AP Chemistry Unit 5: Kinetics
Study reaction rates, rate laws, activation energy, reaction mechanisms, catalysts with exam-format practice and rubric-based scoring.
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Inside This Unit: The Full Breakdown
Kinetics studies how fast reactions occur and what factors influence reaction rates. This unit covers rate laws, reaction mechanisms, activation energy, and catalysis — the tools for understanding and controlling reaction speed.
Why it matters
Kinetics is a major topic on the AP Chemistry exam. You must determine rate laws from experimental data, draw and interpret energy diagrams, propose mechanisms, and explain how catalysts work. These skills are tested in both multiple choice and free response.
Key concepts
- Reaction rate depends on concentration, temperature, surface area, and catalysts. Rate = k[A]ᵐ[B]ⁿ, where m and n are determined experimentally.
- The rate-determining step is the slowest step in a multi-step mechanism and controls the overall reaction rate.
- The Arrhenius equation (k = Ae^(-Ea/RT)) shows that rate constants increase with temperature and decrease with higher activation energy.
- Catalysts lower the activation energy by providing an alternative reaction pathway without being consumed.
Rate Laws and Reaction Order
The rate of a reaction measures how quickly reactant concentrations decrease or product concentrations increase over time. The rate law, rate = k[A]ᵐ[B]ⁿ, expresses this relationship mathematically. The exponents m and n are the reaction orders with respect to each reactant and must be determined experimentally — they cannot be read from the balanced equation. If doubling [A] doubles the rate, the reaction is first order in A (m = 1). If doubling [A] quadruples the rate, it is second order (m = 2). If doubling [A] has no effect, it is zero order (m = 0). The overall reaction order is m + n. Integrated rate laws relate concentration to time: first-order reactions give a straight line when ln[A] is plotted vs. time; second-order reactions give a straight line for 1/[A] vs. time.
Reaction Mechanisms
Most reactions occur through a series of elementary steps called a reaction mechanism. Each elementary step has a rate law that can be written directly from its stoichiometry (unlike the overall reaction). The rate-determining step (RDS) is the slowest step and limits the overall reaction rate. A valid mechanism must satisfy two criteria: the elementary steps must sum to the overall balanced equation, and the rate law derived from the mechanism must match the experimentally observed rate law. Reaction intermediates are species produced in one step and consumed in a later step — they appear in the mechanism but not in the overall equation. Catalysts appear in early steps and are regenerated in later steps.
Activation Energy and Catalysis
Every reaction requires a minimum energy input called the activation energy (Ea) to break bonds in the reactants and reach the transition state. Energy diagrams (potential energy vs. reaction progress) show Ea as the energy difference between reactants and the transition state peak. Exothermic reactions have products lower in energy than reactants; endothermic reactions have products higher. The Arrhenius equation (k = Ae^(-Ea/RT)) quantifies the relationship between temperature and rate constant. Plotting ln(k) vs. 1/T gives a straight line with slope -Ea/R. Catalysts increase reaction rate by providing an alternative pathway with lower Ea. Enzymes are biological catalysts that are highly specific to their substrates.
AP exam tip
When determining rate laws from experimental data on the AP exam, use the method of initial rates: compare pairs of experiments where only one reactant concentration changes. The ratio of rates equals the concentration ratio raised to the reaction order for that reactant.
Connections to other units
- Unit 4 (Chemical Reactions): Balanced equations provide the stoichiometric basis, but kinetics determines how fast those reactions actually proceed.
- Unit 7 (Equilibrium): Equilibrium is reached when forward and reverse reaction rates become equal.
- Unit 6 (Thermodynamics): Thermodynamics tells you whether a reaction is favorable; kinetics tells you how fast it gets there.
