, and Gibbs free energy is at a minimum. This links the concepts of thermodynamics and kinetics.

Answer 64

• the law of mass action states that if the system is at equilibrium at a given temperature, then the following ratio is constant: The law of mass action is actually related to the expressions for the rates of the forward and reverse reactions. * the law of mass action states that if the system is at equilibrium at a given temperature, then the following ratio is constant: The law of mass action is actually related to the expressions for the rates of the forward and reverse reactions. * Because the reaction occurs in one step, the rates of the forward and reverse reactions are given by When ratef = rater, the system is in equilibrium. Because the rates are equal, we can set the rate expressions for the forward and reverse reactions equal to each other: Because kf and kr are both constants, we can define a new constant Kc, where Kc is called the equilibrium constant and the subscript c indicates that it is in terms of concentration. (When dealing with gases, the equilibrium constant is referred to as Kp, and the subscript p indicates that it is in terms of pressure.) For dilute solutions, Kc and Keq are used interchangeably. While the forward and the reverse reaction rates are equal at equilibrium, the concentrations of the reactants and products are not usually equal. This means that the forward and reverse reaction rate constants, kf and kr, are not usually equal. The ratio of kf to kr is Kc (Keq).When a reaction occurs in more than one step, the equilibrium constant for the overall reaction is found by multiplying together the equilibrium constants for each step of the reaction. When you do this, the equilibrium constant for the overall reaction is equal to the concentrations of the products divided by the concentrations of the reactants in the overall reaction, with each concentration term raised to the stoichiometric coefficient for the respective species. The forward and reverse rate constants for the nth step are designated kn and k− n, respectively. * law of mass action defines the position of equilibrium by stating that the ratio of the product of the concentrations of the products, each raised to their respective stoichiometric coefficients, to the product of the concentrations of the reactants, each raised to their respective stoichiometric coefficients, is constant

Answer 65

equilibrium is a state that is achieved only through time. Depending on the actual rates of the forward and reverse reactions, equilibrium might be achieved in minutes or years. At any point in time of a reaction, we can measure the concentrations of all of the reactants and products and calculate the reaction quotient. when calculating a value of Qc for a reaction, the concentrations of the reactants and products may not be constant. In fact, if Qc changes over time because the concentrations of reaction species are changing, the reaction by definition is not at the equilibrium state. Thus, the utility of Qc is not the value itself but rather the comparison that can be made between Qc at any given moment in the reaction to the known Keq for the reaction at a given temperature. Any reaction that has not yet reached the equilibrium state, as indicated by Qc < Keq , will continue spontaneously in the forward direction (consuming reactants to form products) until the equilibrium ratio of reactants and products is reached. Any reaction in the equilibrium state will continue to react in the forward and reverse direction, but the reaction rates for the forward and reverse reactions will be equal and the concentrations of the reactants and products will be constant, such that Qc = Keq. Once a reaction is at equilibrium, any further “ movement” either in the forward direction (resulting in an increase in products) or in the reverse direction (resulting in the re-formation of reactants) will be nonspontaneous. • Qc < Keq, Δ G < 0, reaction proceeds in forward direction • Qc = Keq, Δ G = 0, reaction is in dynamic equilibrium • Qc > Keq, Δ G > 0, reaction proceeds in reverse direction

Answer 66

Properties of the law of mass action- • The concentrations of pure solids and pure liquids do not appear in the equilibrium constant expression because their concentrations do not change in the course of the reaction. • • Keq is characteristic of a particular reaction at a given temperature: The equilibrium constant is temperature dependent. • • Generally, the larger the value of Keq, the farther to the right we'll find the equilibrium and the more complete the reaction. • • If the equilibrium constant for a reaction written in one direction is Keq, the equilibrium constant for the reaction written in reverse is 1/Keq.

Answer 67

•states that a system to which a “ stress” is applied tends to shift so as to relieve the applied stress. No matter what the particular form the stress takes (e.g., change in concentration of one component, change in pressure, or change in temperature), the effect of the stress is to cause the reaction to move temporarily out of its equilibrium state, either because the concentrations or partial pressures of the system are no longer in the equilibrium ratio or because the equilibrium ratio has actually changed as a result of a change in the temperature of the system. The reaction then responds by reacting in whichever direction (either forward or reverse) that results in a re-establishment of the equilibrium state.

Answer 68

• When you add or remove reactants or products from a reaction in equilibrium, you are causing the reaction to no longer be at its minimum energy state the effect is that with the change in concentration of one or more of the chemical species, you have caused the system to have a ratio of products to reactants that is not equal to the equilibrium ratio. In other words, changing the concentration of either a reactant or a product results in Qc≠ Keq. By adding reactant or removing product, you have created a situation in which Qc < Keq, and the reaction will spontaneously move in the forward direction, increasing the value of Qc until Qc = Keq. By removing reactant or adding product, you have created a situation in which Qc > Keq, and the reaction will spontaneously react in the reverse direction, thereby decreasing the value of Qc until once again Qc = Keq. A simple way to remember this is: The system will always react in the direction away from the added species or toward the removed species. in order to improve the yield of chemical reactions. For example, where possible in the industrial production of chemicals, products of reversible reactions are removed as they are formed to prevent the reactions from reaching their equilibrium states. The reaction will continue to go in the forward direction, producing more and more product (assuming continual replenishment of reactants as they are consumed in the reaction). You could also drive a reaction forward by starting with higher concentrations of reactants. This will lead to an increase in the absolute quantities of products formed, but the reaction would still eventually reach its equilibrium state unless product was removed as it formed. In the tissues, there is a lot of CO2, and the reaction shifts to the right. In the lungs, CO2 is lost, and the reaction shifts to the left. Note that blowing off CO2 (hyperventilation) is used as a mechanism of dealing with acidosis (excess H+).

Answer 69

•Because liquids and solids are incompressible, only chemical reactions that involve at least one gas species will be affected by changes to the system's volume and pressure. When you compress a system, its volume decreases, and the total pressure increases. The increase in the total pressure is associated with an increase in the partial pressures of all the gases in the system, and this results in the system no longer being in the equilibrium state, such that Qp≠ Keq. The system will move forward or in reverse, always toward whichever side has the lower total number of moles of gas. This result is a consequence of the ideal gas law, which tells us that there is a direct relationship between the number of moles of gas and the pressure of the gas. If you increase the pressure on a system, it will respond by decreasing the pressure, by decreasing the number of gas moles present. (In this case, the volume of the system was decreased and then held constant while the system returned to its equilibrium state.) When you expand a system, its volume increases, and the total pressure and partial pressures decrease. The system is no longer in its equilibrium state and will react in the direction of the side with the greater number of moles of gas. Changing the pressure of a system without changing the partial pressures of the reactants and products will have no effect on the system

Answer 70

* changing the temperature of a system will also cause the system to react in a particular way so as to “ return” to its equilibrium state. the result of changing temperature is not simply a change in the reaction quotient, Qc or Qp, but a change in Keq. The change in temperature doesn't cause the concentrations or partial pressures of the reactants and products to change immediately, so the Q immediately after the temperature change is the same before the temperature change. Before the temperature change, the system was at equilibrium, and Q was equal to Keq; after the temperature change, Keq is a different value (it depends on temperature), so Q≠ Keq. The system has to move in whichever direction allows it to reach its new equilibrium state at the new temperature. That direction is determined by the enthalpy of the reaction. You can think of heat as a reactant if the reaction is endothermic (+Δ H) and as a product if the reaction is exothermic (− Δ H). Thinking about heat as a reactant or product allows you to apply the principle that we used with concentration changes to temperature changes. if placed in ice bath temp decrease, driving reaction to right to replace heat lost, if put in boiling reaction go to left due to increased “concentration” of heat. * Will shift to the right if: A or B added, C removed, pressure increased or volume reduced, temperature reduced: Will shift to the left if: C added, A or B removed, volume increased or pressure reduced, temperature increased

Answer 71

- the system is the matter that is being observed. It's the total amount of reactants and products in a chemical reaction. It's the amount of solute and solvent used to create a solution. It could even be the gas inside a balloon. Then the surroundings, or environment, are everything outside of what you're looking at. However, the boundary between system and surroundings is not permanently fixed, and can be moved. 1. Isolated— The system cannot exchange energy (heat and work) or matter with the surroundings; for example, an insulated bomb calorimeter. 2. Closed— The system can exchange energy (heat and work) but not matter with the surroundings; for example, a steam radiator. 3. Open— The system can exchange both energy (heat and work) and matter with the surroundings; for example, a pot of boiling water.

Answer 72

When a system experiences a change in one or more of its properties (such as concentration of reactant or product, temperature, or pressure), it undergoes a process. While processes, by definition, are associated with changes of state of systems, some processes are uniquely identified by some property that is constant throughout the process. For example, isothermal processes occur when the system's temperature is constant. Constant temperature implies that the total internal energy of the system is constant throughout the process. Adiabatic processes occur when no heat is exchanged between the system and the environment; thus, the heat content of the system is constant throughout the process. Finally, isobaric processes occur when the pressure of the system is constant

Answer 73

•A spontaneous process is one that can occur by itself without having to be driven by energy from an outside source. Calculating the change in the Gibbs free energy (Δ G) for a process, such as a chemical reaction, allows us to predict whether the process will be spontaneous or nonspontaneous. The same quantities that are used to calculate the change in the Gibbs free energy, Δ H and Δ S, can also tell us whether or not the process will be temperature-dependent; that is, spontaneous at some temperatures and nonspontaneous at others. will not necessarily happen quickly and may not go to completion. Many spontaneous reactions have very high activation energies and, therefore, rarely take place. Combustion, the combination of the chemical components of the match with molecular oxygen in the air, will not need any additional external energy input in order to proceed once the energy of activation has been supplied. Some spontaneous reactions proceed very slowly. The role of enzymes— biological catalysts— is to selectively enhance the rate of certain spontaneous but slow chemical reactions, so that the biologically necessary products can be formed at a rate sufficient for sustaining life. some reactions do not go to completion but settle into a low-energy state called equilibrium. Spontaneous reactions may go to completion, but many simply reach equilibrium with dynamically stable concentrations of reactants and products.

Answer 74

* State and State functions- properties of the system. These properties, or state functions, describe the system in an equilibrium state. They cannot describe the process of the system; that is, how the system got to its current equilibrium. They are useful only for comparing one equilibrium state to another. The pathway taken from one equilibrium state to another is described quantitatively by the process functions, the most important of which are mechanical work (W) and heat (Q). Enthalpy is a state function and is a property of the equilibrium state, so the pathway taken for a process is irrelevant to the change in enthalpy from one equilibrium state to another. * state functions- include temperature (T), pressure (P), volume (V), density (ρ ), internal energy (E or U), enthalpy (H), entropy (S), and Gibbs free energy (G). When the state of a system changes from one equilibrium to another, one or more of these state functions will change. In addition, while state functions are independent of the path (process) taken, they are not necessarily independent of one another. For example, Gibbs free energy is related to enthalpy, entropy, and temperature. Because systems can be in different equilibrium states at different temperatures and pressures, a set of standard conditions has been defined for measuring the enthalpy, entropy, and Gibbs free energy changes of a reaction. The standard conditions are defined as 25° C (298 K) and 1 atm. Don't confuse standard conditions with standard temperature and pressure (STP), for which the temperature is 0° C (273 K) and pressure is 1 atm. You'll use standard conditions for thermodynamic problems of enthalpy or free energy, but you'll use STP for ideal gas calculations.

Answer 75

•the most stable form of a substance is called the standard state of that substance. , H2 (g), H2O (l), NaCl (s), O2 (g), and C (s) (graphite) are the most stable forms of these substances under standard conditions. Recognizing whether or not a substance is in its standard state is important for thermochemical calculations, such as heats of reactions and, in particular, the heat of formation. The changes in enthalpy, entropy, and free energy that occur when a reaction takes place under standard conditions are called the standard enthalpy, standard entropy, and standard free energy changes, respectively, and are symbolized by Δ H° , Δ S° , and Δ G°.

Answer 76

is related to the average kinetic energy of the particles of the substance whose temperature is being measured. Temperature is the way that we scale how hot or cold something is. Fahrenheit, Celsius, and Kelvin. The average kinetic energy of the particles in a substance is related to the thermal energy of the substance, but because we must also include consideration of how much substance is present to calculate total thermal energy content, the most we can say about temperature is that when a substance's thermal energy increases, its temperature also increases. Nevertheless, we cannot say that something that is hot necessarily has greater thermal energy (in absolute terms) than a substance that is cold. For example, we might determine that a large amount of cool water has greater total heat content than a very small amount of very hot water.

Answer 77

* is the transfer of energy from one substance to another as a result of their difference in temperature. In fact, the zeroth law of thermodynamics implies that objects are in thermal equilibrium only when their temperatures are equal. Heat is therefore a process function, not a state function: We can quantify how much thermal energy is transferred between two or more objects as a result of their difference in temperatures by measuring the heat transferred. * This equation is used to determine energy transfer to or from a system. It is most commonly encountered in reference to a gaseous system. * The first law of thermodynamics states that the change in the total internal energy (Δ U) of a system is equal to the amount of heat (thermal energy) transferred (Q) to the system minus the amount of work (W) (another form of energy transfer by the application of force through displacement) done by the system. * heat and work are measured independently, we can assess the transfer of energy in the form of heat through any process regardless of the work done (or not done). Processes in which the system absorbs heat are called endothermic (+Δ Q), while those processes in which the system releases heat are called exothermic (− Δ Q). The unit of heat is the unit of energy: joule (J) or calorie (cal), for which 1 cal =4.184 J. * Endothermic: positive Δ H * Exothermic: negative Δ H substances of different temperatures are brought into thermal contact with each other (that is, some physical arrangement that allows for the transfer of heat energy), energy in the form of heat will transfer from the warmer substance to the cooler substance. When a substance undergoes a chemical reaction that is exothermic or endothermic, heat energy will be exchanged between the system and the environment.

Answer 78

* Process of measuring transferred heat is constant-pressure calorimetry and constant-volume calorimetry. The coffee-cup calorimeter, is a low-tech example of a constant-pressure calorimeter, while the bomb calorimeter is an example of a constant-volume calorimeter. Constant-pressure and constant-volume are terms used to describe the conditions under which the heat changes are measured. * The heat (q) absorbed or released where m is the mass, c is the specific heat of the substance, and Δ T is the change in temperature (in either Celsius or Kelvin). Specific heat is the amount of energy required to raise the temperature of one gram or kilogram of a substance by one degree Celsius or one unit Kelvin. specific heat of H2O (l): one calorie per gram per Celsius degree (1 cal/g° C).When walking barefoot, blacktop feels much hotter than a wooden walkway even when they are the same temperature. This is because they have different specific heats.

Answer 79

an insulated container covered with a lid and filled with a solution in which a reaction or some physical process, such as dissolution, is occurring. The pressure, which is the atmospheric pressure, remains constant throughout the process. The bomb calorimeter or decomposition vessel A sample of matter, typically a hydrocarbon, is placed in the steel decomposition vessel, which is then filled with almost pure O2 gas. The decomposition vessel is then placed in an insulated container holding a known mass of water. The contents of the decomposition vessel are ignited by an electric ignition mechanism. The material combusts (burns) in the presence of the oxygen, and the heat that evolves in the combustion is the heat of the reaction. Because W = PΔ V, no work is done in an isovolumic (Δ V = 0) process, so Wcalorimeter = 0. because of the insulation, the whole calorimeter can be considered isolated from the rest of the universe, so we can identify the “ system” as the sample plus the oxygen and steel vessel, and the surroundings as the water. Because no heat is exchanged between the calorimeter and the rest of the universe, Qcalorimeter is 0. So, Δ Usystem + Δ Usurroundings = Δ Ucalorimeter = Qcalorimeter− Wcalorimeter = 0. Therefore, Δ Usystem = − Δ Usurroundings. Because no work is done, qsystem = − qsurroundings, and msteelcsteelΔ T + moxygencoxygenΔ T = − mwatercwaterΔ T. no heat is exchanged between the calorimeter and the rest of the universe, but it is exchanged between the steel decomposition vessel and the surrounding water. As the previous derivation shows, heat exchange between the system and its surroundings makes it possible for us to calculate the heat of the combustion. calculate q during phase changes, when Δ T = 0. If we used q = mcΔ T, we'd erroneously think q = 0. •evaporation of the sweat that helps cool the body. Evaporation (vaporization) from the liquid to gas phase is an endothermic process: Energy must be absorbed for the particles of the liquid to gain enough kinetic energy to escape into the gas phase. So the sweat that is excreted onto the skin must absorb energy in order to evaporate. Where does that necessary energy come from? It comes from the body itself. Hot, arid desert air has a lower partial pressure of water vapor than humid, tropical air, so sweat vaporizes more readily in the dry air than it does in the humid air. Although it might be hard to believe that any temperature in excess of 100° F could ever be considered comfortable, it probably is true that most people will feel more comfortable in “ dry heat” than in “ tropical heat.”

