AP Chemistry Glossary: Key Terms with FRQ-Ready Definitions
Every term below is defined the way the AP Chemistry rubric wants it, with a short note on how it typically appears on the FRQ. Learn the definitions the way you'll be asked to use them.
Index
- Activation Energy (Eₐ)
- Buffer
- Gibbs Free Energy (ΔG)
- Electronegativity
- Equilibrium (K, Q)
- Hydrogen Bonding
- ICE Table
- Intermolecular Forces (IMFs)
- Le Châtelier's Principle
- pH
- Stoichiometry
- Titration
Activation Energy (Eₐ)
The minimum energy that colliding particles must have for a reaction to occur. Catalysts lower Eₐ by providing an alternative pathway; they do not change ΔH or the equilibrium position.
On the FRQ: "A catalyst speeds up the reaction" earns partial credit. Full credit names the alternative pathway and the lower Eₐ.
Buffer
A solution that resists changes in pH when small amounts of acid or base are added. Composed of a weak acid and its conjugate base (or a weak base and its conjugate acid) in roughly equal concentrations.
On the FRQ: Justify buffering by naming both components and explaining that the conjugate base neutralizes added H⁺ while the weak acid neutralizes added OH⁻.
Gibbs Free Energy (ΔG)
The thermodynamic potential that determines reaction spontaneity: ΔG = ΔH − TΔS. Negative ΔG means the reaction is thermodynamically favored; positive ΔG means it is not.
On the FRQ: Never write "spontaneous" without stating the sign of ΔG and the values or signs of ΔH and ΔS that produced it.
Electronegativity
The tendency of an atom in a bond to attract shared electrons. Increases across a period and up a group. Differences in electronegativity determine bond polarity and, in turn, molecular polarity and IMFs.
On the FRQ: Justify bond polarity by comparing the two atoms' electronegativities, not by intuition about "which one is more electron-hungry."
Equilibrium (K, Q)
The state at which forward and reverse reaction rates are equal. K is the equilibrium constant; Q is the reaction quotient at any moment. If Q < K the reaction shifts right; if Q > K it shifts left.
On the FRQ: Predict equilibrium shifts by computing Q, comparing to K, and citing LeChatelier — all three, not just the last.
Hydrogen Bonding
An especially strong dipole-dipole attraction between an H atom bonded to N, O, or F and a lone pair on N, O, or F in another molecule. Responsible for water's unusually high boiling point and surface tension.
On the FRQ: Say "hydrogen bond" by name. "Strong dipole-dipole" is not the same and often loses the point.
ICE Table
A tool for solving equilibrium problems: Initial concentrations, Change in concentration, Equilibrium concentration. Substitute into the equilibrium expression to solve for K or a missing concentration.
On the FRQ: Show the ICE table on the FRQ — it's usually a setup point regardless of whether your final answer is correct.
Intermolecular Forces (IMFs)
Attractive forces between molecules: London dispersion (all molecules), dipole-dipole (polar molecules), and hydrogen bonding (with N-H, O-H, F-H). Strength: H-bonding > dipole-dipole > LDF, though large nonpolar molecules can have strong LDFs.
On the FRQ: Justify boiling point differences by naming the specific IMFs present, in order of strength — not by "more polar" or "bigger molecule."
Le Châtelier's Principle
A system at equilibrium disturbed by a change in concentration, pressure/volume, or temperature will shift to partially counteract the disturbance and re-establish equilibrium.
On the FRQ: State the disturbance, the direction of the shift, and why. All three are required for full credit.
pH
The negative logarithm of hydrogen-ion concentration: pH = −log[H⁺]. Pure water at 25 °C has pH 7. A change of 1 pH unit is a 10× change in [H⁺].
On the FRQ: Calculate pH from [H⁺] and vice versa; don't confuse pH with pOH or Ka with pKa.
Stoichiometry
The quantitative relationships between reactants and products in a chemical reaction, derived from a balanced equation. Uses mole ratios to convert between amounts of substances.
On the FRQ: Always balance the equation before doing stoichiometry. An unbalanced setup usually loses all downstream points.
Titration
A technique for determining the concentration of an unknown solution by reacting it with a solution of known concentration. Equivalence point is where moles of acid = moles of base.
On the FRQ: On titration-curve FRQs, name the region (buffer, equivalence, past-equivalence) and cite what dominates the pH there.
Keep going
Definitions that earn points.
Cramapple's per-criterion grading tells you when your definition earns the rubric point and when it doesn't.