AP Chemistry · Unit 5 of 9

Kinetics

Model reaction rates with data, rate laws, and mechanism-level reasoning about collisions and activation energy.

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Why this unit matters

Kinetics explains how fast processes occur and which variables control speed, a core part of scientific prediction.

What you will learn

  • Determine reaction order and rate law from initial-rate data.
  • Calculate rate constants and use integrated-rate relationships for simple systems.
  • Explain how temperature, concentration, and catalysts influence rate through collision theory and activation energy.

Understand the core ideas

Kinetics asks how fast a reaction proceeds and what variables control that speed. Collision theory gives the microscopic picture: reacting particles must collide with enough energy and proper orientation to form products. Concentration increases collision frequency, temperature changes the fraction of particles above activation energy, and catalysts lower pathway barriers without changing overall reaction stoichiometry. AP Chemistry defines rate as concentration change per unit time, so sign conventions matter: reactant concentration decreases while product concentration increases. Interpreting concentration-time graphs requires attention to slope, not just endpoint values, because slope magnitude indicates instantaneous rate. This chapter also links particle-level reasoning to measurable data, which helps explain why two reactions with similar equations can proceed at very different speeds under different conditions. Strong answers connect observed rate trends directly to collision frequency and activation-energy distribution, instead of listing factors without mechanistic explanation. It also helps to compare rate changes using explicit numeric factors.

Rate laws are empirical relationships determined by experiment, not read from coefficients of overall balanced equations. Initial-rate comparisons isolate one reactant at a time so reaction order can be inferred from multiplicative changes in rate when concentration is doubled or tripled. After exponents are established, the rate constant k is solved from any consistent trial and checked against other trials for agreement. Units of k depend on overall order, so correct dimensional analysis is part of the solution, not an optional add-on. Proposed mechanisms are evaluated by comparing predicted slow-step rate law to observed data and by checking whether summed elementary steps reproduce the net equation. This chapter rewards careful trial selection, explicit ratio statements, and numerical checks that prevent arithmetic drift. If two different trial pairs imply different orders, that inconsistency signals either a calculation error or data that does not support the proposed law. Clear table annotation helps keep comparisons accurate under exam timing.

Key terms

rate law
Equation relating reaction rate to reactant concentrations raised to experimentally determined orders.
rate constant
Proportionality constant k in a rate law, dependent mainly on temperature and catalyst presence.
activation energy
Minimum energy barrier that reacting particles must overcome to form products.
reaction order
Exponent of a reactant concentration term in the rate law, indicating sensitivity of rate to that reactant.

Determine rate law exponents from initial rates

Suppose data at constant temperature: Trial 1 has [A]=0.10 M, [B]=0.10 M, rate=0.020 M/s. Trial 2 has [A]=0.20 M, [B]=0.10 M, rate=0.080 M/s. Trial 3 has [A]=0.20 M, [B]=0.20 M, rate=0.080 M/s. Assume rate =k[A]m[B]n= k[A]^m[B]^n.

  1. 1) Compare Trials 1 to 2: [A] doubles while [B] constant, rate changes 0.020 to 0.080, a factor of 4, so 2m=42^m = 4 and m = 2.
  2. 2) Compare Trials 2 to 3: [B] doubles while [A] constant, rate unchanged at 0.080, so 2n=12^n = 1 and n = 0.
  3. 3) Write rate law: rate =k[A]2[B]0=k[A]2= k[A]^2[B]^0 = k[A]^2.
  4. 4) Solve k using Trial 1: k = rate/[A]2=0.020/(0.10)2=2.0M−1s−1/[A]^2 = 0.020/(0.10)^2 = 2.0 M^-1 s^-1.
Result: A consistent law is rate =(2.0M−1s−1)[A]2= (2.0 M^-1 s^-1)[A]^2, second order in A and zero order in B.

A common misconception

Claim: Reaction orders must match stoichiometric coefficients from the balanced equation.

Correction: For overall reactions, orders come from experiment. Coefficients determine mole ratios, but they do not by themselves determine kinetic exponents unless the step is elementary and identified as such.

Lessons in this unit

  1. Rates from concentration-time dataExtract average and instantaneous rates with correct sign and units.
  2. Rate laws from experimentsInfer reaction orders using controlled concentration changes.
  3. Mechanisms and elementary stepsRelate proposed mechanisms to observed rate laws and intermediates.
  4. Temperature effects and Arrhenius modelConnect activation energy changes to rate constant behavior.

Study task

Use a three-trial initial-rate table to determine a rate law, solve for k, and evaluate whether a proposed mechanism is consistent.

Unit checkpoint

If doubling [A] quadruples the rate while doubling [B] leaves rate unchanged, what rate law form fits the data?

The reaction is second order in A and zero order in B, so rate =k[A]2= k[A]^2.

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