Ch 4, 5- Compounds/ Stoichiometry/Kinetics

Question 1

“is a combination of two or more atoms held together by covalent bonds”

Answer 1

Molecules

ionic compounds do not form true molecules because of the way in which the oppositely charged ions arrange themselves in the solid state. As solids, they can be considered as nearly infinite three-dimensional arrays of the charged particles that comprise the compound.”

Question 2

NaCl in the solid state forms 3d arrays of the charged particle. This makes it rather difficult to clearly define a sodium chloride molecule. The term ____________, representing the empirical formula of the compound, is used instead. Because no molecule actually exists, molecular weight becomes meaningless, and the term ______ _____ is used instead.”

Answer 2

Formula Unit; formula weight

Question 3

“Ionic compounds form from combinations of elements with large electronegativity differences, such as sodium and chlorine. ______ compounds form from elements of similar electronegativity, such as carbon with oxygen”

Answer 3

Molecular

Question 4

“Remember that the term atomic weight is a misnomer because it is actually a weighted average of the masses of the naturally occurring isotopes of an element, not their weights. The same applies here to our discussion of ______. It is simply the sum of the atomic weights of all the atoms in a molecule, and its units are atomic mass units (amu) per molecule. ”

Answer 4

Molecular Weight

“Similarly, the formula weight of an ionic compound is found by adding up the atomic weights of the constituent ions according to its empirical formula, and its units are also amu per molecule.

Question 5

the _______ of an ionic compound is found by adding up the atomic weights of the constituent ions according to its empirical formula, and its units are also amu per molecule.

Answer 5

formula

Question 6

“The term molecular weight is sometimes used incorrectly to imply molar mass; remember, molecular weight is measured in ______

Answer 6

amu/molecule

Question 7

“Sodium will donate one mole of electrons (one _______), but magnesium will donate two moles of electrons (two ________).”

Answer 7

Equivalent; 2 equivalent

Question 8

Formula for equivalents is

Answer 8

Mass of Compound (g)/Gram Equivalent weight (g)

Question 9

The gram equivalent weight is what

Answer 9

The mass that provides 1 mole of the particle of interest

Gram eq weight=Molar mass/N

“where n is the number of particles of interest produced or consumed per molecule of the compound in the reaction. For example, one would need 31 grams of H2CO3 ) to produce one equivalent of hydrogen ions because each molecule of H2CO3 can donate two hydrogen ions (n = 2)”

Question 10

“Normality (N) is a measure of concentration, given in the units ______

Answer 10

Equivalent/L

“Thus, a 1 N solution of acid contains a concentration of hydrogen ions equal to 1 mole per liter; a 2 N solution of acid contains a concentration of hydrogen ions equal to 2 moles per liter. The actual concentration of the acidic compound may be the same or different from the normality because different compounds are able to donate different numbers of hydrogen ions. In a 1 N HCl solution, the molarity of HCl is 1 M because HCl is a monoprotic acid; in a 1 N H2CO3 solution, the molarity of H2CO3 is 0.5 M because H2CO3 is a diprotic acid”

Question 11

What is the conversion from normality to molarity of a solute?

Answer 11

Molarity=Normality/n

“where n is the number of protons, hydroxide ions, electrons, or ions produced or consumed by the solute.”

Question 12

“In acid–base chemistry, the gram equivalent weight of an acid represents

Answer 12

the mass that yields one mole of protons, or one mole of hydroxide ions if a base.”

Question 13

“What is the normality of a 2 M Mg(OH)2 solution?”

Answer 13

4 N Mg(OH)2

Normality=molarityxn=2 M x 2

Question 14

“There are two ways to express the formula of a compound. The ________ gives the simplest whole-number ratio of the elements in the compound. The _________ gives the exact number of atoms of each element in the compound and is a multiple of the empirical formula.”

Answer 14

empirical formula; molecular formula

Question 15

“The percent composition of an element (by mass) is the percent of a specific compound that is made up of a given element. To determine the percent composition of an element in a compound, the following formula is used:
\

Answer 15

% comp= mass of element/molar mass of compound x 100%

Question 16

“A _________ has two or more reactants forming one product. ”

Answer 16

Combination reaction

“Combination reactions have more reactants than products: A + B → C”

Question 17

A ________ is the opposite of a combination reaction: a single reactant breaks down into two or more products, usually as a result of heating, high-frequency radiation, or electrolysis.”