Answer 80

most reactions occur under contant pressure of 1 atm in closed thermodynamic systems. express heat changes at constant pressure, chemists use the term enthalpy(H). Enthalpy is a state function, so we can calculate the change in enthalpy (Δ H) for a system that has undergone a process— for example, a chemical reaction— by comparing the enthalpy of the final state to the enthalpy of the initial state, irrespective of the path taken. The change in enthalpy is equal to the heat transferred into or out of the system at constant pressure. To find the enthalpy change of a reaction, Δ Hrxn, you must subtract the enthalpy of the reactants from the enthalpy of the products. A positive Δ Hrxn corresponds to an endothermic process, and a negative Δ Hrxn corresponds to an exothermic process. It is not possible to measure enthalpy directly; only Δ H can be measured, and only for certain fast and spontaneous processes.

Answer 81

standard enthalpy of formation of a compound, Δ H° f, is the enthalpy change that would occur if one mole of a compound in its standard state were formed directly from its elements in their respective standard states. Remember that standard state is the most stable physical state of an element or compound at 298 K and 1 atm. Note that Δ H° f of an element in its standard state, by definition, is zero.

Answer 82

- the hypothetical enthalpy change that would occur if the reaction were carried out under standard conditions. What this means is that all reactants must be in their standard states and all products must be in their standard states. This can be calculated by taking the difference between the sum of the standard heats of formation for the products and the sum of the standard heats of formation of the reactants

Answer 83

Enthalpy is a state function and is a property of the equilibrium state, so the pathway taken for a process is irrelevant to the change in enthalpy from one equilibrium state to another. Hess's law states that enthalpy changes of reactions are additive. When thermochemical equations (chemical equations for which energy changes are known) are added to give the net equation for a reaction, the corresponding heats of reaction are also added to give the net heat of reaction. You can think of Hess's law as being embodied in the enthalpy equations we've already introduced. For example, we can describe any reaction as the result of breaking down the reactants into their component elements, then forming the products from these elements. The enthalpy change for the reverse of any reaction has the same magnitude, but the opposite sign, as the enthalpy change for the forward reaction. make sure to switch signs when you reverse the equation. Also, make sure to multiply by the correct stoichiometric coeffi cients when performing your calculations. Because it takes energy to pull two atoms apart, bond breakage is always endothermic. The reverse process, bond formation, must always be exothermic. The larger the alkane reactant, the more numerous the combustion products. important to realize that Hess's law applies to any state function, including entropy and Gibbs free energy. • The enthalpy change for the phase change is called the heat of vaporization (Δ H° vap). As long as the initial and final states exist at standard conditions, the Δ H° rxn will always equal the Δ H° vap, irrespective of the particular pathway that the process takes in vaporization. For example, it's possible that Br2 (l) could first decompose to Br atoms, which then recombine to form Br2 (g), giving the following reaction mechanism. However, because the net reaction is the same as the one shown previously, the change in enthalpy will be the same.

Answer 84

- Hess’s law experessed in bond enthalpies or bond dissociation energies, Bond dissociation energy is the average energy that is required to break a particular type of bond between atoms in the gas phase (remember, bond dissociation is an endothermic process). Bond dissociation energy is given as kJ/mol of bonds broken. For example, the bond enthalpy of the double bond in the diatomic oxygen molecule O2 is 498 kJ/mol. The tabulated bond enthalpies for bonds found in compounds other than diatomic molecules are the average of the bond energies for the bonds in many different compounds. For example, the C– H bond enthalpy (415 kJ/mol) is averaged from measurements of the individual C– H bond enthalpies of thousands of different organic compounds. Please note that bond formation, the opposite of bond breaking, has the same magnitude of energy but is negative rather than positive; that is, energy is released when bonds are formed. The enthalpy change associated with a reaction. decomposition reaction, diatomic hydrogen gas is cleaved to produce monoatomic hydrogen gas. For each mole of H2 cleaved, 436 kJ of energy is absorbed by the system in order to overcome the bonding force. Since energy is absorbed, the bond-breaking reaction is endothermic. You have to exert pulling forces (invest energy) to pull apart two bar magnets (endothermic). On the other hand, the two bar magnets, once separated, “ want” to come back together because they exert an attractive force between their opposite poles. Allowing them to stick together reduces their potential energy (exothermic).

Answer 85

standard enthalpy change. Δ H° comb. Because measurements of enthalpy change require a reaction to be spontaneous and fast, combustion reactions are the ideal process for such measurements. With most reaction occurring the in the presence of O2 in the atmosphere. Diatomic fluorine, for example, can be used as an oxidant. In addition, hydrogen gas will combust with chlorine gas to form gaseous hydrochloric acid and, in the process, will evolve a large amount of heat and light characteristic of combustion reactions. The reactions listed in the C3H8 (g) example shown earlier are combustion reactions with O2 (g) as the oxidant. Therefore, the enthalpy change listed for each of the three reactions is the Δ Hcomb for each of the reactions.

Answer 86

freezing is accompanied by a decrease in entropy, as the relatively disordered liquid becomes a wellordered solid. Meanwhile, boiling is accompanied by a large increase in entropy, as the liquid becomes a much more disordered gas. For any substance, sublimation will be the phase transition with the greatest entropy change. hot tea cools down, frozen drinks melt, iron rusts, buildings crumble, balloons deflate, living things die, energy of some form is going from being localized or concentrated to being spread out or dispersed. The thermal energy in the hot tea is spreading out to the cooler air that surrounds it. The thermal energy in the warmer air is spreading out to the cooler frozen drink. The chemical energy in the bonds of elemental iron and oxygen is released and dispersed as a result of the formation of the more stable (lower-energy) bonds of iron oxide (rust). The potential energy of the building is released and dispersed in the form of light, sound, and heat (motional energy) of the ground and air as the building crumbles and falls. The motional energy of the pressurized air is released to the surrounding atmosphere as the balloon deflates. The chemical energy of all the molecules and atoms in living flesh is released into the environment during the process of death and decay With entropy is the measure of the spontaneous dispersal of energy at a specific temperature, how much the energy is spread out or how widely spread out the energy becomes. where Δ S is the change in entropy, Qrev is the heat that is gained or lost in a reversible process (a process that proceeds with infinitesimal changes in the system), and T is the temperature in Kelvin. The units of entropy are usually kJ/mol· K. When energy is distributed into a system at a given temperature, its entropy increases. When energy is distributed out of a system at a given temperature, its entropy decreases. second law states that energy will spontaneously disperse; it does not say that energy can never be localized or concentrated. However, the concentration of energy will rarely happen spontaneously in a closed system. Work usually must be done to concentrate energy. For example, refrigerators move thermal energy against a temperature gradient (that is, they cause heat to be transferred from “ cool” to “ warm” ), thereby “ concentrating” energy outside of the system in the surroundings. As a result, refrigerators consume a lot of energy to accomplish this movement of energy against the temperature gradient. * Entropy is a state function, so a change in entropy from one equilibrium state to another is pathway-independent and only depends upon the difference in entropies of the final and initial states. * The standard entropy change for a reaction, Δ S° rxn, is calculated using the standard entropies of reactants and products.

Answer 87

Second law of thermodynamics- Energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so. Pay attention to this: The usual way of thinking about entropy as “ disorder” must not be taken too literally, a trap that many students fall into. Be very careful in thinking about entropy as disorder. The old analogy between a messy (disordered) room and entropy is arguably deficient and may not only hinder understanding but actually increase confusion. The second law has been described as “ time's arrow,” because there is a unidirectional limitation on the movement of energy by which we recognize “ before and after” or “ new and old.” energy in a closed system will spontaneously spread out and entropy will increase if it is not hindered from doing so. Remember that a system can be variably defined to include the entire universe; in fact, the second law ultimately claims that the entropy of the universe is increasing.

Answer 88

Δ G = Δ H− TΔ S. Get High Test Scores! This state function is a combination of the two that we've just examined: enthalpy and entropy. The change in Gibbs free energy, Δ G, is a measure of the change in the enthalpy and the change in entropy as a system undergoes a process, and it indicates whether a reaction is spontaneous or nonspontaneous. The change in the free energy is the maximum amount of energy released by a process, occurring at constant temperature and pressure, that is available to perform useful work. This equation is used to determine change in free energy. It is most commonly used to determine whether or not a reaction is spontaneous. Where T is the temperature in Kelvin and TΔ S represents the total amount of energy that is absorbed by a system when its entropy increases reversibly. any system, including chemical reactions, will move in whatever direction results in a reduction of the free energy of the system. Movement toward the equilibrium position is associated with a decrease in Gibbs free energy (− Δ G) and is spontaneous, while movement away from the equilibrium position is associated with an increase in Gibbs free energy (+Δ G) and is nonspontaneous. Once at the energy minimum state, the position of equilibrium (the bottom of the valley), the system will resist any changes to its state, and the change in free energy is zero for all systems at equilibrium. 1. If Δ G is negative, the reaction is spontaneous. 2. If Δ G is positive, the reaction is nonspontaneous. 3. If Δ G is zero, the system is in a state of equilibrium; thus Δ H = TΔ S. a reaction is thermodynamically spontaneous, it has no bearing on how fast it goes. It only means that it will proceed eventually without external energy input. Because the temperature in Gibbs free energy is in units of Kelvin, it is always positive. Therefore, the effects of the signs on Δ H and Δ S and the effect of temperature on the spontaneity of a process.

Answer 89

spontaneous at all temperatures: Δ H=-, Δ S=+. nonspontaneous at all temperatures: Δ H=+ Δ S=-. spontaneous only at high temperatures: ΔH=+ Δ S=+. spontaneous only at low temperatures: ΔH=- ΔS=-. Δ H = - exothermic Δ H = + endothermic Δ S=- decrease in entropy Δ S=+ increase in entropy

Answer 90

are examples of temperature-dependent processes. When water boils, hydrogen bonds (H-bonds) are broken, and the water molecules gain sufficient potential energy to escape into the gas phase. Thus, boiling (vaporization) is an endothermic process, and Δ H is positive. As thermal energy is transferred to the water molecules, energy is distributed through the molecules entering the gas phase, and entropy is positive and TΔ S is positive. Both Δ H and TΔ S are positive, so the reaction will be spontaneous only if TΔ S is greater than Δ H, giving a negative Δ G. These conditions are met only when the temperature of the system is greater than 373 K (100° C). Below 100° C, the free energy change is positive, and boiling is nonspontaneous; the water remains a liquid. At 100° C, Δ H = TΔ S and Δ G = 0; equilibrium between the liquid and gas phases is established in such a way that the water's vapor pressure equals the ambient pressure. This is the definition of the boiling point: the temperature at which the vapor pressure equals the ambient pressure.

Answer 91

• depends on the activation energy Ea, not the Δ G. Spontaneous reactions may be fast or slow. Sometimes a reversible reaction may produce two products that differ both in their stability, as measured by the change in the Gibbs free energy associated with their production, and in their kinetics, as measured by their respective energies of activation. Sometimes, the thermodynamically more stable product will have the slower kinetics due to higher activation energy. In this situation, we talk about kinetic versus thermodynamic reaction control. For a period of time after the reaction begins, the “ dominant” product— the major product— will be the one that is produced more quickly as a result of its lower energy of activation. The reaction can be said to be under kinetic control at this time. Given enough time, however, and assuming a reversible process, the dominant product will be the thermodynamically more stable product as a result of its lower free energy value. The reaction can then be said to be under thermodynamic control. Eventually, the reaction will reach its equilibrium, as defined by its Keq expression.

Answer 92

free energy change of reactions can be measured under standard state conditions to yield the standard free energy, Δ G° rxn. For standard free energy determinations, the concentrations of any solutions in the reaction are 1 M. The standard free energy of formation of a compound, Δ G° f, is the free energy change that occurs when 1 mole of a compound in its standard state is produced from its respective elements in their standard states under standard state conditions. The standard free energy of formation for any element under standard state conditions is, by definition, zero. The standard free energy of a reaction, Δ G° rxn, is the free energy change that occurs when that reaction is carried out under standard state conditions; that is, when the reactants in their standard states are converted to the products in their standard states, at standard conditions of temperature (298 K) and pressure (1 atm). For example, under standard state conditions, conversion of carbon in the form of diamond to carbon in the form of graphite is spontaneous (graphite is the standard state for carbon). However, the reaction rate is so slow that the conversion is never actually observed.

Answer 93

Free Energy, Keq, and Q- where Keq is the equilibrium constant, R is the gas constant, and T is the temperature in K. allows you to make not only quantitative evaluations of the free energy change of a reaction that goes from standard state concentrations of reactants to equilibrium concentrations of reactants and products, but also qualitative assessment of the spontaneity of the reaction. The greater the value of Keq, the more positive the value of its natural log. The more positive the natural log, the more negative the standard free energy change. The more negative the standard free energy change, the more spontaneous the reaction

Answer 94

Once a reaction begins, however, the standard state conditions (i.e., 1 M solutions) no longer hold. The value of the equilibrium constant must be replaced with another number that is reflective of where the reaction is in its path toward equilibrium. To determine the free energy change for a reaction that is in progress, we relate Δ Grxn (not Δ G° rxn) to the reaction quotient Q. where R is the gas constant and T is the temperature in Kelvin. By calculating the value of the reaction quotient and then comparing that value to the known equilibrium constant for the reaction, you will be able to predict whether the free energy change for the reaction is positive (a nonspontaneous reaction) or negative (a spontaneous reaction). That is, if the ratio of Q/Keq is less than one (Q < Keq), then the natural log will be negative and the free energy change will be negative, so the reaction will spontaneously proceed forward until equilibrium is reached. If the ratio of Q/Keq is greater than one (Q > Keq) , then the natural log will be positive, and the free energy change will be positive. In that case, the reaction will spontaneously move in the reverse direction until equilibrium is reached. Of course, if the ratio is equal to one, the reaction quotient is equal to the equilibrium constant; the reaction is at equilibrium, and the free energy change is zero (natural log of 1 is 0).

Answer 95

Bingham fluids do not begin to flow immediately upon application of shear stress. Unlike Newtonian fluids, such as water or vegetable oil, which begin to flow as soon as a finite amount of shear stress is applied, Bingham fluids will only begin to flow when a minimum force value called the yield value is exceeded. Essentially, Bingham fluids behave like solids under static conditions and flow as fluids only when a shear stress at least equal to the yield value is applied. Ketchup also belongs to a class of liquids known as pseudoplastic fluids, which demonstrate the property of shear thinning. Shear-thinning liquids display reducing viscosity with increasing shear rate (related to fluid velocity). That ketchup is stuck, like a solid, in that bottle until it isn't (because the yield value has been exceeded). At that point it begins to flow, and the faster it flows, the more it “ thins” and becomes less viscous. The result of this combination of physical properties is a mess on your plate and lap. hen the attractive forces between molecules (i.e., van der Waals forces, etc.) overcome the kinetic energy that keeps them apart in the gas phase, the molecules move closer together, entering the liquid or solid phase. The liquid and solid phases are often referred to as the condensed phases because of their higher densities compared to that of the gaseous phase. Molecules in the liquid and solid phases have lower degrees of freedom of movement than those in the gaseous phase as a result of the stronger intermolecular forces that dominate in the liquid and solid phases.

Answer 96

• rigidity and resistance to flow intermolecular attractive forces among atoms, ions, or molecules of solid matter holding them in rigid arrangement, no linear motion but posses kinetic limited to vibration in definite shapes independent of shape of container and volumes, incompressible; that is, a given mass of any solid or liquid will have a constant volume regardless of changes in pressure. Most substances water is densest. Water in its solid phase (ice) is less dense than it is in its liquid phase due to the greater spacing between the molecules in the crystalline structure of ice. The spacious lattice of ice crystals is stabilized by the hydrogen bonds between water molecules. Water molecules in the liquid phase also interact through hydrogen bonds, but because the water molecules are moving around, the lattice arrangement is absent, and the molecules are able to move closer to each other. In fact, water's density reaches a maximum around 4° C. The density decreases at temperatures above 4° C because the increasing kinetic energy of the water molecules causes the molecules to move further apart. Between 4° C and 0° C, the density decreases because the lattice organization of hydrogen bonds is beginning to form. Crystalline structures allow for a balance of both attractive and repulsive forces to minimize energy. The molecular arrangement of particles in the solid phase can be either crystalline or amorphous. Crystalline solids, such as the ionic compounds (e.g., NaCl), possess an ordered structure; their atoms exist in a specific three-dimensional geometric arrangement or lattice with repeating patterns of atoms, ions, or molecules. Amorphous solids, such as glass, plastic, and candle wax, lack an ordered three-dimensional arrangement. The particles of amorphous solids are fixed in place but not in the lattice arrangement that characterizes crystalline solids. Most solids are crystalline in structure. The two most common forms of crystals are metallic and ionic crystals. Ionic solids often have extremely strong attractive forces, thereby causing extremely high melting points.

