A chemist investigates the reaction between bromate(V) ions and bromide ions in acidic solution. The overall equation for the reaction is shown below. BrO₃⁻(aq) + 5Br⁻(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l) Experiments show that the rate equation for this reaction is: rate = k[BrO₃⁻][Br⁻][H⁺]² Explain what the rate equation tells us about the reaction mechanism, and explain why the overall equation cannot represent the mechanism for this reaction.

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 rate equation for the reaction between bromate(V) ions and bromide ions in acidic solution is: rate = k[BrO₃⁻][Br⁻][H⁺]² The overall equation is: BrO₃⁻(aq) + 5Br⁻(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l)

Model answer (5 marks)

The rate equation shows that the reaction is fourth order overall, with orders 1 in BrO₃⁻, 1 in Br⁻ and 2 in H⁺. This indicates that the rate‑determining step involves one BrO₃⁻ ion, one Br⁻ ion and two H⁺ ions colliding. The rate law is determined solely by the slowest step in the mechanism, so the slow step must contain exactly these species.

The overall stoichiometric equation contains five Br⁻ ions and six H⁺ ions, but only one Br⁻ and two H⁺ appear in the rate law. Therefore the overall equation cannot represent a single elementary step. An elementary step would require the simultaneous collision of 12 particles (BrO₃⁻ + 5Br⁻ + 6H⁺), which is statistically improbable and would give a rate law of order 12, not 4.

Examiner tips

  • State the overall order (4) and the individual orders (1,1,2).
  • Explain that the rate law reflects only the slow step and list the species involved.
  • Show why the overall equation cannot be the slow step – mismatch of stoichiometry and improbability of a 12‑body collision.

Common mistakes

  • Confusing overall order with stoichiometric coefficients.
  • Assuming the overall equation is the elementary step without checking the rate law.
  • Failing to mention the improbability of a 12‑body collision.

Mark scheme (5 marks)

  1. The reaction is fourth order overall (or: the overall order is 4, found by summing the individual orders: 1 + 1 + 2 = 4)
  2. The rate-determining step (slow step) involves the species whose concentrations appear in the rate equation: one BrO₃⁻ ion, one Br⁻ ion, and two H⁺ ions
  3. The rate equation is determined by (only) the slowest step in the mechanism / the rate-determining step
  4. The overall equation shows 5 Br⁻ ions and 6 H⁺ ions reacting, but only 1 Br⁻ and 2 H⁺ appear in the rate equation, so the overall equation cannot represent a single elementary step / the mechanism
  5. A single-step mechanism would require the simultaneous collision of 12 particles (BrO₃⁻ + 5Br⁻ + 6H⁺), which is statistically improbable / effectively impossible

Key terms in this question

rate equation · mechanism

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