Answer 17

Decomposition Reaction

“Decomposition reactions generally have more products than reactants. A → B + C”

Question 18

“A _______ is a special type of reaction that involves a fuel—usually a hydrocarbon—and an oxidant (normally oxygen). In its most common form, these reactants form the two products of carbon dioxide and water”

Answer 18

Combustion Reaction

“Combustion involves oxidation (using O2 or similar) of a fuel (typically a hydrocarbon).”

“Combustion reactions are usually conducted with hydrocarbon fuels, but they can also use elements like sulfur or other compounds like sugars. The products can differ, but carbon dioxide and water are almost always present.”

Question 19

“A _________ occurs when an atom or ion in a compound is replaced by an atom or ion of another element.”

Answer 19

single-displacement reaction

“Cu (s) + AgNO3 (aq) → Ag (s) + CuNO3 (aq)”

Question 20

“In ___________, also called metathesis reactions, elements from two different compounds swap places with each other to form two new compounds. ”

Answer 20

Double Displacement Reactions

Question 21

“Solutes that enable solutions to carry currents are called

Answer 21

electrolytes”

Question 22

_________ make good electrolytes because they dissolve most readily. _________ are the weakest because they do not form current-carrying ions.”

Answer 22

“Ionic compounds; Nonpolar covalent compounds

Question 23

“The tendency of an ionic solute to dissolve, or solvate, into its constituent ions in water may be high or low. A solute is considered a _________ if it dissociates completely into its constituent ions.

Answer 23

Strong Electrolyte

Question 24

“Many compounds do not ionize at all in aqueous solution, retaining their molecular structure in solution, which may also limit their solubility. These compounds are called _______ and include many nonpolar gases and organic compounds, such as O2 (g), CO2 (g), and glucose.”

Answer 24

Nonelectrolytes

Question 25

“are substances composed of two or more elements in a fixed composition.”

Answer 25

Compounds

Question 26

“is a measure of the mass of a substance that can donate one equivalent of the species of interest.”

Answer 26

Gram Equivalent Weight

Question 27

“is the ratio of equivalents per liter; it is related to molarity by multiplying the molarity by the number of equivalents present per mole of compound.”

Answer 27

Normality

Question 28

“are moles of the species of interest; equivalents are most often seen in acid–base chemistry (hydrogen ions or hydroxide ions) and oxidation–reduction reactions (moles of electrons or other ions).”

Answer 28

Equivalents

Question 29

“The __________ states that any pure sample of a compound will contain the same elements in the same mass ratio”

Answer 29

law of constant composition

Question 30

“To calculate percent composition by mass,

Answer 30

determine the mass of the individual element and divide by the molar mass of the compound.”

Question 31

“occur when two or more reactants combine to form one product.”

Answer 31

Combination reactions

Question 32

“occur when one reactant is chemically broken down into two or more products.”

Answer 32

Decomposition Reaction

Question 33

“occur when a fuel and an oxidant (typically oxygen) react, forming the products water and carbon dioxide (if the fuel is a hydrocarbon).”

Answer 33

Combustion Reaction

Question 34

_______ reactions occur when one or more atoms or ions of one compound are replaced with one or more atoms or ions of another compound.

Answer 34

Displacement

Single-displacement reactions occur when an ion of one compound is replaced with another element.
Double-displacement reactions occur when elements from two different compounds trade places with each other to form two new compounds.”

Question 35

“are those in which an acid reacts with a base to form a salt (and, usually, water).”

Answer 35

Neutralization reactions

Question 36

“is the amount of product generated if all of the limiting reactant is consumed with no side reactions.”

Answer 36

Theoretical Yield

Question 37

Percent Yield is calculated how

Answer 37

“is calculated by dividing actual yield by theoretical yield and converting to a percentage.”

Question 38

____ endings can also be used to indicate lesser charge, while ____ endings indicate greater charge.”

Answer 38

“–ous; –ic

Question 39

“Reactions can be spontaneous or nonspontaneous; the change in ______ determines whether or not a reaction will occur by itself without outside assistance.”

Answer 39

Gibbs free energy (ΔG)

Question 40

“Many reactions proceed by more than one step, the series of which is known as the ______ of a reaction, and the sum of which gives the overall reaction.”