Answer 97

Ionic solids are aggregates of positively and negatively charged ions that repeat according to defined patterns of alternating cations and anions. As a result, the solid mass of an ionic compound, such as NaCl, does not contain discrete molecules. high melting points, high boiling points, and poor electrical conductivity in the solid state but high conductivity in the molten state or in aqueous solution. These properties are due to the compounds' strong electrostatic interactions, which also cause the ions to be relatively immobile in the solid phase. Ionic structures are given by empirical formulas that describe the ratio of atoms in the lowest possible whole numbers. For example, the empirical formula BaCl2 gives the ratio of barium to chloride atoms within the crystal.

Answer 98

metallic solids- consist of metal atoms packed together as closely as possible. Metallic solids have high melting and boiling points as a result of their strong covalent attractions. Pure metallic masses (consisting of a single metal element) are usually described as layers of spheres of roughly similar radii, stacked layer upon layer in a “ staggered” arrangement such that one sphere in one layer fits into the indented space between the spheres that sit above and below it. These staggered arrangements (body-centered cubic and face-centered cubic re more common than layers of spheres stacked to form perfectly aligned columns of spheres because the staggered arrangement minimizes the separation between the atoms. The repeating units of crystals (both ionic and metallic) are represented by unit cells

Answer 99

Compounds and Stoichiometry, referred to the geometric arrangement of Na+ and Cl− ions in table salt as 6:6 coordinated, meaning that each sodium ion is surrounded by (coordinated by) six chloride ions, and each chloride ion is surrounded by (coordinated by) six sodium ions. This particular arrangement is also known as face-centered cubic. There are three cubic unit cells simple cubic, body-centered cubic, and face-centered cubic. the spaces between the anions are occupied by the smaller cations. In most ionic compounds, the anion, which has gained one or more electrons, is much larger than the cation, which has lost one or more electrons.

Answer 100

liquid phase, along with the solid phase, is considered a condensed phase because the spacing between the particles is reduced in comparison to that between gas particles. ncompressible, meaning that their volumes do not change in any significant way as a result of moderate pressure changes. Liquids are categorized as fluids (along with gases) because they do not resist shearing forces and flow when subjected to them. They also conform to the shapes of their containers. These behaviors are a result of the high degree of freedom of movement liquids possess. Like gas molecules, liquid molecules can move about in random motion and are disordered in their arrangement. Both liquids and gases are able to diffuse. Liquid molecules near the surface of the liquid can gain enough kinetic energy to escape into the gas phase; this is called evaporation. most important properties of liquids is their ability to mix— both with each other and with other phases— to form solutions. The degree to which two liquids can mix is called their miscibility. While ethanol and water are completely miscible, oil and water are almost completely immiscible; that is, their molecules tend to repel each other due to their polarity differences. You're certainly familiar with the expression “ Like dissolves like.” Oil and water normally form separate layers when mixed, with the oil layer above the water because it is less dense. Organic chemists take advantage of the solubility differences of immiscible liquids to separate compounds through the method of liquid-liquid extraction. Agitation of two immiscible liquids can result in the formation of a fairly homogenous mixture called an emulsion. Although they look like solutions, emulsions are actually mixtures of discrete particles too small to be seen distinctly.

Answer 101

Phase Equilibria- phase changes reversible with an equilibrium eventually reached, at 1 atm and 0° C in an isolated system, an ice cube and the water in which it floats are in equilibrium. In other words, some of the ice may absorb heat (from the liquid water) and melt, but since that heat is being removed from the liquid water, an equal amount of the liquid water will freeze and form ice relative amounts of water and ice constant. Equilibrium between the liquid and gas states of water will be established in a closed container, such as a plastic water bottle with the cap screwed on tightly. At room temperature and atmospheric pressure, most of the water in the bottle will be in the liquid phase, but a small number of molecules at the surface will gain enough kinetic energy to escape into the gas phase; likewise, a small number of gas molecules will lose sufficient kinetic energy to re-enter the liquid phase. After a while, equilibrium is established, and the relative amounts of water in the liquid and gas phases become constant— at room temperature and atmospheric pressure, equilibrium occurs when the air above the water has about 3 percent humidity. Phase equilibria are analogous to the dynamic equilibria of reversible chemical reactions for which the concentrations of reactants and products are constant because the rates of the forward and reverse reactions are equal.

Answer 102

gas-liquid equilibrium- The temperature of any substance in any phase is related to the average kinetic energy of the molecules that make up the substance not all the molecules have exactly the same instantaneous speeds. Therefore, the molecules possess a range of instantaneous kinetic energy values. In the liquid phase, the molecules have relatively large degrees of freedom of movement. Some of the molecules near the surface of the liquid may have enough kinetic energy to leave the liquid phase and escape into the gaseous phase. This process is known as evaporation (or vaporization). Each time the liquid loses a high-energy particle, the temperature of the remaining liquid decreases. Evaporation is an endothermic process for which the heat source is the liquid water. Of course, the liquid water itself may be receiving thermal energy from some other source, that given enough energy all liquid will freeze or evaporate. In covered or closed container, the escaping molecules are trapped above the solution. These molecules exert a countering pressure, which forces some of the gas back into the liquid phase; this process is called condensation. Condensation is facilitated by lower temperature or higher pressure. Atmospheric pressure acts on a liquid in a manner similar to that of an actual physical lid. As evaporation and condensation proceed, the respective rates of the two processes become equal, and equilibrium is reached. The pressure that the gas exerts over the liquid at equilibrium is the vapor pressure of the liquid. Vapor pressure increases as temperature increases, since more molecules have sufficient kinetic energy to escape into the gas phase. The temperature at which the vapor pressure of the liquid equals the ambient (also known as external, applied, or atmospheric) pressure is called the boiling point: During boiling, vaporization happens throughout the entire volume of the liquid, not just near its surface

Answer 103

liquid-solid equilibrium- Even though the atoms or molecules of a solid are confined to definite locations, each atom or molecule can undergo motions about some equilibrium position. These vibrational motions increase when heat is applied. From our understanding of entropy, we can say that the availability of energy microstates increases as the temperature of the solid increases. In basic terms, this means that the molecules have greater freedom of movement, and energy disperses. If atoms or molecules in the solid phase absorb enough energy, the three-dimensional structure of the solid will break down, and the atoms or molecules will escape into the liquid phase. The transition from solid to liquid is called fusion or melting. The reverse process, from liquid to solid, is called solidification, crystallization, or freezing. The temperature at which these processes occur is called the melting point or freezing point, depending on the direction of the transition. Whereas pure crystalline solids have distinct, very sharp melting points, amorphous solids, such as glass, plastic, and candle wax, tend to melt (or solidify) over a larger range of temperatures due to their less-ordered molecular distribution.

Answer 104

• gas-solid equilibrium- The final phase equilibrium is that which exists between the gas and solid phase. When a solid goes directly into the gas phase, the process is called sublimation. Dry ice (solid CO2) sublimes at room temperature and atmospheric pressure; the absence of the liquid phase. The reverse transition, from the gaseous to the solid phase, is called deposition. In organic chemistry, a device known as the cold finger is used to purify a product that is heated under reduced pressure to cause it to sublimate. The desired product is usually more volatile than the impurities, so the gas is purer than the original product and the impurities are left in the solid state. The gas then deposits onto the cold finger, which has cold water flowing through it, yielding a purified solid product that can be collected.

Answer 105

Gibbs function- the thermodynamic criterion for each of the phase equilibria is that the change in Gibbs free energy must be equal to zero (G = 0). The same is true of the Gibbs functions for any other phase equilibria.

Answer 106

Heating curves- When a compound is heated, the temperature rises until the melting or boiling points are reached. Then the temperature remains constant as the compound is converted to the next phase (i.e., liquid or gas, respectively). Once the entire sample is converted, then the temperature begins to rise again

Answer 107

• Phase Diagrams- graphs that show the temperatures and pressures at which a substance will be thermodynamically stable in a particular phase. They also show the temperatures and pressures at which phases will be in equilibrium. With every pure substance having a characteristic phase diagram

Answer 108

Single component- lines on a phase diagram are called the lines of equilibrium or the phase boundaries and indicate the temperature and pressure values for the equilibria between phases. The lines of equilibrium divide the diagram into three regions corresponding to the three phases— solid, liquid, and gas— and they themselves represent the phase transformations. the gas phase is found at high temperatures and low pressures, the solid phase is found at low temperatures and high pressures, and the liquid phase is found at moderate temperatures and moderate pressures. The point at which the three phase boundaries meet is called the triple point. This is the temperature and pressure at which the three phases exist in equilibrium. The phase boundary that separates the solid and the liquid phases extends indefinitely from the triple point. The phase boundary between the liquid and gas phases, however, terminates at a point called the critical point. This is the temperature and pressure above which there is no distinction between the phases. Although this may seem to be an impossibility— after all, it's possible always to distinguish between the liquid and the solid phase— such “ supercritical fluids” are perfectly logical. As a liquid is heated in a closed system its density decreases and the density of the vapor sitting above it increases. The critical point is the temperature and pressure at which the two densities become equal and there is no distinction between the two phases. The heat of vaporization at this point and for all temperatures and pressures above the critical point values is zero.

Answer 109

multiple components- phase diagram for a mixture of two or more components is complicated by the requirement that the composition of the mixture, as well as the temperature and pressure, must be specified. vapor above the solution is a mixture of the vapors of A and B. The pressures exerted by vapor A and vapor B on the solution are the vapor pressures that each exerts above its individual liquid phase. Raoult's law enables one to determine the relationship between the vapor pressure of gaseous A and the concentration of liquid A in the solution. Curves such as this show the different compositions of the liquid phase and the vapor phase above a solution for different temperatures. The upper curve is the composition of the vapor, while the lower curve is that of the liquid. It is this difference in composition that forms the basis of distillation, an important separation technique in organic chemistry. For example, if we were to start with a mixture of A and B at a proportion of 40 percent A and 60 percent B and heat it to that solution's boiling point (85° C), the resulting vapor would not have the same composition as the liquid solution because the two compounds have different volatilities. Compound B is more volatile because it has the lower boiling point. Therefore, vapor B should be a larger proportion of the vapor than vapor A and will differ from the proportion of B to A in the liquid phase. Because the boiling point for this 60– 40 mixture is 85° C, the vapor will also be at 85° C. At this temperature, we can tell from the graph that the vapor composition will be 30 percent A and 70 percent B. Indeed, the proportion of the more volatile compound, in this case compound B, has been enhanced. Repeated rounds of boiling (vaporization) and condensation will ultimately yield a pure sample of compound B.

Answer 110

Properties- physical properties of solutions that are dependent on the concentration of dissolved particles but not on the chemical identity of the dissolved particles. These properties— vapor pressure depression, boiling point elevation, freezing point depression, and osmotic pressure— are usually associated with dilute solutions

Answer 111

vapor pressure depression- When you add solute to a solvent and the solute dissolves, the solvent in solution has a vapor pressure that is lower than the vapor pressure of the pure solvent for all temperatures. The lowering of a solution's vapor pressure would mean that a higher temperature is required to overcome atmospheric pressure, thereby raising the boiling point. If the vapor pressure of A above pure solvent A is designated by P° A and the vapor pressure of A above the solution containing B is PA, the vapor pressure decreases. where XB is the mole fraction of the solute B in solution with solvent A. Because XB = 1 − XA and P = P° A− PA, substitution into the previous equation leads to the common form of Raoult's law: where XA is the mole fraction of the solvent A in the solution. Similarly, the expression for the vapor pressure of the solute in solution (assuming it is volatile). Raoult's law holds only when the attraction between the molecules of the different components of the mixture is equal to the attraction between the molecules of any one component in its pure state. When this condition does not hold, the relationship between mole fraction and vapor pressure will deviate from Raoult's law. Solutions that obey Raoult's law are called ideal solutions. Raoult's law is used to calculate vapor pressure depression and plot the phase diagram for a solution of two liquids. It is most often encountered in reference to distillation

Answer 112

• Boiling point elevation- When a nonvolatile solute is dissolved into a solvent to create a solution, the boiling point of the solution will be greater than that of the pure solvent. If the vapor pressure of a solution is lower than that of the pure solvent, then more energy (and consequently a higher temperature) will be required before its vapor pressure equals the ambient pressure. where Tb is the boiling point elevation, Kb is a proportionality constant characteristic of a particular solvent, m is the molality of the solution [molality = moles of solute per kilogram of solvent (mol/kg). and i is the van't Hoff factor, which is the moles of particles dissolved into a solution per mole of solute molecules. For example, i = 2 for NaCl because each molecule of sodium chloride dissociates into two particles, a sodium ion and a chloride ion, when it dissolves. This equation is used to calculate boiling point elevation given molality or molality given boiling point elevation

Answer 113

• Freezing point depression- equation is used to calculate freezing point depression given molality or molality given freezing point depression. presence of solute particles in a solution interferes with the formation of the lattice arrangement of solvent molecules associated with the solid state. Thus, a greater amount of energy must be removed from the solution (resulting in a lower temperature) in order for the solution to solidify. , pure water freezes at 0° C, but for every mole of solute dissolved in 1 kg (1 liter) of water, the freezing point is lowered by 1.86° C. Therefore, the Kf for water is − 1.86° C/m. As is the case for Kb, the values for Kf are unique to each solvent. where Tf is the freezing point depression, Kf is the proportionality constant characteristic of a particular solvent, m is the molality of the solution, and i is the van't Hoff factor. Freezing point depression is a colligative property that depends only on the concentration of particles, not on their identity.

Answer 114

salt mixes with the snow and ice and initially dissolves into the small amount of liquid water that is in equilibrium with the solid phase (the snow and ice). The solute in solution causes a disturbance to the equilibrium such that the rate of melting is unchanged (because the salt can't interact with the solid water that is stabilized in a rigid lattice arrangement), but the rate of freezing is decreased (the solute displaces some of the water molecules from the solid-liquid interface and prevents liquid water from entering into the solid phase). imbalance causes more ice to melt than water to freeze. Melting is an endothermic process, so heat is initially absorbed from the liquid solution, causing the solution temperature to fall below the ambient temperature. Now, there is a temperature gradient and heat flows from the “ warmer” air to the “ cooler” aqueous solution; this additional heat facilitates more melting— even though the temperature of the solution is actually colder than it was before the solute was added! The more the ice melts into liquid water, the more the solute is dispersed through the liquid. The resulting salt solution, by virtue of the presence of the solute particles, has a lower freezing point than the pure water and remains in the liquid state even at temperatures that would normally cause pure water to freeze.

Answer 115

• Osmotic pressure- This equation is used to calculate osmotic pressure given molarity or molarity given osmotic pressure. one compartment contains pure water, while the other contains water with dissolved solute. The membrane allows water but not solute to pass through. Because substances tend to flow, or diffuse, from higher to lower concentration (which results in an increase in entropy), water will diffuse from the compartment containing pure water into the compartment containing the water-solute mixture. This net flow will cause the water level in the compartment containing the solution to rise above the level in the compartment containing pure water solute cannot pass through the membrane, the concentrations of solute in the two compartments can never be equal. However, the hydrostatic pressure exerted by the water level in the solute-containing compartment will eventually oppose the influx of water; thus, the water level will rise only to the point at which it exerts a sufficient pressure to counterbalance the tendency of water to flow across the membrane. This pressure, defined as the osmotic pressure (Π ) where M is the molarity of the solution, R is the ideal gas constant, T is the absolute temperature (in Kelvin), and i is the van't Hoff factor. The equation clearly shows that osmotic pressure is directly proportional to the molarity of the solution. Thus, osmotic pressure, like all colligative properties, depends only on the presence of the solute, not its chemical identity.

Answer 116

reverse osmosis, impure water is placed into one container separated from another container by a semipermeable membrane. High pressure is applied to the impure water, which forces it to diffuse across the membrane, filling the compartment on the other side of the membrane with purified water. Because the water is being forced across the membrane in the direction opposite its concentration gradient (that is, the water is being forced from the compartment with the lower concentration of water to the compartment with the higher concentration of water), large pressures (higher than the solution's osmotic pressure) are needed to accomplish the purification.

Answer 117

• Solutions are homogenous (the same throughout) mixtures of two or more substances that combine to form a single phase, usually the liquid phase. gases can be dissolved in liquids (e.g., the carbonation of soda); liquids can be dissolved in other liquids (e.g., ethanol in water); solids can even be dissolved in other solids (e.g., metal alloys). Incidentally, gases “ dissolved” into other gases can be thought of as solutions but are more properly defined as mixtures, because gas molecules don't really interact all that much (one of the postulates of the kinetic molecular theory of gases). All solutions are considered mixtures, but not all mixtures are considered solutions. A solution consists of a solute (e.g., NaCl, NH3, C6H12O6, CO2, etc.) dissolved (dispersed) in a solvent (e.g., H2O, benzene, ethanol, etc.). The solvent is the component of the solution whose phase remains the same after mixing. If the two substances are already in the same phase (for example, a solution of two liquids), the solvent is the component present in a greater quantity. And if the two same-phase components are in equal proportions in the solution, the component considered the solvent is the one that is more commonly identified as a solvent. Solute molecules move about freely in the solvent and interact with the solvent by way of interparticle forces such as ion– dipole, dipole– dipole, or hydrogen bonding. Dissolved solute molecules are also relatively free to interact with other dissolved molecules of different chemical identity; consequently, chemical reactions occur easily in solution.

