Eta Flashcards
Polarizability
Polarizability is the extent to which an electron cloud of an atom can be distorted by an external charge or by an applied electric field to produce a dipole.
Electronegativity
It is the tendency of an atom to attract electrons within a bond.
Electron Affinity
It assesses the tendency of an atom to accept an additional electron by measuring the energy change when an electron is added to an atom.
Ionization energy
It is the opposite of electron affinity and measures the energy required to remove an electron from an atom. The smaller the ionization energy, the higher the reactivity.
Important note about Telomeres and Centromeres
Telomeres are regions at chromosomal ends that are repeatedly truncated with each round of cell division. Centromeres join two sister chromatids and are required for proper chromosome division during mitosis. Despite their different chromosomal locations, both telomeres and centromeres are composed of heterochromatin, a tightly condensed complex of DNA wrapped around histones. Because of its structurally restrictive form, heterochromatin is transcriptionally inactive, meaning that proteins responsible for regulating gene expression cannot access the tightly packed DNA. As a result, hetechromatic regions are often gene-poor and contain repetitive DNA.
(Choice A) Telomeres contain repeats of only TTAGGG, a single DNA sequence of six nucleotides that is added by the enzyme telomerase. Multiple repeats of differing DNA sequences are not present in telomeres.
(Choice C) Telomeres are shortened with each round of cell division. However, only embryonic stem cells (not somatic cells) express telomerase and therefore have very long telomeres; this allows them to proliferate indefinitely in a controlled manner.
(Choice D) DNA polymerase, which is responsible for carrying out DNA synthesis, cannot replicate chromosomal ends.
Origin of replication
Prokaryotes typically have circular DNA with a single origin of replication in the cytoplasm whereas eukaryotes have linear DNA with multiple origins of replication in the nucleus. An origin of replication expands to form a replication bubble, which contains two replication forks that move apart in opposite directions during DNA synthesis.
Projection area
Projection areas are areas in the four lobes (frontal, parietal, temporal, occipital) of the brain where sensory processing occurs.
Allport’s Three basic types of Traits
Cardinal
Central
Secondary
Eysenck’s Three Major Traits
Psychoticism
Extraversion
Neuroticism
Big Five Trait Theorists
Openness Conscientiousness Extraversion Agreeableness Neuroticisim
Gas Chromatography
Think the total number of molecular weight and the polarity, in addition to intermolecular attractions
Ionic Radii, Neutral radii, and Isoelectronic species
Ionic radii tend to decrease in size across a period (row) of the periodic table (left to right) and increase moving down a group (column). This trend occurs for metal cations, and then resets and repeats for anions beginning near the division between metals and nonmetals, past which anions tend to preferentially form.
Compared to the neutral atom of a given element, its cation will be smaller but its anion will be larger. Losing electrons to form a cation causes the remaining electrons to experience a greater effective nuclear charge (Zeff), pulling the electrons closer to the nucleus. Conversely, gaining electrons to form an anion produces greater electronic repulsion and nuclear shielding (lesser Zeff), which pushes electrons farther from the nucleus.
Na+, F−, Mg2+, and O2− ions are isoelectronic (have the same number of electrons), but because the number of protons is different in each ion, the electrons in each ion experience a different Zeff. Therefore, in an isoelectronic series, ionic radii decrease with increasing atomic number. Because magnesium has the highest atomic number (greatest number of protons) in the isoelectronic series, Zeff is greatest in this ion, making it the smallest within the given series.
Atomic Mass Unit and Molecular Weight
An atomic mass unit (amu) is defined as one-twelfth of the mass contained in a carbon-12 atom (the average mass of 6 protons and 6 neutrons). On this basis, the amu provides a useful measure for assessing the masses of different atoms relative to the carbon-12 standard.
Mass measurements on the macroscale (grams) must account for a large quantity of amu. For this purpose, a mole is defined as the number of carbon-12 atoms required to yield exactly 12 grams. This number of atoms is called Avogadro’s number (NA) and is equal to 6.022 × 1023 atoms. Because each carbon-12 atom contains 12 amu, the mole can also be viewed as the number of amu required to yield 1 gram of mass.