Answer 40

Mechanism

Question 41

“One of the most important points to remember is that the slowest step in any proposed mechanism is called the ________ because it acts like a kinetic bottleneck, preventing the overall reaction from proceeding any faster than that slowest step.

Answer 41

rate-determining step

Question 42

“The __________ states that the rate of a reaction is proportional to the number of collisions per second between the reacting molecules.”

Answer 42

collision theory of chemical kinetics

“The theory suggests, however, that not 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 their existing bonds and form new ones.”

Question 43

“The minimum energy of collision necessary for a reaction to take place is called the

Answer 43

activation energy, Ea, or the energy barrier. ”

Question 44

“The rate of a reaction can therefore be expressed as”

Answer 44

Rate=Z x f

“where Z is the total number of collisions occurring per second and f is the fraction of collisions that are effective. ”

Question 45

What is the Arrhenius Equation?

Answer 45

k= Ae^-Ea/RT

“where k is the rate constant of a reaction, A is the frequency factor, Ea is the activation energy of the reaction, R is the ideal gas constant, and T is the temperature in kelvins. The frequency factor, also known as the attempt frequency of the reaction, is a measure of how often molecules in a certain reaction collide, with the unit s^–1.”

“Low activation energy and high temperatures make the negative exponent of the Arrhenius equation smaller in magnitude and thus increase the rate constant k.”

“Overall, what is important here in studying the Arrhenius equation is not the actual calculation (because those involving Euler’s number, e, and natural logs, ln, are not commonly found on the MCAT), but rather the relationships between the variables and the exponent rules that govern the equation.”

Question 46

How can we increase the frequency factor (A) in the arrhenius equation

Answer 46

“The frequency factor can be increased by increasing the number of molecules in a vessel. When there are more molecules, the opportunities for collision are increased”

Question 47

“The _______, has greater energy than both the reactants and the products and is denoted by the symbol ‡. ”

Answer 47

transition state, also called the activated complex

“The energy required to reach this transition state is the activation energy.”

“Once an activated complex is formed, it can either dissociate into the products or revert to reactants without any additional energy input.”

“Relative to reactants and products, transition states have the highest energy. They are only theoretical structures and cannot be isolated. Nevertheless, we can still use the proposed structures to better understand the reactions in which they are involved.”

Question 48

“The energy required to reach this transition state is the

Answer 48

activation energy.”

“Once an activated complex is formed, it can either dissociate into the products or revert to reactants without any additional energy input.”

“Relative to reactants and products, transition states have the highest energy. They are only theoretical structures and cannot be isolated. Nevertheless, we can still use the proposed structures to better understand the reactions in which they are involved.”

Question 49

“The ________ is the difference between the free energy of the products and the free energy of the reactants

Answer 49

free energy change of the reaction (ΔGrxn)

A negative free energy change indicates an exergonic reaction (energy is given off), and a positive free energy change indicates an endergonic reaction (energy is absorbed)

Question 50

“The difference in free energy between the transition state and the reactants is the

Answer 50

activation energy of the forward reaction”

“the difference in free energy between the transition state and the products is the activation energy of the reverse reaction.”

“Kinetics and thermodynamics should be considered separately. Note that the free energy of the product can be raised or lowered, thereby changing the value of ΔG without affecting the value of forward Ea.”

Question 51

What are factors that affect reaction rate?

Answer 51

Reactant concentrations, Temperature, Medium, catalyst

Question 52

This leads to an increase in the frequency factor (A) of the Arrhenius equation. Therefore, the reaction rate will increase for all but ________ reactions, which will be discussed shortly. For reactions occurring in the gaseous state, the partial pressures of the gas reactants serve as a measure of concentration”

Answer 52

Zero-Order

The greater the concentrations of the reactants, the greater the number of effective collisions per unit time.

Question 53

“For nearly all reactions, the reaction rate will _______ as the temperature increases.

Answer 53

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”

“the proportion of reactants gaining enough energy to surpass Ea (and thus capable of undergoing reaction) increases with higher temperature. All reactions are temperature-dependent and experience an optimal temperature for activity.”

“at extreme temperatures, the nucleus itself begins to break down.”

Question 54

“Generally, _____ solvents are preferred for faster reactions because their molecular dipole tends to polarize the bonds of the reactants, thereby lengthening and weakening them, permitting the reaction to occur faster.”