Answer 118

•solvation- is the electrostatic interaction between solute and solvent molecules. This is also known as dissolution, and when water is the solvent, it is called hydration, and the resulting solution is an aqueous solution. Solvation involves breaking intermolecular interactions between solute molecules and between solvent molecules and forming new intermolecular interactions between solute and solvent molecules. When the new interactions (attractions) are stronger than the original ones, solvation is exothermic, and the process is favored at low temperatures. The dissolution of gases into liquids, such as CO2 into water, is an exothermic process because the only significant interactions that must be broken are those between water molecules. CO2, as a gas, demonstrates minimal intermolecular interaction, and thus the dissolution of CO2 gas into water is exothermic overall (and Le Châ telier's principle tells us this is the reason that lowering the temperature of a liquid favors solubility of a gas in the liquid.) new interactions (attractions) are weaker than the original ones, solvation is endothermic, and the process is favored at high temperatures. Most dissolutions are of this type. Two such examples have already been given: dissolving sugar or ammonium nitrate into water. Since the new interactions between the solute and solvent are weaker than the original interactions between the solute molecules and between the solvent molecules, energy (heat) must be supplied to facilitate the formation of these weaker, less stable interactions. Sometimes the overall strength of the new interactions is approximately equal to the overall strength of the original interactions. In this case, the overall enthalpy change for the dissolution is close to zero. These types of solutions approximate the formation of an ideal solution, for which the enthalpy of dissolution is equal to zero. When NaCl dissolves in water, its component ions dissociate from each other and become surrounded by water molecules. For this new interaction to occur, ion– ion interactions between Na+ and Cl− must be broken, and hydrogen bonds between water molecules must also be broken. This step requires energy and is therefore endothermic. Because water is polar, it can interact with each of the component ions through ion– dipole interactions: The partially positive hydrogen end of the water molecules will surround the Cl− ions, and the partially negative oxygen end of the water molecules will surround the Na+ ions. The formation of these ion– dipole bonds is exothermic, but not as much as the endothermicity of breaking the old ones (although it is quite close). As a result, the overall dissolution of table salt into water is endothermic (+0.93 J/mol) and favored at high temperatures. When solid sodium chloride dissolves into water, the rigidly ordered arrangement of the sodium and chloride ions is broken up as the ion– ion interactions are disrupted and new ion– dipole interactions with the water molecules are formed. The ions, freed from their lattice arrangement, have a greater number of energy microstates available to them (in simpler terms, they are freer to move around in different ways), and, consequently, their energy is more distributed and their entropyincreases. The water, however, becomes more restricted in its movement because it is now interacting with the ions. The number of energy microstates available to it (that is, the water molecules' ability to move around in different ways) is reduced, so the entropy of the water decreases. But the increase in the entropy experienced by the dissolved sodium chloride is greater than the decrease in the entropy experienced by the water, so the overall entropy change is positive— energy is, overall, disbursed by the dissolution of sodium chloride in water. Because of the relatively low endothermicity and relatively large positive change in entropy, sodium chloride will spontaneously dissolve in liquid water.

Answer 119

• solubility- the solubility of a substance is the maximum amount of that substance that can be dissolved in a particular solvent at a particular temperature. When this maximum amount of solute has been added, the dissolved solute is in equilibrium with its undissolved state, and we say that the solution is saturated. If more solute is added, it will not dissolve. For example, at 18º C, a maximum of 83 g of glucose (C6H12O6) will dissolve in 100 mL of H2O. Thus, the solubility of glucose is 83 g/100 mL. If more glucose is added to an already saturated glucose solution, it will not dissolve but rather will remain in solid form, precipitating to the bottom of the container. A solution in which the proportion of solute to solvent is small is said to be dilute, and one in which the proportion is large is said to be concentrated. Note that both dilute and concentrated solutions are still considered unsaturated if the maximum equilibrium concentration (saturation) has not been reached. solubility of substances into different solvents is ultimately a function of thermodynamics. When the change in Gibbs free energy is negative at a given temperature for the dissolution of a given solute into a given solvent, the process will be spontaneous, and the solute is said to be soluble. When the change in Gibbs free energy is positive at a given temperature for the dissolution of a given solute into a given solvent, the process will be nonspontaneous, and the solute is said to be insoluble. Some solute/solvent systems have very large negative Δ Gs, so dissolution is very spontaneous and a lot of solute can be dissolved into the solvent. Others have very small negative Δ Gs, so dissolution is only slightly spontaneous and as a result only a little solute can be dissolved into the solvent. Those solutes that dissolve minimally in the solvent (usually water) are called sparingly soluble salts

Answer 120

aqueous solutions- the aqueous state is denoted by the symbol (aq). Because aqueous solutions are so common and so important to biological systems (e.g., you). All sodium salts are completely soluble, and all nitrate salts are completely soluble. 1. All salts of alkali metals are water soluble. 2. All salts of the ammonium ion (NH4+) are water soluble.3. All chlorides, bromides, and iodides are water soluble, with the exceptions of those formed with Ag+, Pb2+, and Hg22+. 4. All salts of the sulfate ion (SO42− ) are water soluble, with the exceptions of those formed with Ca2+, Sr2+, Ba2+, and Pb2+. 5. All metal oxides are insoluble, with the exception of those formed with the alkali metals and CaO, SrO, and BaO, all of which hydrolyze to form solutions of the corresponding metal hydroxides. 6. All hydroxides are insoluble, with the exception of those formed with the alkali metals and Ca2+, Sr2+, and Ba2+. 7. All carbonates (CO32− ), phosphates (PO43− ), sulfides (S2− ), and sulfites (SO32− ) are insoluble, with the exception of those formed with the alkali metals and ammonium.

Answer 121

Ions/ cations and anions- Ionic compounds are made up of positively charged cations and negatively charged anions. Ionic compounds are held together by the ionic bond, which is the force of electrostatic attraction between oppositely charged particles. 1. For elements (usually metals) that can form more than one positive ion, the charge is indicated by a Roman numeral in parentheses following the name of the element. 2. An older but still commonly used method is to add the endings -ous or -ic to the root of the Latin name of the element to represent the ions with lesser or greater charge, respectively. 3. Monatomic anions are named by dropping the ending of the name of the element and adding -ide.4. Many polyatomic anions contain oxygen and are therefore called oxyanions. When an element forms two oxyanions, the name of the one with less oxygen ends in -ite and the one with more oxygen ends in -ate. 5. When the series of oxyanions contains four oxyanions, prefixes are also used. Hypo- and per- are used to indicate less oxygen and more oxygen, respectively. 6. Polyatomic anions often gain one or more H+ ions to form anions of lower charge. The resulting ions are named by adding the word hydrogen or di-hydrogen to the front of the anion's name. An older method uses the prefix bi- to indicate the addition of a single hydrogen ion.

Answer 122

• Ion charges- have charge, Cations have positive charge, and anions have negative charge. Some elements are found naturally only in their charged forms, while others may exist naturally in the charged or uncharged state. Furthermore, some elements can have several different charges or oxidation states. the charged atoms or molecules- the alkali metals (Group IA) and the alkaline earth metals (Group IIA), which have charges of +1 and +2, respectively, in the natural state. Many of the transition metals, such as copper, iron, and chromium, can exist in different positively charged states. Nonmetals, which are found on the right side of the periodic table, generally form anions. all the halogens (Group VIIA) form monatomic anions with a charge of − 1. All elements in a given group tend to form monatomic ions with the same charge (e.g., Group IA elements have a charge of +1). Note that there are anionic species that contain metallic elements (e.g., MnO4− [permanganate] and CrO42− [chromate]); even so, the metals have positively charged oxidation states.

Answer 123

electrolytes- solid ionic compounds tend to be poor conductors of electricity because the charged particles are rigidly set in place by the lattice arrangement that serves as the basic framework for crystalline solids. In aqueous solutions, however, the lattice arrangement is disrupted by the ion– dipole interactions between the ionic constituents and the water molecules. The freed ions are now able to move around, and as a result, the solution of ions is able to conduct electricity. Solutes that enable their solutions to carry currents are called electrolytes. The electrical conductivity of aqueous solutions is governed by the presence and concentration of ions in the solution. Pure water, which has no ions other than the very few hydrogen ions and hydroxide ions that result from water's low-level autodissociation, is a very poor conductor. tendency of an ionic solute to dissociate into its constituent ions in water may be high or low. A solute is considered a strong electrolyte if it dissociates completely into its constituent ions. Examples of strong electrolytes include certain ionic compounds, such as NaCl and KI, and molecular compounds with highly polar covalent bonds that dissociate into ions when dissolved, such as HCl in water. A weak electrolyte, on the other hand, ionizes or hydrolyzes incompletely in aqueous solution, and only some of the solute is dissolved into its ion constituents. Examples include Hg2I2 (Ksp = 4.5 × 10− 29), acetic acid and other weak acids, and ammonia and other weak bases (see Chapter 10, Acids and Bases). Many compounds do not ionize at all in aqueous solution, retaining their molecular structure in solution, which usually limits their solubility. These compounds are called nonelectrolytes and include many nonpolar gases and organic compounds, such as O2 (g), CO2 (g), and glucose.

Answer 124

Concentration denotes the amount of solute dissolved in a solvent. There are many different ways of expressing concentration, and different units have been standardized that you may encounter in everyday situations.

Answer 125

units of concentration: Percent Composition by Mass- The percent composition by mass (w/w%) of a solution is the mass of the solute divided by the mass of the solution (solute plus solvent), multiplied by 100 percent.

Answer 126

Mole Fraction- mole fraction (X) of a compound is equal to the number of moles of the compound divided by the total number of moles of all species in the system. The sum of the mole fractions in a system will always equal 1. Mole fraction is used to calculate the vapor pressure depression of a solution, as well as the partial pressures of gases in a system.

Answer 127

molarity (M)- of a solution is the number of moles of a solute per liter of solution. Solution concentrations are usually expressed in terms of molarity, and you will be working mostly with molarity on the MCAT. The molarity of a solute in solution is often represented by brackets (e.g., [Na+]). Please note that the volume term in the denominator of molarity refers to the solution volume, not the solvent volume used to prepare the solution— although often the two values are close enough that we can approximate the solution volume by the solvent volume. We use molarity for the law of mass action, rate laws, osmotic pressure, pH and pOH, and the Nernst equation.

Answer 128

Molality (m) of a solution is the number of moles of solute per kilogram of solvent. For dilute aqueous solutions at 25° C, the molality is approximately equal to molarity, because the density of water at this temperature is 1 kilogram per liter (1 kg/L). However, note that this is an approximation and true only for dilute aqueous solutions. (As aqueous solutions become more concentrated with solute, their densities become significantly different from that of pure water; most water-soluble solutes have molecular weights significantly greater than that of water, so the density of the solution increases as the concentration increases.) You won't use molality very often, so be mindful of the special situations when it is required: boiling point elevation and freezing point depression

Answer 129

Normality- normality of a solution is equal to the number of equivalents of solute per liter of solution. An equivalent, or gram equivalent weight, is a measure of the reactive capacity of a molecule. Most simply, an equivalent is equal to a mole of charge to calculate the normality of a solution, you need to know for what purpose the solution is being used, because it is the concentration of the reactive species with which we are concerned. For example, in acid-base reactions, we are most concerned with the concentration of hydrogen ions; in oxidation-reduction reactions, we are most concerned with the concentration of electrons. Normality is unique among concentration units in that it is reaction-dependent. For example, in acidic solution, 1 mole of the permanganate ion (MnO4− ) will readily accept 5 moles of electrons, so a 1 M solution would be 5 N. However, in alkaline solution, 1 mole of permanganate will accept only 3 moles of electrons, so in alkaline solution, a 1 M permanganate solution would be 3 N.

Answer 130

dilution- A solution is diluted when solvent is added to a solution of high concentration to produce a solution of lower concentration. The concentration of a solution after dilution can be conveniently determined where M is molarity, V is volume, and the subscripts i and f refer to the initial and final values, respectively.

Answer 131

• Solution Equilibria- solvation, like other reversible chemical and physical processes, tends toward an equilibrium position defined as the lowest energy state of a system under given conditions of temperature and pressure. Related to determinations of equilibrium is the characterization of processes as spontaneous or nonspontaneous: Systems tend to move spontaneously toward the equilibrium position, but any movement away from equilibrium is nonspontaneous. In the process of creating a solution, the equilibrium is defined as the saturation point, where the solute concentration is at its maximum value for the given temperature and pressure. Immediately after solute has been introduced into a solvent, most of the change taking place is dissociation, because no dissolved solute is initially present. However, once solute is dissolved, the reverse process, precipitation of the solute, will begin to occur. When the solution is dilute (unsaturated), the thermodynamically favored process is dissolution, and initially, the rate of dissolution will be greater than the rate of precipitation. As the solution becomes more concentrated and approaches saturation, the rate of dissolution lessens while the rate of precipitation increases. Eventually, the saturation point of the solution is reached, and the solution exists in a state of dynamic equilibrium for which the rates of dissolution and precipitation are equal and the concentration of dissolved solute reaches a steady state (that is, constant) value. Neither dissolution nor precipitation is more thermodynamically favored at equilibrium than the other (because favoring either one of them would necessarily result in the solution no longer being in a state of equilibrium), so the change in free energy is zero, as is the case for all systems at equilibrium. An ionic solid introduced into a polar solvent dissociates into its component ions, and the dissociation of such a solute in solution

Answer 132

the solubility product constant- The degree of solubility is determined by the relative changes in enthalpy and entropy associated with the dissolution of the ionic solute at a given temperature and pressure. One common sparingly soluble salt is silver chloride, AgCl,. units of Ksp depend on the solid. The concentration of the ions is generally in units of molarity (mol/L). This formula is used to determine Ksp or molar solubility. It is most often used in the context of the ion product.law of mass action can be applied to a solution at equilibrium; that is to say, when the solution is saturated and the solute concentration is at a maximum and dynamically stable. For a saturated solution of an ionic compound with the formula AmBn, the equilibrium constant for its solubility in aqueous solution, called the solubility product constant Ksp, where the concentrations of the ionic constituents are equilibrium (saturation) concentrations. For example, we can express the Ksp of silver chloride. Since the silver chloride solution was formed by adding pure solid silver chloride to pure water, neither the solid silver chloride nor the water is included. Solubility product constants, like all other equilibrium constants (Keq, Ka, and Kb) are temperature-dependent. When the solution consists of a gas dissolved into a liquid, the value of the equilibrium constant, and hence the “ position” of equilibrium (saturation), will also depend on pressure. Generally speaking, the solubility product constant increases with increasing temperature for nongas solutes and decreases for gas solutes. Higher pressures favor dissolution of gas solutes, and therefore the Ksp will be larger for gases at higher pressures than at lower ones. solute dissolves into solvent, the system approaches saturation, at which point no more solute can be dissolved and any excess will precipitate to the bottom of the container. You may not know whether the solution has reached saturation, and so to determine “ where” the system is with respect to the equilibrium position, you will calculate a value called the ion product (I.P.), which is analogous to the reaction quotient Q for chemical reactions. The ion product equation has the same form as the equation for the solubility product constant. The difference is that the concentrations that you use are the concentrations of the ionic constituents at that given moment in time.

Answer 133

Because gases become more soluble in solution as pressure increases, a diver who has spent time at significant depths under water will have more nitrogen gas dissolved in her blood. (Nitrogen gas is the main inert gas in the air we breathe.) If she rises to the surface too quickly, the abrupt decompression will lead to an abrupt decrease in gas solubility in the plasma, resulting in the formation of nitrogen gas bubbles in her bloodstream. The gas bubbles can get lodged in the small vasculature of the peripheral tissue, mostly around the large joints of the body, causing pain and tissue damage (hence the name of the condition is the “ bends” ). The condition is painful and dangerous, and can be fatal if not properly prevented or treated.

Answer 134

•The units of I.P. depend on the solid. The concentration of the ions is generally in units of molarity (mol/L). I.P. is compared to Ksp to determine the behavior of a solution.where the concentrations are not necessarily equilibrium (saturation) concentrations. As with the reaction quotient Q, the utility of the I.P. lies in comparing its value to that attained at equilibrium, in this case, the known Ksp. Each salt has its own distinct Ksp at a given temperature and pressure. If, at a given set of temperature and pressure conditions, a salt's I.P. is less than the salt's Ksp, then the solution is not yet at equilibrium and we say that it is unsaturated. For unsaturated solutions, dissolution is thermodynamically favored over precipitation. If the I.P is greater than the Ksp, then the solution is beyond equilibrium and we say that it is supersaturated. It's possible to create a supersaturated solution by dissolving solute into a hot solvent and then slowly cooling the solution. A supersaturated solution is thermodynamically unstable, and any disturbance to the solution, like the addition of more solid solute or other solid particles or further cooling of the solution, will cause spontaneous precipitation of the excess dissolved solute. If the calculated I.P. is equal to the known Ksp, then the solution is at equilibrium, the rates of dissolution and precipitation are equal, and the concentration of solute is at the maximum (saturation) value. If the solution is supersaturated, Qsp > Ksp, precipitation will occur. If the solution is unsaturated, Qsp < Ksp, the solute will continue to dissolve. If the solution is saturated, Qsp = Ksp, then the solution is at equilibrium.

