A student carries out experiments to find the order of reaction with respect to hydrogen peroxide (H₂O₂) in a decomposition reaction. In experiment 1, the initial concentration of H₂O₂ is 0.10 mol/dm³ and the initial rate is 2.0 × 10⁻³ mol/dm³/s. In experiment 2, the concentration is doubled to 0.20 mol/dm³ and the initial rate doubles to 4.0 × 10⁻³ mol/dm³/s. In experiment 3, the concentration is tripled to 0.30 mol/dm³ and the initial rate triples to 6.0 × 10⁻³ mol/dm³/s. Explain what these results show about the order of reaction with respect to H₂O₂, and state what a rate equation for this reaction would look like.

AQA A-Level Chemistry (7405) — 3.1.9 Rate equations (A-Level only) · Explain · 5 marks · View as Markdown

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

Hydrogen peroxide decomposes in the presence of a catalyst. A student collects initial rate data at three different concentrations of hydrogen peroxide, keeping all other conditions constant.

Model answer (5 marks)

When the concentration of H₂O₂ is doubled from 0.10 to 0.20 mol dm⁻³ the initial rate also doubles from 2.0×10⁻³ to 4.0×10⁻³ mol dm⁻³ s⁻¹, and when the concentration is tripled to 0.30 mol dm⁻³ the rate triples to 6.0×10⁻³ mol dm⁻³ s⁻¹. This shows that the rate is directly proportional to the concentration of H₂O₂. Therefore the reaction is first order with respect to H₂O₂. A first‑order rate law is written as

rate = k[H₂O₂]¹

or simply rate = k[H₂O₂]. The rate constant k can be found from any data point, e.g. k = (2.0×10⁻³)/(0.10) = 2.0×10⁻² dm³ mol⁻¹ s⁻¹.

Examiner tips

  • Show the proportionality by comparing the two data sets; use the word ‘directly proportional’ or ‘rate doubles when concentration doubles’.
  • State clearly that the reaction is first order and write the rate law in the form rate = k[H₂O₂].
  • If asked, calculate k from one data point to demonstrate understanding of the rate constant.

Mark scheme (5 marks)

  1. When concentration doubles, the rate doubles (or equivalent proportional statement from the data)
  2. The reaction is first order with respect to H₂O₂
  3. First order means rate is proportional to [H₂O₂]¹ (or that the exponent/power of concentration in the rate equation is 1)
  4. The rate equation is rate = k[H₂O₂] (accept rate = k[H₂O₂]¹)
  5. k is the rate constant (and its value can be determined from the data, e.g. k = rate ÷ [H₂O₂])

Key terms in this question

order of reaction · rate equation · initial rate

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