AP Chemistry Unit 5 Review: Kinetics
Every Unit 5 topic, what's actually tested, and what the exam explicitly excludes.
Unit 5 asks a different question than the units before it: not whether a reaction happens, but how fast. It's 7–9% of the multiple-choice section across 11 topics and 13–14 class periods, and it introduces the graph-reading and mechanism-analysis skills the AP Exam leans on heavily in free response. Here's what's actually in it, topic by topic, verified against the current CED -- including two exclusion statements that narrow the math you actually need to know.
Unit 5 at a Glance
Unit 5 (Kinetics) builds on the chemical reactions from Unit 4 by asking how quickly they proceed. See the full AP Chemistry units breakdown for how it fits alongside the other 8 units, and the Course and Exam Description for the full framework.
Reaction Rates
Reaction rate is how much reactant converts to product per unit time. The rates of change of individual reactants and products are set by the balanced equation's stoichiometry, and the overall rate is influenced by concentration, temperature, surface area, and catalysts.
Introduction to Rate Law
The rate law expresses rate as proportional to each reactant's concentration raised to a power: rate = k[A]m[B]n. The exponent for each reactant is that reactant's order; the sum of all exponents is the overall reaction order. The proportionality constant k is temperature-dependent, and its units depend on the overall order.
Critical rule: reaction orders (m and n) must be determined from experimental data -- comparing initial rates as concentrations change -- never read off the coefficients of the overall balanced equation. Coefficients only match rate-law exponents for a single elementary step, not for an overall multistep reaction.
Worked example: Using the method of initial rates for A + B → C:
| Trial | [A] (M) | [B] (M) | Initial Rate (M/s) |
|---|---|---|---|
| 1 | 0.10 | 0.10 | 2.0 × 10⁻₃ |
| 2 | 0.20 | 0.10 | 8.0 × 10⁻₃ |
| 3 | 0.10 | 0.20 | 2.0 × 10⁻₃ |
Comparing Trials 1 and 2: doubling [A] quadruples the rate (2² = 4), so the reaction is second order in A. Comparing Trials 1 and 3: doubling [B] has no effect on rate, so the reaction is zero order in B. Rate law: rate = k[A]². Solving for k using Trial 1: 2.0 × 10⁻₃ = k(0.10)², so k = 0.20 M⁻¹s⁻¹.
Concentration Changes Over Time
Each reaction order produces a different linear plot: zero order gives a straight line for [A] vs. time; first order gives a straight line for ln[A] vs. time; second order gives a straight line for 1/[A] vs. time. Whichever plot is linear tells you the order, and its slope gives you k.
Worked example: A first-order reaction has a rate constant k = 0.0231 s⁻¹. Using t1/2 = 0.693/k: t1/2 = 0.693 / 0.0231 s⁻¹ = 30.0 s. This half-life formula only applies to first-order reactions -- half-life is not constant for zero- or second-order reactions, since it depends on starting concentration for those orders.
Elementary Reactions
For a single elementary (one-step) reaction, the rate law's exponents do match the reaction's stoichiometric coefficients, since the rate law follows directly from the stoichiometry of the colliding particles. Elementary reactions requiring three or more particles to collide simultaneously are rare -- termolecular steps are uncommon in real mechanisms.
Collision Model
A collision only produces product if it has both sufficient energy to overcome the activation energy and the correct orientation for bonds to rearrange. The Maxwell-Boltzmann distribution describes how particle energies spread at a given temperature, and it's the basis for why raising temperature increases reaction rate: a larger fraction of collisions exceed the activation energy threshold.
Reaction Energy Profile
A reaction energy profile plots energy along the reaction coordinate, from reactants through a transition state to products. The energy difference between reactants and the transition state is the activation energy for the forward reaction.
Not tested: the Arrhenius equation is on the required equation sheet for its qualitative relationship between rate, temperature, and activation energy -- but calculations using it are explicitly excluded from the AP Exam.
Introduction to Reaction Mechanisms
A mechanism is a sequence of elementary steps whose sum equals the overall balanced equation. Components include reactants, products, catalysts, and intermediates -- species produced by an early step and fully consumed by a later one, so they never appear in the overall equation.
Not tested: collecting or interpreting experimental data used to detect a reaction intermediate is explicitly excluded. You need to identify intermediates from a given mechanism, not design an experiment to find one.