Answer 54

Polar

Question 55

How do catalysts increases reaction rate?

Answer 55

“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. ”

Question 56

Catalysts, Consequently, have no impact whatsoever on the

Answer 56

equilibrium position or the measurement of Keq”

“ The presence of the catalyst also has no impact on the free energies of the reactants or the products or the difference between them.”

Question 57

“Remember that, as useful as catalysts are in biological and nonbiological systems, catalysts are not miracle workers: they will not transform a nonspontaneous reaction into a spontaneous one; they only make ________ reactions move more quickly toward equilibrium.

Answer 57

Spontanious

Question 58

“Equilibria are dynamic, meaning that they do undergo change but their ______ will be zero.”

Answer 58

Net change

Question 59

“If we consider a generic reaction, 2 A + B → C, in which one mole of C can be produced from two moles of A and one mole of B, we can describe the rate of this reaction in terms of either ______ or _____

Answer 59

the disappearance of reactants over time or the appearance of products over time. ”

“For the above reaction, the rate of the reaction with respect to A is -[A]/T with respect to B is -[B]/T and with respect to C is [C]/T

Question 60

“Rate is expressed in the units of moles per liter per second or

Answer 60

molarity per second ” M/s

Question 61

“For nearly all forward, irreversible reactions, the rate is proportional to the ________, with each concentration raised to some experimentally determined exponent”

Answer 61

the concentrations of the reactants

“For the general reaction
aA + bB → cC + dD

“the rate is proportional to [A]x[B]y.

So rate is determined by
rate = k[A]x[B]y

“where k is the reaction rate coefficient or rate constant and the exponents x and y are the orders of the reaction”

This expression is called the rate law

Question 62

“The overall order of the reaction is

Answer 62

the sum of x and y

“The MCAT will focus almost exclusively on zero-, first-, second-, and third-order reactions. In most cases, the exponents will be integers.”

Question 63

A common trap in chemical kinetics is what assumption

Answer 63

“the assumption that the orders of a reaction are the same as the stoichiometric coefficients in the balanced overall equation. Pay close attention: On the MCAT, the values of x and y are almost never the same as the stoichiometric coefficients.”

Only 2 cases which it is true. 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 equal to the orders of the reaction.”

Question 64

“There are only two cases in which the stoichiometric coefficients match the orders of the reaction.

Answer 64

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 equal to the orders of the reaction.”

Question 65

“The expression for _________ includes the concentrations of all the species in the reaction, both reactants and products. The expression for chemical ______—the rate law expression—includes only the reactants. ”

Answer 65

equilibrium; Kinetics

“Keq indicates where the reaction’s equilibrium position lies. The rate indicates how quickly the reaction will get there.”

Question 66

“The second common trap is mistaking the equilibrium constant expression (law of mass action) for ______. The expressions for both look similar

Answer 66

Rate law

“if you’re not alert on Test Day, you may mistake one for the other or use one when you should be using the other. The expression for equilibrium includes the concentrations of all the species in the reaction, both reactants and products.

Question 67

“The third trap regards the rate constant, k. Technically speaking, k is 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. However, for a specific reaction, at a specific temperature, the rate constant is

Answer 67

indeed a constant!

Question 68

“For a reversible reaction, the Keq is equal to what in terms of k (from kinetics)

Answer 68

the ratio of the rate constant for the forward reaction, k, divided by the rate constant for the reverse reaction, k–1.”

Question 69

“To determine the value of the rate constant k,

Answer 69

plug in actual values from any one of the trials; pick whichever trial has the most arithmetically convenient numbers.

Question 70

“We classify chemical reactions as

Answer 70

zero-order, first-order, second-order, higher-order, or mixed-order on the basis of kinetics”

Question 71

“A ______ reaction is one in which the rate of formation of product C is independent of changes in concentrations of any of the reactants, A and B.”

Answer 71

Zero-Order

“These reactions have a constant reaction rate equal to the rate constant (rate coefficient), k. The rate law for a zero-order reaction is:

rate = k[A]^0[B]^0 = k”

k has units of M/s

“Remember that the rate constant itself is dependent on temperature; thus, it is possible to change the rate for a zero-order reaction by changing the temperature or simply adding a catalyst!

