1,800+ MCAT terms, defined.

Electron affinity is the energy released when a neutral gaseous atom gains one electron to form an anion. It generally increases across a period and decreases down a group, with exceptions. Halogens have the largest values, so they readi...

Cell potential (E°cell, or emf) is the voltage an electrochemical cell can produce, found from the cathode's and anode's reduction potentials. A positive E°cell usually means a spontaneous galvanic cell; a negative value means the reacti...

A covalent bond forms when two atoms share electron pairs, typically between nonmetals. The most testable point: equal sharing gives a nonpolar bond, unequal sharing (an electronegativity difference) gives a polar bond, and more shared p...

Le Châtelier's principle says a system at equilibrium that is disturbed shifts in the direction that partly offsets the disturbance. The most testable point: concentration and pressure changes shift the position but not K, while a temper...

A galvanic (voltaic) cell uses a spontaneous redox reaction (E°cell > 0, ΔG° < 0) to produce electrical energy. Oxidation occurs at the anode (negative) and reduction at the cathode (positive), so electrons flow anode → cathode through t...

The reaction quotient Q has the same form as the equilibrium constant expression but uses the concentrations or pressures present at any moment. Compare Q with K: Q < K, the reaction proceeds forward; Q > K, in reverse; Q = K, at equilib...

A weak acid only partially ionizes in water, so HA ⇌ H⁺ + A⁻ lies mostly toward HA and Ka is small (Ka < 1). The most testable point: a weaker acid has a smaller Ka, a larger pKa, and a stronger conjugate base.

Formal charge is the charge an atom would have in a Lewis structure if every bonding pair were shared equally. The best Lewis structure usually has formal charges closest to zero, with any negative charge on the more electronegative atom.

Graham's law says that at the same temperature, a gas's effusion rate (escape through a tiny hole) is inversely proportional to the square root of its molar mass, so lighter gases effuse faster. It is also applied, approximately, to diff...

A first-order reaction has a rate proportional to the concentration of one reactant raised to the first power (rate = k[A]), so doubling [A] doubles the rate. Its most testable features are a constant half-life, t½ = 0.693/k, and a strai...

An ideal gas is a model in which particles have negligible volume and no attractions or repulsions, so PV = nRT applies. Real gases behave most ideally at high temperature and low pressure, and small nonpolar gases such as He come closest.

A rate law is an experimentally determined equation relating a reaction's rate to reactant concentrations raised to powers (orders): rate = k[A]^m[B]^n. The orders come from experiment, not from the balanced equation, except for a single...

A photon is a quantum (packet) of electromagnetic radiation with energy E = hf = hc/λ. Energy rises with frequency and falls with wavelength, and it does not depend on the light's intensity, which only sets how many photons arrive.

An oxidation number is the charge an atom would have if all bonding electrons went to the more electronegative atom. It tracks electron transfer in redox reactions. Oxidation is an increase in oxidation number and reduction is a decrease.

Mass number (A) is the total number of protons plus neutrons in a nucleus. It is a whole number that identifies an isotope, and isotopes of one element share the same atomic number Z but have different mass numbers.

The rate-determining step is the slowest elementary step in a multistep mechanism, and it limits how fast the overall reaction goes. The overall rate law follows the molecularity of this step, with any intermediates replaced by the react...

A non-spontaneous process has ΔG > 0 under the given conditions, so it will not proceed on its own and needs an energy input or coupling to a favorable process. Its reverse is spontaneous, and ΔG says nothing about how fast either direct...

A cation is a positively charged ion, formed when an atom loses electrons. It is smaller than its parent atom, and in both galvanic and electrolytic cells cations migrate toward the cathode.

Keq is the equilibrium constant: the ratio of product to reactant concentrations (each raised to its coefficient) once forward and reverse rates are equal. It depends only on temperature. Keq > 1 favors products; Keq < 1 favors reactants.

Reduction potential (E°red) is a measure, in volts, of how strongly a species tends to gain electrons, compared with the standard hydrogen electrode (0 V). The more positive it is, the stronger the oxidizing agent. In a galvanic cell, th...

Molarity (M) is the moles of solute per liter of total solution (mol/L). Because solution volume changes with temperature, molarity does too, while molality does not. Diluting keeps moles constant: M₁V₁ = M₂V₂.

Internal energy (U) is the total kinetic and potential energy of a system's particles, and it is a state function. Its change follows the first law, ΔU = q + w (w is work on the system; physics texts write Q − W with W by the system). Fo...

A second-order reaction has a rate law whose concentration exponents add to 2, such as rate = k[A]² or rate = k[A][B]. Most testable: for rate = k[A]², a plot of 1/[A] versus time is a straight line with slope k, and k has units M⁻¹s⁻¹.

Standard reduction potential (E°red) is the voltage of a reduction half-reaction under standard conditions (1 M, 1 atm, 25 °C), measured relative to the standard hydrogen electrode, defined as 0 V. The more positive it is, the stronger t...

An anion is an atom or group of atoms that has gained one or more electrons, giving it a net negative charge. Anions are larger than their parent atoms, and in electrochemical cells they migrate toward the anode.