Answer 135

* common ion effect- The solubility of a substance varies depending on the temperature of the solution, the solvent, and, in the case of a gas-phase solute, the pressure. The solubility of a salt is considerably reduced when it is dissolved in a solution that already contains one of its constituent ions (compared to its solubility in the pure solvent). This reduction in molar solubility is called the common ion effect. Molar solubility (M) is the concentration, in moles per liter (mol/L), of the solute in the solution at equilibrium at a given temperature. If X moles of AmBn (s) can be dissolved in Y liters of solution to reach saturation, then the molar solubility of AmBn (s) is X mol/Y L. Let us repeat the important effect of the common ion: Its presence results in a reduction in the molar solubility of the salt. Note that the presence of the common ion has no effect on the value of the solubility product constant for the salt. For example, if a salt such as CaF2 is dissolved into a solvent already containing Ca2+ ions (from some other salt, perhaps CaCl2), the solution will dissolve less CaF2 compared to the amount that would be dissolved in the pure solvent (before the I.P. equals Ksp). The common ion effect is really nothing other than Le Châ telier's principle in action. Because the solution already contains one of the constituent ions from the right side of the dissociation equilibrium, we can see that the system will shift away from that side toward the left side, where we find the solid salt. A solution system shifting toward the left (solid salt reactant) is not going to favor dissolution. As a result, molar solubility for the solid is reduced, and less of the solid dissolves in the solution (resulting in the same Ksp). Solubility is also affected by the addition of other substances to the solution. * Every slightly soluble salt of general formula MX3 will have Ksp = 27x4, where x is the molar solubility. Every slightly soluble salt of general formula MX2 will have Ksp = 4x3, where x is the molar solubility. Every slightly soluble salt of general formula MX will have Ksp = x2, where x is the molar solubility

Answer 136

• The Law of Conservation of Matter- states that matter can neither be created nor destroyed. When applied to chemical reactions, this law implies that the number of atoms of each element must be the same on the reactant and the product side. When we balance an equation, we may only change the coefficient appearing before the formula of each species. We may not change the subscripts within a formula; doing so would alter the chemical nature of the species. Remember that no coefficient in front of a species in the reaction implies that a one is present.

Answer 137

* Identify the reactants and products of the reaction if they are not completely specified. Write the formula(s) of the reactants on the left side of the arrow and the formula(s) of the products on the right side of the arrow.Balance the element that appears in the fewest number of molecules. Continue balancing the elements as they appear. Balance last the element that appears in the most number of molecules.As necessary, multiply all coefficients on both sides of the reaction by the same amount in order to clear fractional coefficients. * First write the unbalanced equation. * Second, balance the carbons by multiplying carbon dioxide by 2. * Third, balance the oxygens by multiplying the oxygen molecule by 5/2. * Finally, remove fractions by multiplying all substances by 2.

Answer 138

composiiton of reactants and products, and the driving force of the reaction

Answer 139

Combination reactions are reactions in which two or more reactants combine to form one product. Recognize a combination reaction by observing one product that is a combination of both of the reactants. Specific types of combination reactions are combustion, formation, and corrosion reactions.

Answer 140

The combustion reaction is a type of combination reaction, where a substance is burned in the presence of a gas. Magnesium burns in the presence of nitrogen gas to form solid magnesium nitride. Sodium burns in the presence of chlorine gas to form solid sodium chloride. Sulfur burns in the presence of oxygen to form sulfur dioxide. These three reactions have several things in common. First, there are two reactants combining to make one product, making each reaction a combination reaction. Also, in each reaction, one of the reactants is a gas, making each reaction a combustion reaction. The above reactions also have another important property in common – each reaction has an element as one of the reactants that changes oxidation state during the course of the reaction. The change of oxidation state also allows us to classify these reactions as reduction-oxidation, or redox, reactions as well. (We will review oxidation states and redox reactions later in this workshop.) It is common for reactions to fall into several different categories. Although not technically a combination reaction, the most frequently seen example of a combustion reaction is the burning of hydrocarbons in the presence of oxygen. If oxygen is not present in excess, incomplete combustion occurs, and side reactions yield carbon monoxide rather than carbon dioxide. The reactions of hydrocarbons will be covered in more detail in the organic chemistry portion of the course.

Answer 141

• Formation reactions are types of combination reactions in which a single product is formed from elements in their standard states. Gaseous propane can be formed from its elements in their standard states.Carbon dioxide gas can be formed from its elements in their standard states.The above examples of formation reactions have several things in common. They both have the generic formula for combination reactions, A + B AB. Also, each of the reactants is an element in its standard state. The standard state is defined as the phase, or state, an element is in at one atmosphere of pressure.

Answer 142

* Metals deteriorate in corrosion reactions when a liquid or gas, usually oxygen, chemically attacks the surface of the metal. The most common example of corrosion is the rusting of iron. The silver of silverware that comes into contact with sulfur in food corrodes to silver sulfide. * Decomposition reactions occur when a compound breaks down into two or more substances, usually as a result of heating, electrolysis, or light. The reverse of a decomposition reaction is always a combination reaction, and vice versa. Mercury(II) oxide breaks down into mercury and oxygen in the presence of heat. In order to depict the method of decomposition, a symbol can be placed above the arrow of the chemical equation. The delta above represents the addition of heat. Another example of decomposition is the breakdown of ozone by ultra-violet light. Decomposition reactions can also be reversible, as is the case with the decomposition of limestone (calcium carbonate). Double arrows, like the ones above, are used to depict a reversible reaction. In a reversible reaction both the forward and reverse reaction can occur.

Answer 143

Reactions can also be classified according to the driving force of the reaction. There are various forces that drive a reaction to occur. Among these forces are an increase in entropy due to gas formation, transfer of electrons to achieve greater stability, and the formation of a solid precipitate. These driving forces will be discussed in more detail in other portions of the course. Examples of categories based on the driving force of a reaction are single displacement reactions and double displacement reactions. Single displacement reactions include reduction-oxidation reactions. Double displacement reactions include neutralization and precipitation reactions.

Answer 144

• Single displacement reactions occur when an atom (or ion) of one compound is replaced by an atom of another element. An example of an actual single displacement reaction is the displacement of copper ions by zinc metal. Single displacement reactions are often further classified as redox reactions. In redox reaction, electrons are transferred between species. In order to determine if electrons have been transferred among reactants, it is necessary to recognize the oxidation states, or oxidation numbers, of the reactants and products. The oxidation number is the number assigned to an atom in an ion or molecule that denotes its real or hypothetical charge. The oxidation number of a free element is 0. The oxidation number of a monatomic ion equals the charge of the ion. The sum of the oxidation numbers in a neutral compound is 0. For a polyatomic ion, the sum of the oxidation numbers equals the overall charge of the ion. The more electronegative element in a species is assigned its typical negative oxidation number; the more electropositive element has a positive oxidation number. Fluorine, the most electronegative element, always has a -1 oxidation state in its compounds. Oxygen, second only to fluorine in electronegativity, generally has an oxidation state of -2 in its compounds; the two exceptions are in peroxides, where the oxidation state of oxygen is -1, and in superoxides, where it is -1/2. Hydrogen usually has an oxidation state of +1, except when paired with more electropositive elements, which make it have an oxidation state of -1. The oxidation state of H2 is zero because it is a free element. The oxidation state of Fe3+ is +3 because the oxidation number of a monatomic ion equals the charge of the ion. The oxidation state of the I- ion is – 1 because the oxidation number of a monatomic ion equals the charge of the ion. In HCl, H has an oxidation state of +1 and Cl- has an oxidation state of – 1 because chlorine is more electronegative and takes its typical negative oxidation number. In LiH, Li has an oxidation state of +1 and H has an oxidation state of – 1 because hydrogen is more electronegative than lithium and takes the negative charge.

Answer 145

* , the oxidation number of at least one element will change. Magnesium is in its elemental state, so its oxidation number is zero. Nitrogen, as well, is in its elemental state, so its oxidation number is also zero. In magnesium nitride, magnesium has an oxidation number of +2 and nitrogen has an oxidation number of – 3. (There are three magnesiums in each molecule, for a total contribution of +6 to the oxidation state of the molecule. There are two nitrogens with a charge of – 3 each, or a contribution of – 6 to the oxidation state of the molecule. The sum of oxidation states is zero, as it must be for a neutral molecule.) The oxidation state in at least one of the reacting species has changed, so the reaction is classified as a redox reaction. * recognizing that a reaction is a reduction-oxidation reaction, you should also be able to determine which species is being oxidized and which species is being reduced. Oxidation is the loss of electrons by a species. Reduction is a gain of electrons by a species. OILRIG, magnesium changes oxidation state from 0 to +2, which means that it lost two electrons. In this reaction, then, magnesium has been oxidized. Nitrogen changes oxidation state from 0 to -3, which means that it gained three electrons. Nitrogen was reduced. In redox reactions, we can also define reducing and oxidizing agents. An oxidizing agent is a substance that accepts electrons from another species. A reducing agent is a substance that donates electrons to another species. If we look again at the reaction of magnesium and nitrogen, magnesium has donated electrons to another species, specifically nitrogen. Thus, magnesium is the reducing agent. Nitrogen has accepted electrons from another species, magnesium, and is thus the oxidizing agent. In general, an oxidized species is the reducing agent and the reduced species is an oxidizing agent. Reduction-oxidation reactions are typically seen in the context of electrochemical cells. oxidation reduction reactions Redox- reactions that involve the transfer of electrons from one chemical species to another, OIL RIG stands for “ Oxidation Is Loss, Reduction Is Gain,” because, as we often do, we are talking about those all-important electrons. Alternatively, reduction is just what it sounds like: reduction of charge • oxidation and reduction- law of conservation of charge states that an electrical charge can be neither created nor destroyed. Thus, an isolated loss or gain of electrons cannot occur; oxidation (loss of electrons) and reduction (gain of electrons) must occur simultaneously, resulting in an electron transfer called a redox reaction. An oxidizing agent causes another atom in a redox reaction to undergo oxidation and is itself reduced. A reducing agent causes the other atom to be reduced and is itself oxidized. There are various memory devices designed to help you remember these terms. One that is especially well known is OIL RIG, which stands for “ Oxidation Is Loss; Reduction Is Gain.” * oxidation reduction reactions Redox- reactions that involve the transfer of electrons from one chemical species to another, OIL RIG stands for “ Oxidation Is Loss, Reduction Is Gain,” because, as we often do, we are talking about those all-important electrons. Alternatively, reduction is just what it sounds like: reduction of charge * oxidation and reduction- law of conservation of charge states that an electrical charge can be neither created nor destroyed. Thus, an isolated loss or gain of electrons cannot occur; oxidation (loss of electrons) and reduction (gain of electrons) must occur simultaneously, resulting in an electron transfer called a redox reaction. An oxidizing agent causes another atom in a redox reaction to undergo oxidation and is itself reduced. A reducing agent causes the other atom to be reduced and is itself oxidized. There are various memory devices designed to help you remember these terms. One that is especially well known is OIL RIG, which stands for “ Oxidation Is Loss; Reduction Is Gain.”

Answer 146

double displacement reactions, also called metathesis reactions, elements from two different compounds displace each other to form two new compounds. reaction is a double displacement reaction because it follows the generic formula. Specific types of double displacement reactions are neutralization and precipitation reactions

Answer 147

Neutralization reactions involve an acid and a base reacting to form a salt and usually water. Another neutralization reaction is: These reactions have several things in common. They are both double displacement reactions, following the general formula: In addition, each shows an acid reacting with a base. According to the Brø nsted-Lowry definition of acids and bases, an acid is a proton donor and a base is a proton acceptor. The Lewis definition defines an acid as an electron pair acceptor, while a base is an electron pair donor. some cases no water is formed in a neutralization reaction. This type of neutralization reaction occurs when the acid or the base is Lewis but not Brø nsted-Lowry. Notice, however, that these examples are NOT double displacement reactions, although they are neutralization reactions.

Answer 148

Precipitation reactions are a specific type of metathesis (double displacement) reaction in which a solid product forms. An example of a precipitation reaction is the reaction of sodium hydroxide with silver nitrate. When NaOH (aq) is in solution, what really is present in the beaker are Na+ and OH- ions. So for the example reaction given, what really occurs is: We did not separate AgOH because it is a solid precipitate and thus, it does not dissociate into ions.The sodium and nitrate ions are called spectator ions. They are present on both sides of the reaction and, thus, do not participate in the reaction. The overall reaction is therefore: Although the overall reaction appears to follow the formula for a combination reaction, the reacting ions are from two different species, hence it is still a double displacement reaction.

Answer 149

* A rate is the change in a property per unit time. more common definition of rate uses Δ [X] to represent the change in molar concentration of a substance X, where [X] denotes the molar concentration (in moles per liter) of X, and Δ t is the time it takes for that change to occur. Bringing this all together with our initial definition of rate we have * and find that during an interval of 100s the concentration of HI decreases by 0.50 mol/L. Then the reaction rate would be With reaction rates it is important to specify the substance for which the reaction rate is defined. In the example above, the rate of decomposition of HI is not the same as the rate of formation of H2 or I2: only one H2 molecule is formed for every two HI molecules that react, so in a given time the concentration of H2 changes only half as much as that of HI; consequently, the rate of formation of H2 is half the rate of reaction of HI * a lower rate of change of H2 concentration, because only one molecule of H2 is formed for every 2 HI molecules that decompose. Because we know the change in molar concentration of the HI, we can calculate the change in concentration of H2 by using the stoichiometric relation 2 moles HI = 1 moles H2. The H2 formation rate, 2.5 X 10-3 moles H2/(L s), is half the rate of decomposition of HI. * exponents x and y are called the orders of reaction; x is the order with respect to A and y is the order with respect to B. These exponents may be integers, fractions, or zero and must be determined experimentally. overall order of reaction or reaction order is defined as the sum of the exponents, which is equal to X+Y.

Answer 150

• 1. Zero-Order Reactions- Zero-order reactions have a constant rate that is independent of the reactants' concentrations. Therefore the rate law is: rate = k. 2. First-Order Reactions- First-order reactions have a rate proportional to the concentration of one reactant.First order reactions frequently appear in the form of radioactive decay, a concept which we will study in more detail in physics. 3. Second-Order Reactions- Second order reactions have a rate proportional to the product of the concentrations of two reactants, or to the square of the concentration of a single reactant.

Answer 151

Rates of chemical reactions are affected by concentration, the exposed surface area of the reactants, the temperature, and the presence of catalysts. In this workshop we have already discussed the effect of concentration, which is taken into account in rate laws.

Answer 152

• 1. The Temperature Dependence of Reaction Rates. Reaction rates almost always increase when the temperature is raised. The reason that temperature has such a profound effect on reaction rates comes from the collision theory. Collision Theory supposes that molecules react only if they smash together with at least enough kinetic energy for bonds to be broken. And we know that temperature is a measure of average kinetic energy— therefore, if we increase temperature, we are increasing the average kinetic energy of the particles. In some respect, molecules behave like billiard balls; they bounce apart when they collide at slow speeds and smash into pieces when the impact is really powerful. In collision theory, we assume that a collision is successful only if the molecules collide with an energy that is equal or greater than the activation energy of the reaction. In other words, the activation energy is the minimum energy required for a reaction to occur. As a rule of thumb the rate of reaction will approximately double for every 10° C increase in temperature.

Answer 153

• 2. Medium Dependence of Reaction Rates- The rate of a reaction may also be affected by the medium in which it takes place. Certain reactions proceed rapidly in aqueous solution, whereas other reactions may proceed more rapidly in organic solutions, such as benzene. The state of the medium— solid, liquid, gas— can also have a significant effect.

Answer 154

• 3. Catalysis- The rates of some reactions are increased by the addition of small amounts of certain substances that can often be recovered at the end of the reaction. These substances, called catalysts, increase the reaction rate without themselves being consumed or altered in the reaction process.A catalyst speeds up a reaction by providing an alternative pathway from reactants to products. This new pathway has a lower activation energy, than that of the original pathway. What this means is that at the same temperature, a greater fraction of reactant molecules can cross the barrier and turn into products. There are two classifications of catalysts. A homogenous catalyst is a catalyst that is present in the same phase as the reactants. So for reactants that are in the gas phase, a homogenous catalyst is also a gas. A catalyst is heterogeneous if it is in a phase different from that of the reactants.