Question 72

“_____ and ____ are the only factors that can change the rate of a zero-order reaction”

Answer 72

Temperature and the addition of a catalyst

Question 73

for the Kinetics of a Zero-Order Reaction the rate of reaction, k, is

Answer 73

the opposite of the slope.”

k= - slope

Question 74

“A ________ reaction has a rate that is directly proportional to only one reactant, such that doubling the concentration of that reactant results in a doubling of the rate of formation of the product.

Answer 74

First order

The rate law for a first-order reaction is”

“rate = k[A]1 or rate = k[B]1”

“where k has units of s–1. A classic example of a first-order reaction is the process of radioactive decay”

Question 75

A classic example of a first-order reaction is

Answer 75

the process of radioactive decay”

“The concentration of radioactive substance A at any time t can be expressed mathematically as:

[A]t = [A]0e^–kt”

Question 76

In a first order reaction, Plotting ______ reveals a straight line; the slope of such a line is ______

Answer 76

ln [A] vs. time; opposite of the rate constant, k.”

“The concentration of radioactive substance A at any time t can be expressed mathematically as:

[A]t = [A]0e^–kt”

Excerpt From: Kaplan. “Kaplan MCAT General Chemistry Review: Created for MCAT 2015 (Kaplan Test Prep).” iBooks.

Question 77

“The concentration of radioactive substance A at any time t can be expressed mathematically as:

Answer 77

[A]t = [A]0e^–kt”

Question 78

“A ______ has a rate that is proportional to either the concentrations of two reactants or to the square of the concentration of a single reactant.”

Answer 78

Second Order

“rate = k[A]1[B]1 or rate = k[A]2 or rate = k[B]2”

“k has units of M^–1 s^–1”

Question 79

For a second order reaction Plotting _____ vs time reveals ______. The slope is equal to _____

Answer 79

1/[A], a linear curve; the rate constant k

Question 80

_______sometimes refer to non-integer orders (fractions) and in other cases to reactions with rate orders that vary over time

Answer 80

Mixed order reactions

Fractions are more specifically described as broken-order. In recent times, the term mixed-order has come to refer solely to reactions that change order over time. Knowing those two definitions will be sufficient for Test Day”

Question 81

“Reaction rates can be affected by a number of factors.”

Answer 81

1. Increasing the concentration of reactant will increase reaction rate (except for zero-order reactions) because there are more effective collisions per time.
2. Increasing the temperature will increase reaction rate because the particles’ kinetic energy is increased.
3. Changing the medium can increase or decrease reaction rate, depending on how the reactants interact with the medium.”
4. Adding a catalyst increases reaction rate because it lowers the activation energy. Homogeneous catalysts are the same phase as the reactants; heterogeneous catalysts are a different phase.”

Question 82

“have a constant rate that does not depend on the concentration of reactant.”

Answer 82

Zero-Order reactions

1. The rate of a zero-order reaction can only be affected by changing the temperature or adding a catalyst.
2. A concentration vs. time curve of a zero-order reaction is a straight line; the slope of such a line is equal to –k.

Question 83

“have a nonconstant rate that depends on the concentration of reactant where
“A concentration vs. time curve of a first-order reaction is nonlinear.
The slope of a ln [A] vs. time plot is –k for a first-order reaction.”

Answer 83

First order reactions

Question 84

have a nonconstant rate that depends on the concentration of reactant.
Where
“A concentration vs. time curve of a second-order reaction is nonlinear.
The slope of a vs. time plot is k for a second-order reaction.”

Answer 84

Second Order

Question 85

“are those with noninteger orders.”

Answer 85

Broken-Order reaction

Question 86

“are those that have a rate order that changes over time.”

Answer 86

Mixed-Order reactions

Question 87

“When the reaction system is closed and no reactants or products are added or removed, the system will eventually settle into a state in which

Answer 87

the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of the products and reactants remain constant. ”

“In this dynamic equilibrium, the forward and reverse reactions are still occurring—they haven’t stopped, as they do 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”

Question 88

For A ⇌ B

At equilibrium, the concentrations of A and B are _____ (although not necessarily ____), and the reactions A → B and B → A continue to occur at equal rates.”

Answer 88

Constant; Equal

Question 89

“For a reversible reaction at a given temperature, the reaction will reach equilibrium when what happens in terms of entropy and gibbs free energy

Answer 89

the system’s entropy—or energy distribution—is at a maximum and the Gibbs free energy of the system is at a minimum.”