Kaplan MCAT General Chemistry Review

Freezing point At a given pressure, the temperature at which the solid and liquid phases of a substance coexist in equilibrium.

Freezing point depression Amount by which a given quantity of solute lowers the freezing point of a liquid, a colligative property.

Galvanic cell An electrochemical cell that uses a spontaneous redox reaction to do work (i.e., produce an electrical current). Also called a voltaic cell.

Gas The physical state of matter possessing the most disorder, in which molecules interact only slightly; found at relatively low pressure and high temperatures. Also called vapor. See ideal gas.

Gas constant (R) A proportionally constant that appears in the Ideal Gas Law equation, PV = nRT. Its value depends upon the units of pressure, temperature, and volume used in a given situation.

Gibbs free energy (G) The energy of a system available to do work. The change in Gibbs free energy, G, can be determined for a given reaction from the equation G is used to predict the spontaneity of a reaction: A negative G denotes a spontaneous reaction, while positive G denotes a nonspontaneous reaction. See standard free energy.

Graham’s law The law stating that the rate of effusion of diffusion for a gas is inversely proportional to the square root of the gas’s molecular weight.

Gram-equivalent weight The amount of a compound that contains 1 mole of reacting capacity when fully dissociated. One GEW equals the molecular weight divided by the reactive capacity per formula unit.

Group A vertical column of the periodic table, containing elements that are similar in their chemical properties. Compare period.

Half-life The time required for the amount of a reactant to decrease to one-half of its former value.

Half-reaction Either the reduction half or oxidation half of a redox reaction. Each half-reaction occurs at one electrode of an electrochemical cell.

Halogens The active nonmetals in Group VIIA of the periodic table, which have high electronegativities and highly negative electron affinities.

Heat The energy representing the kinetic energy of molecules that is transferred spontaneously from a warmer sample to a cooler sample. Compare temperature.

Heat of formation (H_f) The heat absorbed or released during the formation of a pure substance from the elements at a constant pressure.

Heat of fusion (H_fus) The H for the conversion of 1 gram or 1 mole of a solid to a liquid at constant temperature and pressure.

Heat of sublimation (H_sub) The H for the conversion of 1 gram or 1 mole of a solid to a gas at constant temperature and pressure.

Heat of vaporization (H_vap) The H for the conversion of 1 gram or 1 mole of a liquid to a vapor at constant temperature and pressure.

Heisenberg uncertainty principle The principle that states that it is impossible to determine simultaneously with perfect accuracy both the momentum and position of a particle.

Henderson-Hasselbalch equation Approximate equation showing the relationship of the pH or pOH of a solution to the pK_a or pK_b and the ratio of the concentrations of the dissociated species.

Henry’s law The law stating that the mass of a gas that dissolves in a solution is directly proportional to the partial pressure of the gas above the solution.

Hess’s law The law stating that the energy change in an overall reaction is equal to the sum of the energy changes in the individual reactions that comprise it.

Heterogeneous Nonuniform in composition.

Homogeneous Uniform in composition.

Hund’s rule The rule that electrons will occupy all degenerate orbitals in a subshell with single electrons having parallel spins before entering half-filled orbitals. See paired electrons.

Hybridization The combination of two or more atomic orbitals to form new orbitals whose properties are intermediate between those of the original orbitals.

Hydrogen bonding The strong attraction between a hydrogen atom bonded to a highly electronegative atom (such as fluorine or oxygen) in one molecule and a highly electronegative atom in another molecule.

Hydrolysis A reaction between water and a species in solution.

Hydronium ion The H₃O⁺ ion in aqueous solution.

Hydroxide ion The OH^- ion.

Ideal gas A hypothetical gas whose behavior is described by the Ideal Gas Law under all conditions. An ideal gas would have particles of zero volume that do not exhibit interactive forces.