Answer 155

irreversible and that the reactions proceed to completion. reversible reactions often do not proceed to completion, because by definition the products can react to reform the reactants. This is particularly true of reactions occurring in closed systems, where products are not allowed to escape. When there is no net change in the concentrations of the products and reactants during a reversible chemical reaction, equilibrium exists. Do not however be misled into thinking that the equilibrium is static; change continues to occur in both the forward and reverse directions. the concentrations of A and B are constant, yet the reaction rates of A to B and vice versa continue to occur at equal rates. rater=ratef, equilibrium has been achieved. Since the rates are equal and kf and kr are both constants, the equilibrium expression for both the forward and reverse reactions.

Answer 156

Equilibrium constant- kc: Pure solids and liquids do not appear in the equilibrium constant expression. Large values of Kc (larger than 103): the equilibrium favors the products strongly.. Intermediate values of Kc (in the range 10-3 to 103): reactants and products are present in similar amounts at equilibrium. Small values of Kc (smaller than 10-3): the equilibrium favors the reactants strongly.

Answer 157

* Le Châ telier's principle: when a stress is applied to a system at equilibrium, the system will adjust to minimize the effect of the stress. * A. Increasing the Concentration- Increasing the concentration of a species will tend to shift the equilibrium away from the species that is added, in order to reestablish its equilibrium concentration, and vice versa. If the concentrations of A and/or B are increased, the equilibrium will shift towards C and D. In other words the equilibrium will shift to the right and favor the production of more products. On the same note, if the concentrations of C and D are increased, the equilibrium will shift away from the production of C and D and favor the production of A and B. Decreasing the concentration of a particular species will also affect the equilibrium by shifting so as to favor the production of that particular species. For example, if A and B are removed, the equilibrium will shift so as to favor increasing the concentration of A and B. * B. Change in Pressure or Volume- In a system at constant temperature, a change in pressure causes a change in volume, and vice versa. Since liquids and solids are virtually incompressible, a change in the pressure or volume of systems involving these phases has little or no effect on their equilibrium. Therefore, reactions that involve changes in pressure and volume significantly affect gases. From our knowledge of gas laws in physics, we know that pressure and volume are inversely related. An increase in the pressure of a system will shift the equilibrium so as to decrease the number of moles of gas present. This reduces the volume of the system and relieves the stress of increased pressure. We see that the left side of the reaction has 4 moles of gaseous molecules, whereas the right side has only 2 moles. When the pressure of this system is increased, the equilibrium will shift to the side that has fewer moles of gas. Since there are fewer moles on the right, the equilibrium will shift toward the right. Conversely, if the volume of the same system is increased, its pressure immediately decreases, which according to Le Châ telier's principle, leads to a shift in the equilibrium to the left. * C. Change in Temperature- Changes in temperature have a profound impact on equilibrium. In order to predict how changing the temperature affects equilibrium we need to understand what role heat plays in a chemical reaction. To predict the effect, heat may be considered as a product in an exothermic reaction, and as a reactant in an endothermic reaction. Consider the general exothermic reaction: If this system were cooled (placed in an ice bath for example), its temperature would decrease, driving the reaction to the right, to replace the lost heat. Similarly, if the system were placed in boiling-water, the reaction equilibrium will shift to the left due to the increased concentration of heat.

Answer 158

* Arrhenius- Arrhenius defined an acid as a species that dissociates in water to produce a hydrogen ion, H+, and a base as a species that dissociates in water to produce a hydroxide ion, OH− . ). Not accounted for out of solutions * Nomenclature- name of an Arrhenius acid is related to the name of the parent anion (the anion that combines with H+ to form the acid). Acids formed from anions whose names end in – ide have the prefix hydro- and the ending – ic. Acids formed from oxyanions are called oxyacids. If the anion ends in − ite (less oxygen), then the acid will end with – ous acid. If the anion ends in – ate (more oxygen), then the acid will end with – ic acid. Prefixes in the names of the anions are retained. MnO4− is called permanganate even though there are no “ manganate” or “ manganite” ions

Answer 159

Brø nsted-Lowry- acid is a species that donates hydrogen ions, while a Brø nsted-Lowry base is a species that accepts hydrogen ions. The advantage of this definition over Arrhenius's is that it is not limited to aqueous solutions. OH− , NH3, and F− are all Brø nsted-Lowry bases because each has the ability to accept a hydrogen ion. However, neither NH3 nor F− can be classified as Arrhenius bases because they do not dissociate to produce OH− ions in aqueous solutions. According to both of these definitions, there's only one way for a species to be an acid, and that is to produce a hydrogen ion. The only difference between the two definitions for acidic compounds is the requirement of an aqueous medium in the Arrhenius definition. Brø nsted-Lowry acids and bases always occur in pairs because the definitions require the transfer of a proton from the acid to the base. These are conjugate acid-base pairs

Answer 160

Lewis- Lewis defined an acid as an electron-pair acceptor (acid = acceptor) and a base as an electron-pair donor. The Lewis definition encompasses some species not included within the Brø nsted-Lowry definition. For example, BCl3 and AlCl3 are species that can each accept an electron pair, which qualifies them as Lewis acids, but they lack a hydrogen ion to donate, disqualifying them as Brø nsted-Lowry acids (or Arrhenius acids, for that matter). Lewis acids act as catalysts, as in the anti-addition of diatomic halogens to alkenes.

Answer 161

• Every Arrhenius acid (or base) can also be classified as a Brø nsted-Lowry acid (or base). Every Brø nsted-Lowry acid (or base) can also be classified as a Lewis acid (or base

Answer 162

* Properties of acids and bases: * auto-ionization of water and hydrogen ion equilibria: acid-base behavior of water- The H2O molecule can act as either an acid or a base, depending on the nature of the reacting species. Water acts as an acid by donating one of its hydrogen ions, and it acts as a base by accepting a hydrogen ion. Thus, water is an amphoteric species: in the presence of a base it reacts like an acid and, in the presence of an acid, it reacts like a base. As an amphoteric compound, water can react with itself, in a process called auto-ionization, One water molecule donates a hydrogen ion to another water molecule to produce the hydronium ion (H3O+) and the hydroxide ion (OH− ). it's always attached to water or some other species that has the ability to accept it. Auto-ionization of water is a reversible reaction; therefore, the expression above is in equilibrium. For pure water at 298 K, the water dissociation constant, Kw. Each mole of water that auto-ionizes produces one mole each of hydrogen (or hydronium) ions and hydroxide ions, so the concentrations of the hydrogen ions and hydroxide ions are always equal in pure water at equilibrium. Thus, the concentration of each of the ions in pure water at equilibrium at 298 K is 10− 7 mol/L. Only equal when solution is neutral, product of respective concentrations equal to 10^-14 when temperature is 298K, dependent only on temp, At temperatures above 298 K, Kw will increase; this is a direct result of the endothermic nature of the auto-ionization reaction. not volume, conc., pressure etc ex. if a species is added to pure water that donates hydrogen ions to the water, then the hydrogen ion concentration will increase, causing the system to shift toward the reactants in the auto-ionization process. The result is a decrease in the hydroxide ion concentration and a return to the equilibrium state. This is Le Châ telier's principle in action: The addition of product to a system at equilibrium causes the system to shift away from the products and toward the reactants. The shift away from the products necessarily leads to a decrease in the hydroxide ion concentration such that the product of the concentrations of the dissolved ions equals Kw. The addition of a species that accepts hydrogen ions (i.e., a base), resulting in a decrease in the hydrogen ion concentration, will cause the system to instead shift toward the products, replacing the hydrogen ions. This process will necessarily lead to an increase in the hydroxide ion concentration and a return to the equilibrium state. For pure water at equilibrium at 298 K, the concentration of the hydrogen ion equals the concentration of the hydroxide ion and is 10− 7 mol/L. Therefore, pure water at 298 K has a pH of 7 and a pOH of 7. (Note: − log10− 7 = 7.) or any aqueous solution at 298 K, a pH less than 7 (or pOH greater than 7) indicates a relative excess of hydrogen ions, and the solution is acidic; a pH greater than 7 (or pOH less than 7) indicates a relative excess of hydroxide ions, and the solution is basic. A pH (or pOH) equal to 7 indicates equal concentrations of hydrogen and hydroxide ions, resulting in a neutral solution.

Answer 163

pH and pOH scales- the reactivity of an acidic solution is not a function of hydrogen ion concentration but instead of the logarithm of the hydrogen ion concentration. As a result, we often use the logarithmic pH and pOH scales to express the concentrations of the hydrogen and hydroxide ions, respectively. pH and pOH are specific calculations of the more generic “ p-scale.” A p-scale is defined as the negative logarithm of the number of items.

Answer 164

Estimating p-Scale Values- The log of a product is equal to the sum of the logs; that is, log (xy) = log x + log y. log xn = n logx and log 10x = x. From these two properties, one can derive the particularly useful relationship that will be seen on Test Day (and we can see in the example): − log 10− x = x. n× 10− m, where n is a number between 1 and 10. Using the basic log rule that log (xy) = log x + log y, we can express the negative log of this product, − log (n× 10− m), as − log (n) − log (10− m). Since log refers to the common log with base of 10, we can simplify − log (n) − log (10− m) to − log (n) − (− m), or m− log (n). Since n is a number between 1 and 10, its logarithm will be a fraction between 0 and 1 (note: log 1 = 0 and log 10 = 1). Thus, m− log (n) will be between (m− 1) and (m− 0). Furthermore, the larger n is (that is, the closer to 10), the larger the fraction log (n) will be and the closer to (m− 1) our answer will be. When the nonlogarithmic value is n× 10− m, the logarithmic value will be between (m− 1) and m.

Answer 165

•Strong acids and bases- completely dissociate into their component ions in aqueous solution, Hence, in a 1 M solution of NaOH, complete dissociation gives 1 M Na+ and 1 M OH− . The pH and pOH for this solution can be calculated. no undissociated strong acid or base, such as NaOH, remains in solution. This is why the dissociation of strong acids and bases is said to go to completion. In the NaOH example above, we assumed that the concentration of OH− from the auto-ionization of water is negligible due to the addition of the strong base. The contribution of OH− and H+ ions to an aqueous solution from the auto-ionization of water can be neglected only if the concentration of the acid or base is significantly greater than 10− 7 M. Keep this in mind as you solve acid-base problems on Test Day. For example, say you are asked to calculate the pH of a 1 × 10− 8 M solution of HCl (a strong acid), and you determine that the pH is 8 because − log (10− 8) = 8. Always ask yourself if the answer you have predicted is feasible. A pH of 8 can't describe an acidic solution (at least not at 298 K), because the presence of the acid will cause the hydrogen ion concentration to increase above 10− 7, resulting in a pH below 7. acid compound concentration is actually ten times less than the equilibrium concentration of hydrogen ions in pure water generated by water's autodissociation. Consequently, the hydrogen ion concentration from the water itself is significant and can't be ignored. The addition of acid results in the common ion effect (Le Châ telier's principle in ionic solutions) and causes the system to shift away from the ions, thereby reducing the concentration of hydrogen ions and hydroxide ions. The reversal of auto-ionization is thermodynamically favored, returning the system to equilibrium.

Answer 166

Strong acdis- HCl (hydrochloric acid), HBr (hydrobromic acid), HI (hydroiodic acid), H2SO4 (sulfuric acid), HNO3 (nitric acid), and HClO4 (perchloric acid). Strong bases commonly encountered include NaOH (sodium hydroxide), KOH (potassium hydroxide), and other soluble hydroxides of Group IA and IIA metals. Calculation of the pH and pOH of strong acids and bases assumes complete dissociation of the acid or base in solution: [H+] = normality of strong acid and [OH− ] = normality of strong base. e.g., strong bases completely dissociate in aqueous solutions

Answer 167

Weak acids and bases- acids and bases that only partially dissociate in aqueous solutions. These are called weak acids and bases. weak monoprotic acid, HA, will dissociate partially in water to achieve an equilibrium state. Since the system exists in an equilibrium state, we can write the dissociation equation to determine the acid dissociation constant, Ka. The smaller the Ka, the weaker the acid, and consequently, the less it will dissociate. Note that the concentration of water, while seemingly not included in the dissociation constant expression, is incorporated into the value of Ka (Keq [H2O] = Ka).

Answer 168

weak base- monovalent base, BOH, undergoes dissociation to yield B+ and OH− in solution. base dissociation constant, Kb. The smaller the Kb, the weaker the base, and consequently, the less it will dissociate. As with the acid dissociation expression, the base dissociation expression incorporates the concentration of the water in the value of Kb. we can characterize a species as a weak acid if its Ka is less than 1.0 and as a weak base if its Kb is less than 1.0. molecular (nonionic) weak bases are almost exclusively amines.

Answer 169

A conjugate acid is the acid formed when a base gains a proton, and a conjugate base is the base formed when an acid loses a proton. CO32− is the conjugate base of HCO3− , an acid, and H3O+ is the conjugate acid of H2O, acting as a base. To find the Ka, we consider the equilibrium concentrations of the dissolved species. Remember that water is never included in the equilibrium expression! The reaction between bicarbonate (HCO3− ) and water is reversible. a conjugate acid-base pair formed from a weak acid, the conjugate base is generally stronger than the conjugate acid. Note that this does not necessarily mean that a weak acid will produce a strong conjugate base or that a weak base will produce a strong conjugate acid. However, it is always the case that a strong acid will produce a weak conjugate base (e.g., HCl/Cl− ) and a strong base will produce a weak conjugate acid (e.g., NaOH/H2O). As it turns out, for HCO3− and CO32− , the reaction of CO32− with water to produce HCO3− and OH− occurs to a greater extent— is more thermodynamically favorable— than the reaction of HCO3− and water to produce CO32− and H3O+. the net reaction is the auto-ionization of water, the equilibrium constant for the reaction is Kw = [H3O+][OH− ] = 10− 14, which is the product of Ka and Kb. he product of the concentrations of the hydrogen ion and the hydroxide ion must always equal 10− 14 for acidic or basic aqueous solutions. Because water is an amphoteric species (both a weak acid and a weak base), all acid-base reactivity in water ultimately reduces to the acid-base behavior of water, and all acidic or basic aqueous solutions are governed by the dissociation constant for water. Thus, if the dissociation constant for either the acid or its conjugate base is known, then the dissociation constant for the other can be determined. if Ka is large, then Kb is small, and vice versa.

Answer 170

• applications of ka and kb- most common use of the acid and base dissociation constants is for the determination of the concentration of one of the species in solution at equilibrium. you may be asked to calculate the concentration of the hydrogen ion (or pH), the concentration of the hydroxide ion (or pOH), or the concentration of either the original acid or base. To calculate the concentration of H+ in a 2.0 M aqueous solution of acetic acid, CH3COOH (Ka = 1.8 × 10− 5), Acetic acid is a weak acid, so the concentration of CH3COOH at equilibrium is equal to its initial concentration, 2.0 M, less the amount dissociated, x. Likewise, [H+] = [CH3COO− ] = x, because each molecule of CH3COOH dissociates into one H+ ion and one CH3COO− ion. We can approximate that 2.0 − x≈ 2.0 because acetic acid is a weak acid and only slightly dissociates in water. The fact that x is so much less than the initial concentration of acetic acid (2.0 M) validates the approximation; otherwise, it would have been necessary to solve for x using the quadratic formula. That sounds rather unpleasant, doesn't it? Fortunately for you, the MCAT test writers select examples of weak acids and bases that allow you to make this approximation. (A rule of thumb is that the approximation is valid as long as x is less than 5 percent of the initial concentration.)

Answer 171

• Salt Formation- Acids and bases may react with each other to form a salt and often, but not always, water, in what is termed a neutralization reaction. salt may precipitate out or remain ionized in solution, depending on its solubility and the amount produced. In general, neutralization reactions go to completion. The reverse reaction, in which the salt ions react with water to give back the acid or base, is known as hydrolysis.

Answer 172

Four combinations of strong and weak acids and bases are possible: 1. Strong acid + strong base: e.g., HCl + NaOH ↔ NaCl + H2O. 2. Strong acid + weak base: e.g., HCl + NH3↔ NH4Cl. 3. Weak acid + strong base: e.g., HClO + NaOH ↔ NaClO + H2O. 4. Weak acid + weak base: e.g., HClO + NH3↔ NH4ClO.

