A student investigates the reaction between hydrogen peroxide and iodide ions in acidic solution. The overall equation for the reaction is shown below. H₂O₂ + 2H⁺ + 2I⁻ → I₂ + 2H₂O Experimental results show that when the concentration of hydrogen peroxide is doubled (keeping all other concentrations constant), the rate of reaction doubles. When the concentration of iodide ions is doubled (keeping all other concentrations constant), the rate of reaction also doubles. The concentration of H⁺ ions has no effect on the rate of reaction. Explain what these results tell us about the rate equation for this reaction and about the mechanism by which the reaction occurs.

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 is proportional to the concentration of H₂O₂ and to the concentration of I⁻, but independent of [H⁺].

Hence the rate equation is

rate = k[H₂O₂][I⁻]

This shows the reaction is first order in H₂O₂ and first order in I⁻, zero order in H⁺.

Because the overall stoichiometry involves two H⁺ ions, the rate‑determining step cannot be the single‑step overall reaction. The slow step must involve only one molecule of H₂O₂ and one ion of I⁻, with H⁺ not participating in the rate‑determining step.

Examiner tips

  • State the orders for each reactant and give the rate equation explicitly.
  • Explain why the overall stoichiometry does not match the rate law (H⁺ not involved).
  • Use the word ‘first order’ and ‘zero order’ as required.
  • Mention the implication for the mechanism – a slow step with one H₂O₂ and one I⁻.

Common mistakes

  • Writing the rate law with a wrong exponent for H₂O₂ or I⁻.
  • Including H⁺ in the rate equation or claiming it is first order.
  • Not linking the rate law to the mechanism (e.g., saying the overall reaction is the slow step).

Mark scheme (5 marks)

  1. The reaction is first order with respect to hydrogen peroxide (H₂O₂)
  2. The reaction is first order with respect to iodide ions (I⁻)
  3. The reaction is zero order with respect to H⁺ ions
  4. The rate equation is: rate = k[H₂O₂][I⁻]
  5. The rate equation shows the mechanism cannot be a single step matching the overall equation; the rate-determining (slowest) step must involve one molecule of H₂O₂ and one ion of I⁻ only (H⁺ is not involved in the rate-determining step)

Key terms in this question

rate equation · mechanism

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