Kaplan MCAT General Chemistry Review

Electrolysis The process in which an electric current is passed though a solution, resulting in chemical changes that do not otherwise occur spontaneously.

Electrolyte A compound that ionizes in water.

Electrolytic cell An electrochemical cell that uses an external voltage source to drive a nonspontaneous redox reaction.

Electromagnetic radiation A wave composed of electric and magnetic fields oscillating perpendicular to each other and to the direction of propagation.

Electromagnetic spectrum The range of all possible frequencies or wavelengths of electromagnetic radiation.

Electromotive force (emf ) The potential difference developed between the cathode and the anode of an electrochemical cell.

Electron (e^-) A subatomic particle that remains outside the nucleus and carries a single negative charge. In most cases, its mass is considered to be negligible (that of the proton).

Electron configuration The symbolic representation used to describe the electron occupancy of the various energy sublevels in a given atom.

Electronegativity A measure of the ability of an atom to attract the electrons in a bond. A common comparative electronegativity scale is the Pauling scale. Electronegativity is a periodic trend and a direct result of Z_eff.

Electron spin The intrinsic angular momentum of an electron, having arbitrary values of +and -. See spin quantum number.

Element A substance that cannot be further broken down by chemical means. All atoms of a given element have the same number of protons.

Emission spectrum A series of discrete lines at characteristic frequencies, each representing the energy emitted when electrons in an atom relax from an excited state to their ground state.

Empirical formula The simplest whole number ratio of the different elements in a compound.

Endothermic reaction A reaction that absorbs heat from the surroundings as the reaction proceeds (positive H).

End point The point in a titration at which the indicator changes color, showing that enough reactant has been added to the solution to complete the reaction.

Enthalpy (H) The heat content of a system at constant pressure. The change in enthalpy (H) in the course of a reaction is the difference between the enthalpies of the products and the reactants.

Entropy (S) A property related to the degree of disorder in a system. Highly ordered systems have low entropies. The change in entropy (S) in the course of a reaction is the difference between the entropies of the products and the reactants.

Equilibrium The state of balance in which the forward and reverse reaction rates are equal. In a system at equilibrium, the concentrations of all species will remain constant over time unless there is a change in the reaction conditions. See Le Châtelier’s principle.

Equilibrium constant (K_cor K_eq) The ratio of the concentration of the products to the concentration of the reactants for a certain reaction at equilibrium, all raised to their stoichiometric coefficients.

Equivalence point The point in a titration at which the number of equivalents of the species being added to the solution is equal to the number of equivalents of the species being titrated.

Excess reagent In a chemical reaction, any reagent whose amount does not limit the amount of product that can be formed. Compare Limiting reagent.

Excitation The promotion of an electron to a higher energy level by absorption of an energy quantum.

Excited state An electronic state having a higher energy than the ground state.

Exothermic reaction A reaction that gives off heat (negative H) to the surroundings as the reaction proceeds.

fsubshells The subshells corresponding to the angular momentum quantum number l = 3, found in the fourth and higher principal energy levels, each containing seven orbitals.

Face-centered cube A crystalline unit cell, cubic in shape, with atoms at each corner and at the center of each face.

Faraday (F) The total charge on 1 mole of electrons (1 F = 96,487 coulombs). Not to be confused with the farad (also denoted F), a unit of capacitance.

First law of thermodynamics The law stating that the total energy of a system and its surroundings remains constant.

Formal charge The conventional assignment of charges to individual atoms of a Lewis formula for a molecule, used to keep track of valence electrons. Defined as the total number of valence electrons in the free atom minus the total number of nonbonding electrons minus one-half the total number of bonding electrons.