Dalton's law states that the total pressure of a gas mixture equals the sum of the partial pressures of its components, each being the pressure that gas would exert alone in the same container. Most testable: partial pressure = mole frac...

Vapor pressure is the pressure of a vapor in dynamic equilibrium with its liquid (or solid) in a closed container, where evaporation and condensation rates are equal. It rises with temperature, and a liquid boils when its vapor pressure...

An energy level is one of the specific, quantized energies a bound electron in an atom can have. Electrons move between levels by absorbing or emitting a photon with energy equal to the gap, ΔE = hν. Higher n means higher energy, and the...

The limiting reagent is the reactant that is completely used up first, so it sets the maximum amount of product (the theoretical yield). To find it, divide each reactant's moles by its balanced-equation coefficient; the smallest quotient...

An exothermic process releases heat from the system to the surroundings, so the system's enthalpy falls and ΔH is negative. The most testable point is that exothermic describes heat flow, not spontaneity: ΔG also depends on TΔS, so an ex...

A spontaneous process is thermodynamically favorable: it proceeds forward from the given conditions without continuous outside work. At constant T and P, this means ΔG < 0. Spontaneous does not mean fast, because rate is set by kinetics.

A galvanic (voltaic) cell uses a spontaneous redox reaction to produce electrical energy. It has E°cell > 0 and ΔG < 0, with oxidation at the anode, reduction at the cathode, and electrons flowing through the wire from anode to cathode.

The speed of light, c, is the speed of electromagnetic waves in a vacuum, 3.00 × 10⁸ m/s, the same for every wavelength. The key use is c = fλ: frequency and wavelength are inversely related, so shorter wavelength means higher frequency...

Mole fraction (X) is the moles of one component divided by the total moles of all components in a mixture. It is unitless, and the mole fractions in a mixture sum to 1. The most testable use is that it sets partial pressure (Dalton's law...

In collision theory, reactant particles must collide to react, but only collisions with energy of at least the activation energy (Ea) and a suitable orientation form products. Rate tracks the frequency of these effective collisions. High...

Ka is the acid dissociation constant: the equilibrium constant for HA + H₂O ⇌ H₃O⁺ + A⁻. A larger Ka (smaller pKa) means a stronger acid that dissociates more, so Ka and pKa move in opposite directions.

The half-equivalence point is where half of a weak acid (or weak base) in a titration has been neutralized, so [HA] = [A⁻]. There pH = pKa (for a weak base, pOH = pKb), at the midpoint of the buffer region.

Reaction order for a reactant is the exponent of its concentration in the experimentally determined rate law; the overall order is the sum of these exponents. It is found from data, such as initial rates, and matches the balanced-equatio...

Calorimetry is the measurement of heat transferred in a process by tracking the temperature change of a known mass of material, usually water, in an insulated container. Its key point is that, ideally, heat released by the system equals...

Heat capacity is the heat a sample must absorb to raise its temperature by 1 K (or 1 °C), C = q/ΔT. A larger value means a smaller temperature change for the same heat. It depends on sample size; specific heat is the per-mass version.

The atomic number (Z) is the number of protons in an atom's nucleus, and it defines the element. A neutral atom has Z electrons. The most testable point is that isotopes share Z but differ in neutron number, and only nuclear reactions ch...

Freezing point depression is the lowering of a solvent's freezing point when a solute is dissolved in it. It is a colligative property: it depends on the number of dissolved particles (i × molality), not their identity, so 0.1 m CaCl₂ lo...

Spin is an intrinsic property of the electron, described by the spin quantum number ms, which is either +½ or −½. By the Pauli exclusion principle, two electrons in one orbital must have opposite spins, and atoms with unpaired spins are...

An endothermic process absorbs heat from its surroundings, so the system's enthalpy rises (ΔH > 0) and the surroundings cool. Most testable point: raising the temperature shifts an endothermic equilibrium toward products, and K increases.

Effective nuclear charge (Zeff) is the net positive charge a valence electron feels after inner electrons partly shield it from the full nuclear charge. It rises left to right across a period and changes little down a group, which explai...

A weak base accepts a proton from water only partly (B + H₂O ⇌ BH⁺ + OH⁻), so it has a small Kb and most of it stays unprotonated at equilibrium. Most testable: a smaller Kb (larger pKb) means a weaker base and a stronger conjugate acid,...

An electrolytic cell uses an external power source to force a nonspontaneous redox reaction (ΔG > 0, E°cell < 0). Oxidation is still at the anode and reduction at the cathode, but the anode is positive, the opposite of a galvanic cell.

A concentration cell is a galvanic cell with identical electrodes and the same ion in both half-cells at different concentrations, so E° = 0 and the voltage comes from the Nernst equation. The dilute side is the anode, the concentrated s...

A conjugate base is what remains when a Brønsted–Lowry acid donates a proton (HA → H⁺ + A⁻). The stronger the acid, the weaker its conjugate base, and Ka × Kb = Kw for a conjugate pair.

The Bohr model pictures the hydrogen electron in circular orbits with fixed, quantized energies. Light is absorbed or emitted when the electron jumps between levels, and the photon energy equals the energy gap. It fits one-electron speci...