6.022 × 1023 amu = 1 mol amu = 1 g
The molecular weight of a compound is simply the sum of the amu contributions from each atom in a formula unit. For example, the molecular weight of a single (NH4)2CO3 molecule is 96.11 amu. On the macroscale, 6.022 × 1023 formula units (1 mole) result in the molar mass. Because 1 mole of amu contains 1 gram of mass, the molar mass numerically correlates to the molecular weight but indicates the amount of mass (in grams) contained in 1 mole of the formula unit (grams/mole).
96.11 amu × (1 g / 1 mol amu) = 96.11 g/mol
For (NH4)2CO3, a molecular weight of 96.11 amu means that 1.00 mole of (NH4)2CO3 molecules has a mass of 96.11 g (Number I). Likewise, a 96.11 g sample of (NH4)2CO3 will contain 1 mole (6.022 × 1023 units) of (NH4)2CO3 molecules (Number II). Conversely, 96.11 amu of (NH4)2CO3 refers only to a single molecule of (NH4)2CO3, which does not contain the same mass as a mole of (NH4)2CO3 molecules (Number III).
Electronic Configuration - Interesting Concept
An electron configuration sequentially lists the placement of all electrons within the shells and subshells of an atom or ion in order of increasing energy, according to the Aufbau principle. Subshells are labeled by type (s, p, d, f), which indicates the kind of orbitals present within the subshell. Accordingly, an s subshell contains one s orbital, a p subshell contains three p orbitals, a d subshell contains five d orbitals, and an f subshell contains seven f orbitals.
The Pauli exclusion principle states that each orbital within a subshell can hold a maximum of two electrons, but two electrons in the same orbital must have opposite spins. Therefore, s, p, d, and f subshells can hold a maximum of 2, 6, 10, and 14 electrons, respectively. In schematic diagrams, orbitals are often represented by blanks or boxes, and electrons are represented by arrows. The spin of each electron is indicated by the orientation of the arrow (up or down).
Accordingly, a subshell represented as 3s ↑↑
(Number I) cannot exist because the electrons in the s orbital have the same spin. Likewise, a subshell indicated by 2p7 (Number II) also cannot exist because a p subshell holds a maximum of 6 electrons (two electrons in each of the three p orbitals).
(Number III) 6d ↑↓ ↑ ↑↓ ↑ ↑
is a valid subshell configuration. A d subshell contains five d orbitals of equal energy that together can accommodate up to 10 electrons. Because the d orbitals are equivalent in energy, paired electrons of opposite spin can be placed in any of the five orbitals without distinction.
(Number IV) 4f14 is a correct subshell configuration. An f subshell contains seven f orbitals of equal energy that together can accommodate up to 14 electrons in total.
Bohr’s Model Assumption
In the Bohr model of the atom, several assumptions are made regarding electrons and their locations around the nucleus. The Bohr model asserts that:
- Electrons move around the nucleus in fixed circular orbits, which are only allowed at particular intervals from the nucleus.
- Electrons in orbits farther from the nucleus have higher energy than electrons in orbits closer to the nucleus.
- Energy is absorbed by an electron moving from a lower orbit to a higher orbit, but energy is emitted by an electron returning from a higher orbit back to a lower orbit.
- The energy that is absorbed or emitted by an electron equals the energy difference between two orbits.
Nature of Coordination of d-orbitals
When placed within a coordination sphere, the atomic d orbitals of a metal are no longer degenerate, so they will have different energies. This occurs because some orbitals are pointing toward the ligands, which are electron donors, and other d orbitals are pointing away from or between the ligands. The orbitals that point toward the ligands are higher in energy than the orbitals pointing away from or between them because electrons repel each other. The energy of the orbitals determines which wavelengths of light can be absorbed. Some ligands cause greater differences in the energy of the d orbitals than other ligands, resulting in the absorption of different wavelengths of light. These wavelengths are usually in the visible spectrum.
When heme binds O2, the nature of the O2 ligand changes the energy of iron’s d orbitals. This energy change causes heme to absorb blue-green light and reflect red light.
Further information on Coordination Bonds
Coordinate covalent bonds are a special type of bond between a central atom, such as a metal, and a ligand. Both electrons in the bond come from the ligand. Coordination bonds are neither covalent nor ionic but have some properties of both bond types. The metal ion maintains its oxidation state, as in the case of an ionic bond, and the ligands are not charged but are usually electronegative, as in covalent bonds. The metal and its ligands together form a complex, and the number of coordinate bonds to the metal is known as the coordination number.
