A student investigates the decomposition of hydrogen peroxide solution at two different temperatures. At the higher temperature, the reaction produces oxygen gas much more quickly. Using the Maxwell-Boltzmann distribution, explain why increasing the temperature causes the rate of the decomposition reaction to increase.

Edexcel A-Level Chemistry (9CH0) — 9.1 Rates and the Maxwell-Boltzmann distribution · Explain · 5 marks · View as Markdown

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

Hydrogen peroxide decomposes according to the equation: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). The student notices that at a higher temperature the reaction is noticeably faster.

Model answer (5 marks)

The Maxwell‑Boltzmann distribution shows the spread of kinetic energies of the H₂O₂ molecules. A reaction can only occur when a molecule has at least the activation energy, E_a. At a higher temperature the whole distribution shifts to the right and the average kinetic energy increases. Consequently a larger fraction of the molecules now possess energy ≥E_a. This means more collisions are effective, so the rate of decomposition rises.

Examiner tips

  • Use the phrase ‘activation energy’ and ‘effective collisions’ – these are key terms.
  • Show that the distribution shifts right with temperature and link this to a higher proportion of molecules above E_a.
  • Mention that the rate increases because of more successful collisions per unit time.
  • Keep the answer concise – 5 marks can be earned with 3–4 short sentences.

Common mistakes

  • Saying ‘more collisions’ without specifying ‘effective’ or ‘above activation energy’.
  • Confusing temperature with concentration or pressure.
  • Using vague terms like ‘more energy’ without linking to the Maxwell‑Boltzmann distribution or activation energy.

Mark scheme (5 marks)

  1. The Maxwell-Boltzmann distribution shows the spread/range of energies of particles in a reaction mixture
  2. There is a minimum energy required for a reaction to occur, called the activation energy
  3. At a higher temperature, particles have more (kinetic) energy on average / the distribution shifts to the right
  4. A greater proportion / more particles have energy equal to or greater than the activation energy at the higher temperature
  5. There are more successful/effective collisions per unit time, so the rate of reaction increases

Key terms in this question

Maxwell-Boltzmann distribution

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