A chemist investigates the reaction between propanone and iodine in acidic solution. The overall equation for the reaction is: CH₃COCH₃ + I₂ → CH₃COCH₂I + HI. The rate equation for this reaction is: rate = k[CH₃COCH₃][H⁺]. Explain what the rate equation tells us about the mechanism of this reaction, and explain why iodine does not appear in the rate equation.

Edexcel A-Level Chemistry (9CH0) — 16.1 Rate equations and mechanisms · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The proposed mechanism for this reaction involves two steps. The first step is the acid-catalysed formation of an enol intermediate from propanone, which is slow. The second step is the rapid reaction of the enol intermediate with iodine.

Model answer (5 marks)

The rate equation shows that the reaction is first order in propanone and first order in H⁺, indicating that the slow, rate‑determining step involves one molecule of propanone and one proton.

Only the species that take part in the rate‑determining step appear in the rate equation; the rapid step that follows does not influence the rate.

Because iodine is not involved in the slow step, it does not appear in the rate equation.

Thus the overall order is 2 (second order overall), which matches the two reactants in the rate‑determining step.

Examiner tips

  • State the orders with respect to each reactant; link them to the slow step.
  • Explain why only slow‑step species appear in the rate law.
  • Mention that iodine is absent because it reacts in the fast step.
  • Show the overall order by adding the individual orders.

Common mistakes

  • Saying the reaction is first order overall; it is second order.
  • Including iodine in the rate law or claiming it is a catalyst.
  • Failing to connect the order to the slow step or to the mechanism.

Mark scheme (5 marks)

  1. The rate equation shows the reaction is first order with respect to propanone
  2. The rate equation shows the reaction is first order with respect to H⁺ (hydrogen ions / acid)
  3. Only the species in the rate-determining (slowest) step appear in the rate equation
  4. Iodine does not appear in the rate equation because it is not involved in the rate-determining step
  5. The overall order of the reaction is 2 (second order overall), consistent with two species in the rate-determining step

Key terms in this question

rate equation · mechanism

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