The metal ion Fe2+ is positively charged whereas the donor atoms (nitrogen) are neutral, resulting in a net charge of +2 for the complex. Unlike ionic bonds, the donor atoms do not give electrons to the positively charged metal. Instead counterions often surround the complex in solution to balance the metal’s positive charge.
(Choice A) The nitrogen ligands each have a lone pair of electrons that provide the bonding electrons in the coordinate covalent bond.
(Choice C) The lone pairs of nitrogen’s electrons interact with iron’s d orbitals to form the coordinate bond. In a coordination bond, the d orbitals are no longer degenerate and have distinct shapes, some of which point directly at the negatively charged ligands.
(Choice D) The strength of the coordination bond depends on which d orbitals have electrons in them.
Acid-Base Neutralization Reaction
In a titration, a measured amount of a solution with a known concentration (titrant) is added to another solution containing an unknown concentration of the compound to be measured (analyte). In acid-base titrations, an acid is titrated with a base (or vice versa). The resulting acid-base neutralization reaction produces a change in pH, which is monitored by a pH indicator that signals the equivalence point of the neutralization.
The analyte must be fully dissolved before it can be measured. Sebacic acid has low solubility in water due to a high nonpolar hydrocarbon character. A base will convert the carboxylic acid groups into highly polar ionic salts with much higher aqueous solubility.
Once dissolved, the carboxylate ions can then be titrated with an acid. Subtracting the number of moles of base (such as KOH) in the initial solution from the number of moles of acid (such as HCl) required to reach the equivalence point during the titration will give the number of moles of carboxylate groups from sebacic acid in the sample.
(Choices A and B) Sebacic acid is a ten-carbon organic acid and will not significantly dissolve in dilute aqueous acids. Only a base will ionize the carboxylic acid groups and allow the compound to dissolve. Once dissolved, titration of the basic solution should then be performed using an acid.
(Choice C) A dilute base will dissolve the sebacic acid sample, but basic solutions must be titrated with acids rather than with bases.
Educational objective:
Acid-base neutralization reactions can be used in titrations with an indicator to determine the concentration of an acidic (or basic) analyte. By assessing the equivalence point of the neutralization, the volume of titrant required can be correlated to the amount of an analyte present in solution. Acids or bases may also aid the solubility of nonpolar analytes by forming more soluble ionic salts.
Equivalence Point
In an acid-base titration, a measured amount of an acid (or a base) solution (titrant) with a known concentration is slowly added to another solution containing an unknown concentration of a base (or an acid) to be measured. As the titration proceeds, an acid-base neutralization reaction occurs, producing a change in the pH of the solution being titrated.
When the acid (or base) being titrated is fully neutralized, the number of equivalents of acid exactly equals the number of equivalents of base (equivalence point). When passing through this point, the pH changes rapidly. On the titration curve, this is seen as a nearly vertical line, and at its midpoint lies the equivalence point.
To visually detect when a titration is complete, an indicator that has an endpoint (color change) near the pH of the equivalence point can be added to the solution. Different indicators change color across a particular pH range (endpoint range). The best indicator for a given titration is one that has a pH range that corresponds most closely to the pH of the equivalence point.
In the titration of aqueous ammonia, the equivalence point occurs at pH 5.3. Among the four indicators listed, methyl red has an endpoint range that overlaps closest to the equivalence point. Therefore, methyl red is the best indicator for the titration.
Buffers
Buffers are solutions that resist changes in pH when small amounts of acid or base are added. Buffers consist of a mixture of either a weak acid and a salt of its conjugate base, or a weak base and a salt of its conjugate acid. The acidic component of a buffer mixture can neutralize any added base, and the basic component of a buffer mixture can neutralize any added acid. As a result, large changes in pH are impeded.
For example, acetic acid (CH3COOH) is a weak acid, and its conjugate base is the acetate anion (CH3COO−). As such, a buffer could be made by mixing equal concentrations of each into solution. A salt such as sodium acetate (CH3COONa) could function as the source of acetate ions. Once formed, the buffer would establish the following equilibrium:
CH3COOH+H2O⇌CH3COO−+H3O+
Any strong base (OH−) added to the solution would be neutralized by the CH3COOH to form more acetate ions:
CH3COOH+OH−⇌CH3COO−+H2O
Similarly, any strong acid (H3O+) added to the solution would be neutralized by the CH3COO− to form more CH3COOH:
CH3COO−+H3O+⇌CH3COOH+H2O
Therefore, an aqueous mixture of CH3COOH and CH3COONa will function as a buffer.
