Kaplan MCAT General Chemistry Review

Crystal A solid whose atoms, ions, or molecules are arranged in a regular, three-dimensional lattice structure.

d subshells The subshells corresponding to the angular momentum quantum number l = 2, found in the third and higher principal energy levels, each containing five orbitals.

Dalton’s law The law stating that the sum of the partial pressures of the components of a gaseous mixture must equal the total pressure of the sample.

Daniell cell An electrochemical cell in which the anode is the site of Zn metal oxidation and the cathode is the site of Cu²⁺ ion reduction.

Degenerate orbitals Orbitals that possess equal energy.

Delocalized orbitals Molecular orbitals whose electron density is spread over an entire molecule, or a portion thereof, rather than being localized between two atoms.

Density () A physical property of a substance, defined as the mass contained in a unit of volume.

Diamagnetic A condition that arises when a substance has no unpaired electrons and is slightly repelled by a magnetic field.

Diffusion The random motion of gas or solute particles across a concentration gradient, leading to uniform distribution of the gas or solute throughout the container.

Dipole A species containing bonds between elements of different electronegativities, resulting in an unequal distribution of charge in the species.

Dipole–dipole interaction The attractive force between two dipoles whose magnitude is dependent on both the dipole moments and the distance between the two species.

Dipole moment A vector quantity whose magnitude is dependent on the product of the charges and the distance between them. The direction of the moment is from the positive to the negative pole.

Disproportionation A redox reaction in which the same species acts as the oxidizing agent and as the reducing agent. For example,

Hg₂Cl₂ HgCl₂ + Hg

Dissociation The separation of a single species into two separate species; this term is usually used in reference to salts or weak acids or bases.

Effective nuclear charge (Z_eff) The charge perceived by an electron from its orbital. Applies most often to valence electrons and influences periodic properties such as atomic radius and ionization energy.

Electrochemical cell A cell within which a redox reaction takes place, containing two electrodes between which there is an electrical potential difference. See electrolytic cell; galvanic cell.

Electrode An electrical conductor through which an electric current enters or leaves a medium.

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.