Answer 173

The products of a reaction between equal concentrations of a strong acid and a strong base are equimolar concentrations of salt and water. The acid and base neutralize each other, so the resulting solution is neutral (pH = 7), and the ions formed in the reaction do not react with water. strong acid and strong base- titration of 10 mL of a 0.1 N solution of HCl with a 0.1 N solution of NaOH. Strong acid + weak base: equivalence point pH < 7. Strong acid + strong base: equivalence point pH = 7. Strong base + weak acid: equivalence point pH > 7. HCl is a strong acid and NaOH is a strong base, the equivalence point of the titration will be at pH 7, and the solution will be neutral. Note that the end point shown is close to, but not exactly equal to, the equivalence point; selection of a better indicator, one that changes colors at, say, pH 8, would have given a better approximation. Still, the amount of error introduced by the use of an indicator that changes color around pH 11 rather than, say, pH 8 is not especially significant: a mere fraction of a milliliter of excess NaOH solution. early part of the curve when little base has been added, the acidic species predominates, so the addition of small amounts of base will not appreciably change either the [OH− ] or the pH. Similarly, in the last part of the titration curve when an excess of base has been added, the addition of small amounts of base will not change the [OH− ] significantly, and the pH remains relatively constant. The addition of base most alters the concentrations of H+ and OH− near the equivalence point, and thus the pH changes most drastically in that region. The equivalence point for strong acid/strong base titration is always at pH 7 (for monovalent species). pH meter to chart the change in pH as a function of volume of titrant added, you can make a good approximation of the equivalence point by locating the midpoint of the region of the curve with the steepest slope.

Answer 174

The product of a reaction between a strong acid and a weak base is also a salt, but often no water is formed because weak bases are usually not hydroxides. However, in this case, the cation of the salt will react with the water solvent, re-forming the weak base in hydrolysis. NH4+ is the conjugate acid of a weak base (NH3), which is stronger than the conjugate base (Cl− ) of the strong acid, HCl. NH4+ will then transfer a proton to H2O to form the hydronium ion. The increase in the concentration of the hydronium ion will cause the system to shift away from auto-ionization, thereby reducing the concentration of hydroxide ion. Consequently, the concentration of the hydrogen ion will be greater than that of the hydroxide ion at equilibrium, and as a result, the pH of the solution will fall below 7 weak base and strong acid- The appearance of the titration curve for a weak base titrand and strong acid titrant will look like an inversion of the curve for a weak acid titrand and strong base titrant. The initial pH will be in the basic range (typical range: pH 10− 12) and will demonstrate a fairly steep drop in pH with the addition of strong acid. The equivalence point will be in the acidic pH range, because the reaction between the weak base and strong acid will produce a stronger conjugate acid and weaker conjugate base. The stronger conjugate acid will result in an equilibrium state with a concentration of hydrogen ions greater than that of the hydroxide ions. The equivalence point for weak base/strong acid titration is always in the acidic range of the pH scale

Answer 175

when a weak acid reacts with a strong base, the pH of the solution at equilibrium is in the basic range because the salt hydrolyzes to re-form the acid, with the concurrent formation of hydroxide ion from the hydrolyzed water molecules. The increase in the concentration of the hydroxide ion will cause the system to shift away from auto-ionization, thereby reducing the concentration of the hydrogen ion. Consequently, the concentration of the hydroxide ion will be greater than that of the hydrogen ion at equilibrium, and as a result, the pH of the solution will rise above 7. Consider the reaction of acetic acid CH3COOH (weak acid) with sodium hydroxide NaOH (strong base) • weak acid and strong base- Titration of a weak acid, HA (e.g., CH3COOH), with a strong base, such as NaOH, The first difference we should notice is that the initial pH of the weak acid solution is greater than the initial pH of the strong acid solution. Weak acids don't dissociate to the same degree that strong acids do; therefore, the concentration of H3O+ will generally be lower (and pH will be higher) in a solution of weak acid. The second difference is the shapes of the curves. The pH curve for the weak acid/strong base titration shows a steeper rise in pH for given additions of base. The pH changes most significantly early on in the titration, and the equivalence point is in the basic range. The third difference is the position of the equivalence point. While the equivalence point for a strong acid/strong base titration is pH 7, the equivalence point for a weak acid/strong base is above 7. This is because the reaction between the weak acid (HA) and strong base (OH− ) produces a stronger conjugate base (A− ) and a weaker conjugate acid (H2O). This produces a greater concentration of hydroxide ions than hydrogen ions at equilibrium (due to the common ion effect on the auto-ionization of water). The equivalence point for weak acid/strong base titration is always in the basic range of the pH scale.

Answer 176

The pH of a solution containing a weak acid and a weak base depends on the relative strengths of the reactants. For example, the weak acid HClO has a Ka = 3.2 × 10− 8, and the weak base NH3 has a Kb = 1.8 × 10− 5. Thus, an aqueous solution of HClO and NH3 is basic because the Ka for HClO is less than the Kb for NH3 That is, HClO is weaker as an acid than NH3 is as a base, and at equilibrium, the concentration of hydroxide ions will be greater than the concentration of hydrogen ions in the aqueous solution.

Answer 177

• Polyvalence and Normality- The relative acidity or basicity of an aqueous solution is determined by the relative concentrations of acid and base equivalents. An acid equivalent is equal to one mole of H+ (or H3O+) ions; a base equivalent is equal to one mole of OH− ions. Some acids and bases are polyvalent; that is, each mole of the acid or base liberates more than one acid or base equivalent. One mole of H2SO4 can produce two acid equivalents (2 moles of H3O+). You'll notice, if you look closely at the dissociation reaction for sulfuric acid, the first dissociation goes to completion but the second dissociation goes to an equilibrium state. The acidity or basicity of a solution depends upon the concentration of acidic or basic equivalents that can be liberated. The quantity of acidic or basic capacity is directly indicated by the solution's normality. For example, each mole of H3PO4 yields three moles (equivalents) of H3O+.Therefore, a 2 M H3PO4 solution would be 6 N. equivalent weight- The gram equivalent weight is the mass of a compound that produces one equivalent (one mole of charge). For example, H2SO4 (molar mass: 98 g/mol) is a divalent acid, so each mole of the acid compound yields two acid equivalents. The gram equivalent weight is 98/2 = 49 grams. That is, the complete dissociation of 49 grams of H2SO4 will yield one acid equivalent (one mole of H3O+). Common polyvalent acids include H2SO4, H3PO4, and H2CO3. Common polyvalent bases include Al(OH)3, Ca(OH)2, and Mg(OH)2. polyvalent acids and bases- The titration curve for a polyvalent acid or base looks different from that for a monovalent acid or base. titration of Na2CO3 with HCl in which the divalent (diprotic) acid H2CO3 is the ultimate product. In region I, little acid has been added, and the predominant species is CO32− . In region II, more acid has been added, and the predominant species are CO32− and HCO3− , in relatively equal concentrations. The flat part of the curve is the first buffer region (discussed in the next section), corresponding to the pKa of HCO3− (Ka = 5.6 × 10- gives a pKa = 10.25). Region III contains the equivalence point, at which all of the CO32− is finally titrated to HCO3− . As the curve illustrates, a rapid change in pH occurs at the equivalence point; in the latter part of region III, the predominant species is HCO3− . In region IV, the acid has neutralized approximately half of the HCO3− , and now H2CO3 and HCO3− are in roughly equal concentrations. This flat region is the second buffer region of the titration curve, corresponding to the pKa of H2CO3 (Ka = 4.3 × 10− 7 gives a pKa = 6.37). In region V, the equivalence point for the entire titration is reached, as all of the HCO3− is finally converted to H2CO3. Again, a rapid change in pH is observed near the equivalence point as acid is added. titrations of the acidic and basic amino acids (which have acidic or basic side chains, respectively) two equivalence points, there will in fact be three: one corresponding to the titration of the carboxylic acid and a second corresponding to the titration of the amino acid, both of which are attached to the central carbon, and a third corresponding either to the acidic or basic side chain.

Answer 178

• Amphoteric Species- amphoteric, or amphiprotic, species is one that reacts like an acid in a basic environment and like a base in an acidic environment. In the Brø nsted-Lowry sense, an amphoteric species can either gain or lose a proton. water reacts with an acid, it behaves as a base. The partially dissociated conjugate base of a polyvalent acid is usually amphoteric (e.g., HSO4− can either gain an H+ to form H2SO4 or lose an H+ to form SO42− ). The hydroxides of certain metals (e.g., Al, Zn, Pb, and Cr) are also amphoteric. species that can act as either oxidizing or reducing agents are considered to be amphoteric as well, because by accepting or donating electron pairs, they act as Lewis acids or bases, respectively.

Answer 179

• Titration and Buffers- Titration is a procedure used to determine the molarity of a known reactant in a solution. There are different types of titrations, including redox, acid-base, and complexometric (metal ion). Titrations are accomplished by reacting a known volume of a solution of unknown concentration with a known volume of a solution of known concentration (the titrant). In acid-base titrations, the equivalence point is reached when the number of acid equivalents present in the original solution equals the number of base equivalents added, or vice versa. It is important to emphasize that, while a strong acid/strong base titration will have an equivalence point at pH 7, the equivalence point does not always occur at pH 7. when titrating polyprotic acids or bases there are multiple equivalence points, as each acidic or basic conjugate species is titrated separately. In problems involving titration or neutralization of acids and bases, where Na and Nb are the acid and base normalities, respectively, and Va and Vb are the volumes of acid and base solutions, respectively. Note that as long as both volumes use the same units, the units used do not have to be liters. The equivalence point can be found graphically by plotting the pH of the titrand solution as a function of added titrant by using a pH meter, or estimate by watching for a color change of an added indicator.

Answer 180

Indicators- (indicators) will change color as they shift between their conjugate acid and base forms. Adding H+ shifts the equilibrium to the left. Adding OH− removes H+ and therefore shifts the equilibrium to the right. Indicators are weak organic acids or bases that have different colors in their protonated and deprotonated states. Due to their colored states, indicators can be used in low concentrations and therefore do not significantly alter the equivalence point. The indicator must always be a weaker acid or base than the acid or base being titrated; otherwise, the indicator would be titrated first! The point at which the indicator actually changes color is not the equivalence point but rather the end point. If the indicator is chosen correctly and the titration is performed well, the volume difference between the end point and the equivalence point is negligible, and may be corrected for or simply ignored.

Answer 181

Acid-base titrations can be performed for different combinations of strong and weak acids and bases. The most useful combinations are strong acid/strong base, weak acid/strong base, and weak base/strong acid. Weak acid/weak base titrations can be done but are not very accurate and therefore rarely performed. The pH curve for the titration of a weak acid and weak base lacks the sharp change that normally indicates the equivalence point. Furthermore, indicators are less useful because the pH change is more gradual.

Answer 182

• Buffers- buffer solution consists of a mixture of a weak acid and its salt (which consists of its conjugate base and a cation) or a mixture of a weak base and its salt (which consists of its conjugate acid and an anion). solution of acetic acid (CH3COOH) and its salt, sodium acetate (CH3COO− Na+), and a solution of ammonia (NH3) and its salt, ammonium chloride (NH4+Cl− ). The acetic acid/sodium acetate solution is an acid buffer, and the ammonium chloride/ammonia solution is a base buffer. Buffer solutions have the useful property of resisting changes in pH when small amounts of strong acid or base are added. Consider a buffer solution of acetic acid and sodium acetate (note: the sodium ion has not been included because it is not involved in the acid-base reaction). When a small amount of strong base, such as NaOH, is added to the buffer, the OH− ions from the NaOH react with the H+ ions present in the solution; subsequently, more acetic acid dissociates (the system shifts to the right), restoring the [H+]. The weak acid component of the buffer acts to neutralize the strong base that has been added. The resulting increase in the concentration of the acetate ion (the conjugate base) does not yield as large an increase in hydroxide ions as the unbuffered strong base would. Thus, the addition of the strong base does not result in a significant increase in [OH− ] and does not appreciably change the pH. Likewise, when a small amount of HCl is added to the buffer, H+ ions from the HCl react with the acetate ions to form acetic acid. Acetic acid is weaker than the added hydrochloric acid (which has been neutralized by the acetate ions), so the increased concentration of acetic acid does not significantly contribute to the hydrogen ion concentration in the solution. Because the buffer maintains [H+] at relatively constant values, the pH of the solution is relatively unchanged

Answer 183

human body, one of the most important buffers is the H2CO3/HCO3− conjugate pair in the plasma component of the blood. CO2 (g), one of the waste products of cellular respiration, has low solubility in aqueous solutions. The majority of the CO2 transported from the peripheral tissue to the lungs (where it will be exhaled out) is dissolved in the plasma in a “ disguised” form. CO2 (g) and water react. Carbonic acid (H2CO3) and its conjugate base, bicarbonate (HCO3− ), form a weak acid buffer for maintaining the pH of the blood within a fairly narrow physiological range. The most important point to notice about this system for pH homeostasis is its direct connection to the respiratory system. In conditions of metabolic acidosis (excess plasma H+), for example, the respiratory (breathing) rate will increase in order to “ blow off” a greater amount of carbon dioxide gas; this causes the system to shift from the right to the left, thereby restoring the normal physiological pCO2 and in doing so, reducing the [H+] and buffering against dramatic and dangerous changes to the blood pH. One of the more interesting topics to ponder about the blood buffer system is why a weak acid buffer system was selected as a primary mechanism for human blood pH homeostasis around pH 7.4, which is weakly basic. Buffers have a definite and narrow range of optimal buffering capability (pKa± 1), and pH 7.4 is actually slightly above the outer limit of buffering capability for the carbonic acid/bicarbonate system (pKa = 6.1).

Answer 184

Henderson-Hasselbalch equation is used to estimate the pH or pOH of a solution in the buffer region where the concentrations of the species and its conjugate are present in approximately equal concentrations when [conjugate base] = [weak acid] (in a titration, halfway to the equivalent point), the pH = pKa because log 1 = 0.

Answer 185

Buffering capacity is optimal at this pH. For a weak base buffer solution pOH = pKb when [conjugate acid] = [weak base]. Buffering capacity is optimal at this pOH. Clearly, changing the concentrations of the buffer components in such a way that results in a change in their ratio will lead to a change in the pH of the buffer solution. But what about changing the concentrations while maintaining the ratio of the buffer components. doubling the concentrations of the acid and the base (thereby maintaining a constant ratio)? Because we are taking the log of the ratio of the components, the logarithmic value will not change as long as the ratio doesn't change. If the ratio of the buffer components doesn't change, the pH of the buffer solution doesn't change. Nevertheless, something has changed, but what is it? The buffering capacity— the size of that kitchen sponge— has changed. Doubling the concentrations of the buffer components produces a buffer solution with twice the buffering capacity. The kitchen sponge is twice as big and can soak up twice as much acid or base.

Answer 186

• assigning oxidation numbers: Oxidation numbers are assigned to atoms in order to keep track of the redistribution of electrons during chemical reactions. Based on the oxidation numbers of the reactants and products, it is possible to determine how many electrons are gained or lost by each atom. The oxidation number of an atom in a compound is assigned. 1. The oxidation number of free elements is zero. For example, the atoms in N2, P4, S8, and He all have oxidation numbers of zero. 2. The oxidation number for a monatomic ion is equal to the charge of the ion. For example, the oxidation numbers for Na+, Cu2+, Fe3+, Cl– , and N3– are +1, +2, +3, − 1, and − 3, respectively. 3. The oxidation number of each Group IA element in a compound is +1. The oxidation number of each Group IIA element in a compound is +2. 4. The oxidation number of each Group VIIA element in a compound is − 1, except when combined with an element of higher electronegativity. For example, in HCl, the oxidation number of Cl is − 1; in HOCl, however, the oxidation number of Cl is +1. 5. The oxidation number of hydrogen is − 1 in compounds with less electro-negative elements than hydrogen (Groups IA and IIA). Examples include NaH and CaH2. The more common oxidation number of hydrogen is +1. 6. In most compounds, the oxidation number of oxygen is− 2. However, this is not the case in molecules such as OF2. Here, F is more electronegative than O, so the oxidation number of oxygen is +2. Also, in peroxides, such as BaO2, the oxidation number of O is − 1 instead of − 2 because of the structure of the peroxide ion, [O− O]2– . (Note that Ba, a Group IIA element, cannot be a +4 cation.) 7. The sum of the oxidation numbers of all the atoms present in a neutral compound is zero. The sum of the oxidation numbers of the atoms present in a polyatomic ion is equal to the charge of the ion. Thus, for SO42− , the sum of the oxidation numbers must be − 2.

Answer 187

* balancing redox reactions- By assigning oxidation numbers to the reactants and products, you can determine how many moles of each species are required for conservation of charge and mass, which is necessary to balance the equation. To balance a redox reaction, both the net charge and the number of atoms must be equal on both sides of the equation. half-reaction method, also known as the ion-electron method, in which the equation is separated into two half-reactions— the oxidation part and the reduction part. Each half-reaction is balanced separately, and they are then added to give a balanced overall reaction. Step 1: Separate the two half-reactions. * Step 2: Balance the atoms of each half-reaction. First, balance all atoms except H and O. Next, in an acidic solution, add H2O to balance the O atoms and then add H+ to balance the H atoms. (In a basic solution, use OH− and H2O to balance the O's and H's.) To balance the iodine atoms, place a coefficient of 2 before the I− ion. For the permanganate half-reaction, Mn is already balanced. Next, balance the oxygens by adding 4 H2O to the right side. Finally, add H+ to the left side to balance the 4H2O. These two halfreactions are now balanced. * Step 3: Balance the charges of each half-reaction. The reduction half-reaction must consume the same number of electrons as are supplied by the oxidation half. For the oxidation reaction, add 2 electrons to the right side of the reaction. For the reduction reaction, a charge of +2 must exist on both sides. Add 5 electrons to the left side of the reaction to accomplish this. Next, both half-reactions must have the same number of electrons so that they cancel each other out in the next step. In this example, you need to multiply the oxidation half by 5 and the reduction half by 2. * Step 4: Add the half-reactions: with the final equation of… To get the overall equation, cancel out the electrons and simplify any H2Os, H+s, or OH− s that appear on both sides of the equation. * Step 5: Finally, confirm that mass and charge are balanced. There is a +4 net charge on each side of the reaction equation, and the atoms are stoichiometrically balanced.

