A chemist investigates the reaction between peroxodisulfate ions and iodide ions. The overall equation for the reaction is: S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq) Experiments show that the rate equation for this reaction is: rate = k[S₂O₈²⁻][I⁻] A student proposes the following two-step mechanism: Step 1: S₂O₈²⁻(aq) + I⁻(aq) → SO₄²⁻(aq) + SO₄I³⁻(aq) slow Step 2: SO₄I³⁻(aq) + I⁻(aq) → SO₄²⁻(aq) + I₂(aq) fast Explain whether this proposed mechanism is consistent with the rate equation given above.

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).

Model answer (5 marks)

The rate‑determining step is step 1, which contains one S₂O₈²⁻ and one I⁻. Therefore the rate law derived from this step is rate = k[S₂O₈²⁻][I⁻]. This matches the experimentally obtained rate equation. Hence the proposed mechanism is consistent with the data.

Examiner tips

  • Identify the slow step first; the rate law comes from it. Show the stoichiometry of the slow step to justify the rate law. Compare the derived law directly with the experimental law. Mention that the fast step does not affect the rate law.

Common mistakes

  • Assuming the fast step determines the rate. Ignoring that the rate law only involves species in the slow step. Failing to match the derived rate law with the experimental one.

Mark scheme (5 marks)

  1. The slow step (rate-determining step) determines the overall rate of reaction
  2. The rate equation is derived from the species in the rate-determining step only
  3. Step 1 involves one S₂O₈²⁻ and one I⁻, so the predicted rate equation from this step is rate = k[S₂O₈²⁻][I⁻]
  4. The predicted rate equation matches the experimentally determined rate equation
  5. Therefore the mechanism is consistent with the experimental rate equation

Key terms in this question

rate equation · mechanism

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