Reaction Mechanism and Rate Law
When the first step of a mechanism is rate-limiting (or every step is irreversible), the overall rate law is set directly by the molecularity of that slowest step -- the rate-determining step.
Pre-Equilibrium Approximation
When the first step is not rate-limiting -- typically because it's a fast, reversible step followed by a slower second step -- you need the pre-equilibrium approximation to derive the correct overall rate law instead of just reading it off the rate-determining step.
Multistep Reaction Energy Profile
Knowing the energetics of each elementary step in a mechanism lets you construct the full energy profile for the overall multistep reaction -- each step contributes its own hump, with intermediates sitting in the valleys between them.
Catalysis
A catalyst speeds up a reaction by increasing the number of effective collisions and/or opening a pathway with lower activation energy. Its concentration stays constant overall -- it's consumed in one step and regenerated in a later one. Three catalytic mechanisms appear on the exam: enzymes (bind the reactant, forming a new intermediate), acid-base catalysis (covalent proton transfer creates a new intermediate), and surface catalysis (a reactant binds to a solid surface, again forming a new bound intermediate).
Common Mistakes in Unit 5
- Reading reaction order straight from the balanced equation. Order comes from experimental data, not stoichiometric coefficients -- except for a single elementary step, where they do match.
- Assuming half-life is constant for every reaction order. t1/2 = 0.693/k is a first-order-only formula; zero- and second-order half-lives depend on starting concentration.
- Confusing an intermediate with a catalyst. An intermediate is produced then consumed (appears in the middle of a mechanism); a catalyst is consumed then regenerated (appears at the start and reappears at the end).
- Writing the rate law from a slow step that isn't actually first. If a fast, reversible step precedes the slow step, you need the pre-equilibrium approximation -- not a direct read of the rate-determining step's molecularity.
- Thinking a catalyst changes the overall energy released or absorbed. A catalyst only changes the activation energy pathway; the net enthalpy difference between reactants and products stays exactly the same.
How Unit 5 Connects to the Rest of the Course
- The activation energy and energy-profile reasoning here is the same framework Unit 2's potential-energy-vs-distance curves introduced for bond formation.
- The rate-determining-step logic resurfaces directly in Unit 9's electrochemistry, where multi-step electron transfer follows the same rate-limiting-step reasoning.
- Understanding reversible elementary steps here is the direct setup for Unit 7's full treatment of chemical equilibrium.
Related Resources
- AP Chemistry Units (All 9)
- AP Chemistry Unit 1 Review
- AP Chemistry Unit 2 Review
- AP Chemistry Unit 3 Review
- AP Chemistry Unit 4 Review
- AP Chemistry Unit 8 Review
- AP Chemistry Course and Exam Description
- AP Chemistry FRQ Calculator
- AP Chemistry Unit 9 Review
Frequently Asked Questions
What topics are in AP Chemistry Unit 5?
Reaction Rates, Introduction to Rate Law, Concentration Changes Over Time, Elementary Reactions, Collision Model, Reaction Energy Profile, Introduction to Reaction Mechanisms, Reaction Mechanism and Rate Law, Pre-Equilibrium Approximation, Multistep Reaction Energy Profile, and Catalysis -- 11 topics in total.
How much is Unit 5 worth on the AP Chemistry exam?
Seven to nine percent of the multiple-choice section across roughly 13 to 14 class periods.
Can you determine reaction order from a balanced chemical equation?
No. Reaction order must come from experimental data -- initial rates, or a concentration-vs-time graph -- never from the coefficients in a balanced equation. Only for a single elementary step does the rate law's exponents match the stoichiometric coefficients.
Does AP Chemistry require calculations with the Arrhenius equation?
No. Calculations involving the Arrhenius equation are explicitly excluded. You need the qualitative relationship -- higher temperature and lower activation energy both increase rate -- not the numerical form of the equation.
Does a catalyst change the overall energy released or absorbed by a reaction?
No. A catalyst only lowers the activation energy (or increases the frequency of effective collisions) by providing an alternative pathway -- it never changes the net enthalpy difference between reactants and products.
Sourced from College Board's official AP Chemistry Course and Exam Description, Effective Fall 2024. This page describes the document's real content and current exclusion statements; it is not a copy of it and is not affiliated with or endorsed by College Board.