(Choice A) NaNO3 is the salt of the conjugate base of HNO3, but HNO3 is a strong acid and strong acids are not suitable for making buffers.
(Choice C) NaBr with NaCN is a mixture of two salts. A buffer requires a weak acid (or a weak base) to be present with the salt of its corresponding conjugate base (or conjugate acid).
(Choice D) NaOH is a strong base, and strong bases are not suitable for making buffers. Moreover, NaCl is not the salt of the conjugate acid of NaOH.
Four characteristics of an ideal gas
- An ideal gas has no attractive or repulsive forces between the gas molecules
- The size (molecular volume) of the individual gas molecules of an ideal gas is negligible (taken to be zero) compared to the volume (space) of the container the gas occupies.
- Collisions between the molecules of an ideal gas are completely elastic (no energy is lost by interactions or friction)
- Ideal gas molecules have an average kinetic energy (energy of motion) that is directly proportional to the gas temperature.
Equilibrium from Graphs
Equilibrium is achieved in a reversible reaction when the forward reaction and the reverse reaction occur simultaneously at the same rate. Once equilibrium is achieved, the forward reaction generates products as fast as the reverse reaction converts those products back into the original reactants, and this causes the concentrations of the reactants and the products to become constant. Although constant, the equilibrium concentrations are not necessarily equal because equilibrium refers to a state of equal reaction rates (changes in concentration over time) but not to a state of equal concentrations.
In the graph of concentration vs. time for the reaction between H2(g) and I2(g), the constant (horizontal) regions of the curves (after approximately 5 hours) indicate a state of equilibrium in which the concentrations of the chemical species no longer change. From this region of the graph, the equilibrium concentration of HI(g) is seen to be 6.0 M. Therefore, if a 750 mL sample of the reaction mixture were analyzed at equilibrium, the number of moles of HI(g) present in the sample would be determined as follows:
750 mL×1 L1000 mL×6.0 mol1 L=4.5 mol
(Choice A) At equilibrium, a 750 mL sample of the reaction mixture contains 0.8 mol each of H2(g) and I2(g), but the question asks for the moles of HI(g).
(Choice B) A 750 mL sample measured at the intersection point of the lines on the graph (after approximately 1 hour) contains 1.9 moles of HI(g), but this point does not indicate the equilibrium of the reaction.
(Choice D) At equilibrium, the HI(g) has a molar concentration of 6.0 mol/L. This is the number of moles of HI(g) per liter rather than the number of moles in a 750 mL sample of the reaction mixture.
Educational objective:
Equilibrium is achieved when two opposing chemical reactions occur simultaneously at the same rate such that the concentrations of the chemical species become constant. Equilibrium reaction rates are equal, but the equilibrium concentrations of chemical species may be unequal.
Measurement of Pressuree
1 atm=760 mmHg=760 torr=101,325 Pa=101.325 kPa
Reaction Activation
Most chemical reactions can only occur when two or more molecules collide with enough energy to break bonds, allowing new bonds to form. This required energy is called the activation energy. Temperature is a measure of the average kinetic energy of the molecules in a system. At higher temperatures, the molecules move more quickly and collide more often and with more energy. Therefore, an increase in temperature will increase the rate of a reaction by providing more molecules with the required activation energy.
The reaction between hydrogen and oxygen is thermodynamically favorable, meaning that it occurs spontaneously. However, the reaction has a high activation energy. At room temperature, most of the molecules lack the energy necessary to initiate the reaction, and the reaction proceeds so slowly that it effectively does not occur at all. A spark releases heat into the system, raising the temperature and therefore the kinetic energy of nearby molecules. These molecules are then able to react exothermically, releasing more heat into the system and providing more molecules with sufficient energy to react.
(Choice A) The equilibrium position of a reaction is not appreciably altered by the presence of a spark. Instead, the rate at which equilibrium is achieved changes.
(Choice B) The spark consumes a negligible amount of oxygen. Because oxygen is a reactant, its consumption would drive the reaction toward reactant formation based on Le Châtelier principle.
(Choice D) Oxidizers such as oxygen gain electrons in combustion reactions. They do not lose electrons.
Educational objective:
Molecules must collide with sufficient energy, known as the activation energy, for a reaction to occur. Increased temperature increases the kinetic energy of the molecules in a system, and therefore increases the rate of a reaction.
