Reaction mechanism test with answers for AP Chemistry (free PDF download)

Quick answer: A reaction mechanism is a sequence of elementary steps whose sum is the overall equation. This AP Chemistry test on reaction mechanism has 8 questions worth 20 marks, to do in 45 minutes: elementary steps, testing a mechanism and energy profile. The mark scheme is on a separate page; free printable PDF.

This test lasts 45 minutes and covers mechanisms, energy profiles, catalysts and the Arrhenius equation. A calculator is allowed; R = 8.314 J/(mol K). Show your working. The total is 20 marks.

Level: AP Chemistry · Curriculum: AP Chem Unit 5

Time: 45 minutes · 8 questions · Total: 20 marks · Calculator allowed

Contents

Question 1 Elementary steps (2 marks)

For the elementary step 2 NO₂ → NO₃ + NO:

State its molecularity. (1 mark)
Write its rate law. (1 mark)

Question 2 Testing a mechanism (3 marks)

2 NO₂Cl → 2 NO₂ + Cl₂ has the mechanism: step 1 (slow) NO₂Cl → NO₂ + Cl; step 2 (fast) NO₂Cl + Cl → NO₂ + Cl₂.

Show that the steps give the overall equation. (1 mark)
Identify the intermediate. (1 mark)
Write the rate law predicted. (1 mark)

Question 3 Energy profile (3 marks)

Energies are relative to a common zero.

Calculate Eₐ for the forward reaction. (1 mark)
Calculate ΔH and state whether it is endothermic. (1 mark)
Calculate Eₐ for the reverse reaction. (1 mark)

Question 4 Role of a catalyst (2 marks)

Explain how a catalyst increases the rate and state its effect on ΔH.

Question 5 Activation energy (3 marks)

A first-order reaction has k = 2.0 × 10⁻² s⁻¹ at 350 K and 8.0 × 10⁻² s⁻¹ at 370 K. Calculate Eₐ in kJ/mol.

Question 6 Fast equilibrium (3 marks)

For 2 NO + O₂ → 2 NO₂ the proposed steps are: (1) fast equilibrium 2 NO ⇌ N₂O₂; (2) slow N₂O₂ + O₂ → 2 NO₂.

Derive the rate law in terms of reactants only. (2 marks)
State the overall order. (1 mark)

Question 7 Catalyst or intermediate (2 marks)

A mechanism for 2 H₂O₂ → 2 H₂O + O₂ is: step 1, Br⁻ + H₂O₂ + H⁺ → HOBr + H₂O; step 2, HOBr + H₂O₂ → H⁺ + Br⁻ + H₂O + O₂. Identify one catalyst and the intermediate.