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A2 · Practice questions · Rate-determining step and mechanisms

Find the slow step.

Six original Cambridge-style questions on rate-determining steps and mechanisms.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

The slow step writes the rate equation.

01
Analysis
[3 marks]

For NO2 + CO → NO + CO2, rate = k[NO2]2. Propose a two-step mechanism and identify the intermediate.

  • Step 1 (slow): NO2 + NO2 → NO3 + NO (two NO2, so second order). ✓
  • Step 2 (fast): NO3 + CO → NO2 + CO2 (CO after the slow step, so zero order). ✓
  • NO3 is the intermediate; the steps add up to the overall equation. ✓
02
Explain
[3 marks]

Bromoethane is hydrolysed by aqueous OH− with rate = k[CH3CH2Br][OH−]. Explain how this supports an SN2 mechanism.

  • Both species are first order, so one of each is in the rate-determining step. ✓
  • In SN2 the OH− attacks the δ+ carbon as the C–Br bond breaks, in a single step. ✓
  • So the only step contains both species, matching the rate equation. ✓
03
Analysis
[3 marks]

2-bromo-2-methylpropane reacts with OH−(aq). Predict the rate equation and give the two steps of the mechanism.

  • Rate = k[(CH3)3CBr]. ✓
  • Step 1 (slow): (CH3)3CBr → (CH3)3C+ + Br−. ✓
  • Step 2 (fast): (CH3)3C+ + OH− → (CH3)3COH. ✓
04
Explain
[2 marks]

Distinguish between an intermediate and a catalyst in a reaction mechanism.

  • An intermediate is formed in one step and used up in a later step. ✓
  • A catalyst is used up in one step and regenerated in a later step, so it is present at the start and the end. ✓
05
Analysis
[2 marks]

For X + 2Y → Z, rate = k[X][Y]. A student suggests a one-step mechanism. Explain why it is wrong.

  • A one-step mechanism would need one X and two Y colliding, giving rate = k[X][Y]2. ✓
  • The data show first order in Y, so the slow step has only one Y and the second Y reacts in a later fast step. ✓
06
Recall
[2 marks]

On an energy profile for a two-step reaction, how can you identify (a) the rate-determining step, (b) the intermediate?

  • (a) The step with the higher activation energy (the higher hump measured from its starting level). ✓
  • (b) The intermediate sits in the dip (local minimum) between the two transition states. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.