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AP Chemistry Unit 5: Kinetics

7 to 9% of the exam · 180 practice questions · 5 free-response questions

Kinetics asks how fast reactions happen and why. You will write rate laws from experimental data, use integrated rate laws and half-life, and connect collisions and activation energy to reaction speed. The exam also asks you to match a proposed mechanism to a rate law, find intermediates and catalysts, and identify the rate-determining step. Students often misread concentration versus time plots when finding reaction order. Another frequent error is writing a rate law from the coefficients of the overall equation instead of from data or the slow step.

Practice this unit

White belt · 60 questionsRegular · 60 questionsBlack belt · 60 questionsFree response

White belt covers core terms and foundational ideas, Regular is set at exam level, and Black belt has harder multi-step questions.

Topics in this unit

Free-response questions

Sample questions

These are taken from the Regular set, with the answer and explanation shown.

Topic 5.2 Introduction to Rate Law

1. For a reaction involving A, increasing [A] from 0.20 M to 0.30 M, with everything else constant, raises the initial rate from 4.0 × 10⁻³ M/s to 9.0 × 10⁻³ M/s. What is the order of the reaction with respect to A?

  1. AZero order
  2. BFirst order
  3. CSecond order
  4. DThird order

Answer: C. The concentration factor is 0.30 ÷ 0.20 = 1.5. The rate factor is 9.0 ÷ 4.0 = 2.25. Since 1.5² = 2.25, the order is 2.

Common trap: Students used to doubling look for a factor of 4 and, not finding it, guess first order. Compare 2.25 with 1.5ⁿ instead.

Topic 5.4 Elementary Reactions

2. For which of the following can the rate law be written directly from its balanced equation, without experimental data?

  1. AThe overall reaction 2 N₂O₅ → 4 NO₂ + O₂
  2. BThe elementary step O₃ + O → 2 O₂
  3. CThe overall reaction 2 NO₂ + F₂ → 2 NO₂F
  4. DThe overall reaction 2 H₂ + O₂ → 2 H₂O

Answer: B. Only an elementary step shows the actual collision, so its coefficients give its rate law: Rate = k[O₃][O]. An overall reaction may happen through several steps, so its rate law must be measured.

Common trap: Students often treat every equation as elementary. The word "overall" is a warning that the coefficients may not match the orders.

Topic 5.10 Multistep Reaction Energy Profile

3. On a two-step energy profile, the reactants are at 40 kJ/mol, the first peak is at 115 kJ/mol, the intermediate is at 75 kJ/mol, the second peak is at 105 kJ/mol, and the products are at 0 kJ/mol. Which statement is correct?

  1. AStep 1 is exothermic, step 2 is endothermic, and ΔH overall is −40 kJ/mol.
  2. BStep 1 is endothermic, step 2 is exothermic, and ΔH overall is −40 kJ/mol.
  3. CStep 1 is endothermic, step 2 is exothermic, and ΔH overall is +40 kJ/mol.
  4. DBoth steps are exothermic, and ΔH overall is −110 kJ/mol.

Answer: B. Step 1 goes from 40 up to 75, so ΔH₁ = +35 kJ/mol (endothermic). Step 2 goes from 75 down to 0, so ΔH₂ = −75 kJ/mol (exothermic). Overall: 0 − 40 = −40 kJ/mol, which also equals +35 + (−75).

Common trap: The −110 choice adds the drops without noticing that step 1 goes up. Always check each step's direction.

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