Chemistry · College Board CED Unit 5: Kinetics · 16 min read · Updated 2026-05-11
Kinetics — AP Chemistry
AP Chemistry · College Board CED Unit 5: Kinetics · 16 min read
1. Reaction Rate Definition & Measurement★★☆☆☆⏱ 3 min
Kinetics is the study of the speed of chemical reactions, factors that alter reaction rate, and the step-by-step pathways reactants follow to form products. It accounts for 7-9% of your total AP Chemistry exam score, tested in both multiple-choice and free-response sections.
Four common rate measurement methods frequently tested on the AP exam: pressure tracking for gas-phase reactions, colorimetry for reactions with colored species, titration of reaction aliquots, and mass change for reactions producing solids or gaseous products.
2. Rate Laws and Reaction Orders★★★☆☆⏱ 4 min
A rate law is an experimentally determined equation that relates reaction rate to reactant concentrations, with the general form:
rate = k[A]^m[B]^n
Where $k$ = temperature-dependent rate constant, $m$ = reaction order with respect to $A$, $n$ = reaction order with respect to $B$, and overall reaction order = $m + n$. **Critical note**: reaction orders are *not* equal to the stoichiometric coefficients of the overall reaction, and can only be determined from experimental data or the rate-determining step of a mechanism.
To determine orders from initial rate data, compare two trials where only one reactant concentration changes, and calculate how the rate scales with that concentration change.
3. Integrated Rate Equations and Half-Life★★★☆☆⏱ 4 min
Integrated rate laws relate reactant concentration to elapsed time, so you can calculate how much reactant remains after a given time, or how long it takes for concentration to drop to a target value. The three key integrated rate laws tested on the AP exam are:
**Zero order**: $[A]_t = -kt + [A]_0$, linear plot of $[A]$ vs $t$, slope = $-k$, half-life $t_{1/2} = \frac{[A]_0}{2k}$ (half-life increases with initial concentration)
**First order**: $\ln[A]_t = -kt + \ln[A]_0$, linear plot of $\ln[A]$ vs $t$, slope = $-k$, half-life $t_{1/2} = \frac{\ln 2}{k} \approx \frac{0.693}{k}$ (half-life independent of initial concentration, a frequently tested property)
**Second order**: $\frac{1}{[A]_t} = kt + \frac{1}{[A]_0}$, linear plot of $\frac{1}{[A]}$ vs $t$, slope = $k$, half-life $t_{1/2} = \frac{1}{k[A]_0}$ (half-life decreases with initial concentration)
4. Activation Energy and the Arrhenius Equation★★★★☆⏱ 3 min
The Arrhenius equation relates the rate constant $k$ to absolute temperature $T$ (Kelvin) and $E_a$:
k = A e^{-\frac{E_a}{RT}}
Where $A$ = frequency factor (accounts for collision frequency and correct orientation), and $R = 8.314 J \cdot mol^{-1} \cdot K^{-1}$ (always use this value for kinetics calculations, not the $0.0821 L \cdot atm \cdot mol^{-1} \cdot K^{-1}$ value used for gas laws). For calculations with two $k$ values at different temperatures, use the rearranged two-point form:
5. Reaction Mechanisms and Intermediates★★★★☆⏱ 3 min
A reaction mechanism is the sequence of elementary (single-collision) steps that make up an overall reaction. Unlike the overall reaction, the rate law of an elementary step is directly determined by its stoichiometry: unimolecular steps (1 reactant) are first order, bimolecular steps (2 reactants) are second order, and rare termolecular steps (3 reactants) are third order.
The rate-determining step (RDS) is the slowest elementary step, so its rate law matches the experimentally determined overall rate law. For a mechanism to be valid: 1) the sum of elementary steps equals the overall reaction, and 2) the rate law of the RDS matches the experimental rate law.
Common Pitfalls
Why: Students confuse overall reactions with elementary steps, and incorrectly assume order equals stoichiometric coefficient.
Why: Overgeneralization of the R value used for gas law problems, leading to unit mismatch.
Why: Most AP kinetics problems test first-order reactions, leading to overgeneralization of this property.
Why: Students forget intermediates are fully consumed before the reaction completes.
Why: Rushing through problems and skipping required unit conversion.