Unit 5: Kinetics
Practice AP Chemistry Unit 5 kinetics — rate laws, integrated rate laws, Arrhenius equation, reaction mechanisms, and catalysis. AP-style FRQ and MCQ prep.
What Unit 5 Covers in AP Chemistry
Kinetics is one of the most FRQ-heavy units in AP Chemistry. The College Board-style questions here require both quantitative calculation and conceptual reasoning about how and why reaction rates change. Unit 5 rewards students who can move fluently between data, equations, and molecular-level explanations.
Core Topics in Unit 5
- Rate laws — Writing rate laws from experimental data, determining reaction orders for each reactant, and calculating the rate constant k
- Reaction order and overall order — First, second, and zero-order reactions; how rate depends on concentration for each order
- Integrated rate laws — Using concentration-vs-time data to determine reaction order; applying integrated rate law equations to find concentration at a given time
- Half-life — Calculating half-life for first-order reactions and understanding how half-life varies with concentration for other orders
- Arrhenius equation — Relating temperature to rate constant; calculating activation energy from two temperature-rate data points
- Reaction mechanisms — Identifying the rate-determining step, writing rate laws from mechanisms, and validating mechanisms against experimental rate laws
- Catalysis — Homogeneous and heterogeneous catalysis; how catalysts lower activation energy without being consumed
Determining Rate Laws from Data
A foundational AP Chemistry skill in Unit 5 is using a data table of initial concentrations and initial rates to determine the rate law. The method of initial rates requires comparing experiments where one reactant concentration changes while others stay constant. Practice this calculation until it is automatic — it appears in nearly every AP Chemistry exam in some form.
Analysing Reaction Mechanisms in FRQs
FRQs on mechanisms ask you to identify the rate-determining step, write the overall reaction from elementary steps, and confirm whether a proposed mechanism is consistent with the observed rate law. Remember: the rate law for an elementary step can be written directly from its stoichiometry, but the overall rate law cannot.
Common Mistakes in Unit 5
- Writing the rate law from the overall balanced equation rather than from experimental data or the rate-determining step
- Assuming all reactions are first order — always verify order from data before applying integrated rate laws
- Confusing activation energy with the enthalpy of reaction — they are related but not the same
- Forgetting that a catalyst appears in the mechanism but is regenerated and does not appear in the overall rate law as a reactant