Answer 188

• Electrochemical Cells- contained systems where redox reactions occur. There are three types of electrochemical cells: galvanic cells (also known as voltaic cells), electrolytic cells, and concentration cells. Spontaneous reactions occur in galvanic cells and concentration cells, and nonspontaneous reactions in electrolytic cells. Remember that spontaneity is quantified by the change in Gibbs free energy, Δ G. All three types contain electrodes where oxidation and reduction take place. For all electrochemical cells, the electrode where oxidation occurs is called the anode, and the electrode where reduction occurs is called the cathode. Furthermore, we can also generally state that for all electrochemical cells, the movement of electrons is from anode to cathode and current i runs from cathode to anode. AN OX and a RED CAT. Electrons and current move through an electrochemical cell in OPPOSITE directions.

Answer 189

Galvanic (voltaic) cells-all nonrechargeable batteries, also called voltaic cells. If energy is being supplied by the battery, then the redox reaction taking place is giving off energy. This means that the reaction's free energy must be decreasing (– Δ G) and the reaction must, therefore, be spontaneous. With the electromotive force Gibbs free energy is negative; the sign (+ or – ) on emf is always opposite that of the change in free energy. Two electrodes of distinct chemical identity are placed in separate compartments, which are called half-cells. The two electrodes are connected to each other by a conductive material, such as a copper wire. Surrounding each of the electrodes is an aqueous electrolyte solution, composed of cations and anions.

Answer 190

• Daniell cell, the cations in the two half-cell solutions can be of the same element as the respective metal electrode. Connecting the two solutions is a structure called the salt bridge, which consists of an inert salt. When the electrodes are connected to each other by a conductive material, When the electrodes are connected to each other by a conductive material, charge will begin to flow as the result of a redox reaction that is taking place between the two half-cells. The redox reaction in a galvanic cell is spontaneous, and therefore the change in Gibbs free energy for the reaction is negative (– Δ G). As the spontaneous redox reaction proceeds toward equilibrium, the movement of charge (electrons) results in a conversion of electric potential energy into kinetic energy. By separating the reduction and oxidation half-reactions into two compartments, we are able to harness this energy and use it to do work by connecting various electrical devices into the circuit between the two electrodes. In the Daniell cell, a zinc bar is placed in an aqueous ZnSO4 solution, and a copper bar is placed in an aqueous CuSO4 solution. The anode of this cell is the zinc bar where Zn (s) is oxidized to Zn2+ (aq). The cathode is the copper bar, and it is the site of the reduction of Cu2+ (aq) to Cu (s). The half-cell reactions. If the two half-cells were not separated, the Cu2+ ions would react directly with the zinc bar, and no useful electrical work would be obtained. Since the solutions and electrodes are physically separated, they must be connected to complete the circuit. Without a connection, the electrons from the zinc oxidation half-reaction would not be able to get to the copper ions; thus, a wire (or other conductor) is necessary. However, if only a wire were provided for this electron flow, the reaction would soon stop because an excess negative charge would build up in the solution surrounding the cathode and an excess positive charge would build up in the solution surrounding the anode. Eventually, the excessive charge accumulation would provide a counter voltage large enough to prevent the redox reaction from taking place, and the current would cease. This charge gradient is dissipated by the presence of a salt bridge, which permits the exchange of cations and anions. The salt bridge contains an inert electrolyte, usually KCl or NH4NO3, whose ions will not react with the electrodes or with the ions in solution. While the anions from the salt bridge (e.g., Cl− ) diffuse into the ZnSO4 solution to balance out the charge of the newly created Zn2+ ions, the cations of the salt bridge (e.g., K+) flow into the CuSO4 solution to balance out the charge of the SO42− ions left in solution when the Cu2+ ions are reduced to Cu and precipitate out of solution (“ plate out” ) onto the copper cathode. During the course of the reaction, electrons flow from the zinc bar (anode) through the wire and the voltmeter (if one is connected) toward the copper bar (cathode). The anions (Cl− ) flow externally (via the salt bridge) into the ZnSO4, and the cations (K+) flow into the CuSO4. This flow depletes the salt bridge and, along with the finite quantity of Cu2+ in the solution, accounts for the relatively short life of the cell. A cell diagram is a shorthand notation representing the reactions in an electrochemical cell. A cell diagram for the Daniell cell. 1. The reactants and products are always listed from left to right in this form: 2. A single vertical line indicates a phase boundary. 3. A double vertical line indicates the presence of a salt bridge or some other type of barrier.

Answer 191

• Electrolytic cells, in almost all of their characteristics and behavior, are the opposite of galvanic cells. galvanic cells house spontaneous redox reactions, which generate current and deliver electrical energy, electrolytic cells house nonspontaneous reactions, which require the input of energy to proceed. Galvanic cells supply energy; electrolytic cells consume it. The change in Gibbs free energy for the redox reaction of an electrolytic cell is positive. The type of redox reaction driven by an external voltage source is called electrolysis, in which chemical compounds are decomposed. electrolytic cells can be used to drive the nonspontaneous decomposition of water into oxygen and hydrogen gas. molten NaCl is decomposed into Cl2 (g) and Na (l). The external voltage source. supplies energy sufficient to drive the redox reaction in the direction that is thermodynamically unfavorable (i.e., nonspontaneous). In this example, Na+ ions migrate toward the cathode, where they are reduced to Na (l). At the same time, Cl− ions migrate toward the anode, where they are oxidized to Cl2 (g). Notice that the half-reactions do not need to be separated into different compartments; this is because the desired reaction is nonspontaneous. Even though this is a nonspontaneous reaction that is being driven by an external voltage, oxidation occurs at the anode and reduction occurs at the cathode. (Note that sodium is a liquid at the temperature of molten NaCl; it is also less dense than the molten salt and, thus, is easily removed as it floats to the top of the reaction vessel.) This cell is used in industry as the major means of sodium and chlorine production. You may be wondering why you would put in this much work to produce these compounds. Well, think about the thermodynamics of electrolysis. Energy is supplied to drive a nonspontaneous process. . This means that the products of the reaction are not naturally favored. sodium and chlorine are never found naturally in their elemental form because they are so reactive. So before we can use elemental sodium or chlorine gas, we have to make it ourselves first. He theorized that the amount of chemical change induced in an electrolytic cell is directly proportional to the number of moles of electrons that are exchanged during a redox reaction. The number of moles exchanged can be determined from the balanced half-reaction. In general, for a reaction that involves the transfer of n electrons per atom M. According to this equation, one mole of M (s) will be produced if n moles of electrons are supplied. Additionally, the number of moles of electrons needed to produce a certain amount of M (s) can now be related to the measurable electrical property of charge. One electron carries a charge of 1.6 × 10− 19 coulombs (C). The charge carried by one mole of electrons can be calculated by multiplying this number by Avogadro's number, number is called Faraday's constant, and one faraday (F) is equivalent to the amount of charge contained in one mole of electrons (1 F = 96,487 C, or J/V) or one equivalent. electrolytic cell, the anode is positive and the cathode is negative. In a galvanic cell, the anode is negative and the cathode is positive. However, in both types of cells, reduction occurs at the cathode, and oxidation occurs at the anode.

Answer 192

• concentration cell is a special type of voltaic cell: two half-cells connected by a conductive material, allowing a spontaneous redox reaction to proceed, generating a current and delivering energy. Just like a galvanic cell, the concentration cell houses a redox reaction that has a negative Δ G. The distinguishing characteristic of a concentration cell is in its design: The electrodes are chemically identical. For example, if both electrodes are copper metal, they have the same reduction potential. Therefore, current is generated as a function of a concentration gradient established between the two solutions surrounding the electrodes. The concentration gradient results in a potential difference between the two compartments and drives the movement of electrons in the direction that results in equilibration of the ion gradient. The current will stop when the concentrations of ion species in the half-cells are equal. This implies that the voltage (V) or emf of a concentration cell is zero when the concentrations are equal; the voltage, as a function of concentrations, can be calculated using the Nernst equation.

Answer 193

electrode charge designation- galvanic cell, current is spontaneously generated as electrons are released by the oxidized species at the anode and travel through the conductive material to the cathode, where reduction takes place. Because the anode of a galvanic cell is the source of electrons, it is considered the negative electrode; the cathode is considered the positive electrode. Electrons, therefore, move from negative (low electric potential) to positive (high electric potential), while the current (by convention, the movement of positive charge) is from positive (high electric potential) to negative (low electric potential). the anode of an electrolytic cell is considered positive, because it is attached to the positive pole (the cathode) of the external voltage source and attracts anions from the solution. The cathode of an electrolytic cell is considered negative, because it is attached to the negative pole (the anode) of the external voltage source and attracts cations from the solution. spite of this difference in designating charge (sign), oxidation takes place at the anode and reduction takes place at the cathode in both types of cells, and electrons always flow through the wire from the anode to the cathode. A simple mnemonic is that the CAThode attracts the CATions and the ANode attracts the ANions. In the Daniell cell, for example, the electrons created at the anode by the oxidation of the elemental zinc travel through the wire to the copper half-cell. There they attract copper (II) cations to the cathode, resulting in the reduction of the copper ions to elemental copper, and draw cations out of the salt bridge into the compartment. The anode, having lost electrons, attracts anions from the salt bridge at the same time that Zn2+ ions formed by the oxidation process move away from the anode and toward the cathode. his distinction arises, for example, in a type of electrophoresis called isoelectric focusing, a technique often used to separate amino acids based on the isoelectric point (pI) of each amino acid. The positively charged amino acids (protonated at the solution's pH) will migrate toward the cathode; negatively charged amino acids (deprotonated at the solution's pH) will migrate toward the anode.

Answer 194

• Reduction Potentials and the Electromotive Force- galvanic cells, the direction of spontaneous movement of charge is from the anode, the site of oxidation, to the cathode, the site of reduction. The relative tendencies of different chemical species to be reduced have been determined experimentally, using the tendency of the hydrogen ion (H+) to be reduced as an arbitrary zero reference point.

Answer 195

• reduction potentials- A reduction potential is exactly what it sounds like. It tells us how likely a compound is to be reduced. The more positive the value, the more likely it is to be reduced. A reduction potential is measured in volts (V) and defined relative to the standard hydrogen electrode (SHE), which is given a potential of 0.00 volts. The species in a reaction that will be oxidized or reduced can be determined from the reduction potential of each species, defined as the tendency of a species to gain electrons and be reduced. Each species has its own intrinsic reduction potential; the more positive the potential, the greater the species' tendency to be reduced. Standard reduction potential,(Eredo), is measured under standard conditions: 25° C, 1 M concentration for each ion participating in the reaction, a partial pressure of 1 atm for each gas that is part of the reaction, and metals in their pure state. The relative reactivities of different half-cells can be compared to predict the direction of electron flow. A more positive Eredomeans a greater relative tendency for reduction to occur, while a less positive Eredomeans a greater relative tendency for oxidation to occur. For galvanic cells, the electrode with the more positive reduction potential is the cathode, and the electrode with the less positive reduction potential is the anode. Since the species with a stronger tendency to gain electrons is actually gaining electrons, the redox reaction is spontaneous, and Δ G is negative. For electrolytic cells, the electrode with the more positive reduction potential is “ forced” (by the external voltage source) to be oxidized and is, therefore, the anode. The electrode with the less positive reduction potential is “ forced” to be reduced and is, therefore, the cathode. Since the movement of electrons is in the direction against the “ tendency” of the respective electrochemical species, the redox reaction is nonspontaneous, and Δ G is positive. need to multiply each halfreaction by a common denominator to cancel out electrons, do NOT multiply the reduction potential, E° , by that number.

Answer 196

• Standard reduction potentials are also used to calculate the standard electromotive force (emf or E° cell) of a reaction, or the difference in potential between two half-cells under standard conditions. The emf of a reaction is determined by adding the standard reduction potential of the reduced species and the standard oxidation potential of the oxidized species. When adding standard potentials, do not multiply them by the number of moles oxidized or reduced. two ways to express emf. The first allows us to use only reduction potentials. The second asks us to change the sign of the “ Ered” value in order to use an oxidation potential, which is the exact opposite of a reduction potential. where E° ox is the oxidation potential of the anode, which is the negative of the reduction potential. The standard emf of a galvanic cell is positive, while the standard emf of an electrolytic cell is negative.

Answer 197

• Thermodynamics of Redox Reactions: emf and gibbs free energy- the change in Gibbs free energy, Δ G. This is the change in the chemical potential of a reaction, or the change in the amount of energy available in a chemical system to do work. where n is the number of moles of electrons exchanged, F is Faraday's constant, and Ecell is the emf of the cell. Keep in mind that if Faraday's constant is expressed in coulombs (J/V), then Δ G must be expressed in J, not kJ. Notice the similarity of this relationship to that expressed in the physics formula W = qV for the amount of work available or needed in the transport of a charge q across a potential difference Δ V. If the reaction takes place under standard conditions (298 K, 1 atm, solutions at 1 M), then Δ G is more specifically defined as the standard Gibbs free energy, and Ecell is the standard cell potential. Being mindful of it will help you eliminate wrong answer choices that have the wrong sign for either Δ G° or E° cell. For example, if you are asked to calculate the change in Gibbs free energy for a galvanic cell, eliminate the answer choices with positive values; voltaic cells have positive emfs and the equation tells you the change in Gibbs free energy will have the opposite sign.

Answer 198

effect of concentration on emf- electrochemical cells may have ionic concentrations that deviate from 1 M. Also, for the concentration cell, the concentrations of the ions in the two compartments must be different, even if one of them is 1 M, for there to be a measurable voltage and current.Concentration and the emf of a cell are related: emf varies with the changing concentrations of the species involved. It can be determined using the Nernst equation. where Q is the reaction quotient for the reaction at a given point in time. For example, for the following reaction. expression for the reaction quotient Q has two terms for the concentrations of reactants and two terms for the concentrations of products, you need to remember that only the species in solution are included. When considering the case of the Daniell cell, for example, we need to think about which species of the redox reaction are in solution. Upon oxidation, the resulting cation will go into solution, and as a result, the product concentration is the concentration of the oxidized species. Because the electrons are captured by the cations that surround the cathode in the reduction half-reaction, these cations are the reactants of the redox reaction. Therefore, the reactant concentration is the concentration of the species that gets reduced. The emf of a cell can be measured by a voltmeter. A potentiometer is a kind of voltmeter that draws no current, and it gives a more accurate reading of the difference in potential between two electrodes.

Answer 199

• emf and the equilibrium constant (Keq)- where R is the gas constant 8.314 J/(K· mol), T is the absolute temperature in K, and Keq is the equilibrium constant for the reaction. Combining the two expressions that solve for standard free energy change. values for n,T, and Keq are known, then the E° cell for the redox reaction is easily calculated. Analysis of the equations shows us that for redox reactions with equilibrium constants less than 1 (equilibrium state favors the reactants), the E° cell will be negative because the natural log of any number less than 1 is negative. These properties are characteristic of electrolytic cells, which house nonspontaneous redox reactions. Instead, if the Keq for the redox reaction is greater than 1 (equilibrium state favors the products), the E° cell will be positive, because the natural log of any number greater than 1 is positive. These properties are characteristic of galvanic cells, which house spontaneous redox reactions. If the Keq is equal to 1 (concentrations of the reactants and products are equal at equilibrium), the E° cell will be equal to zero, because by definition of standard conditions (all ionic species at the same concentration, 1 M), the reaction is already at equilibrium. An easy way to remember this is that E° cell = 0 V for any concentration cell because, by definition, the equilibrium state of a concentration cell is when the concentrations of the ions in the two half-cells are equal (Keq = 1 for concentration cell). If E° cell is positive, ln K is positive. This means that K must be greater than one and that the equilibrium lies toward the right (i.e., products are favored).

Answer 200

• When two or more gases are found in one vessel without chemical interaction, each gas will behave independently of the other(s). Therefore, the pressure exerted by each gas in the mixture will be equal to the pressure that the gas would exert if it were the only one in the container.he pressure exerted by each individual gas is called the partial pressure of that gas. Dalton's law of partial pressures, which states that the total pressure of a gaseous mixture is equal to the sum of the partial pressure of the individual components

Answer 201

combustion reactions in which there are longer hydrocarbons with less branching to release more products and heat

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