A student investigates the rate of a reaction between marble chips (calcium carbonate) and hydrochloric acid. She carries out the experiment at 25 °C and then repeats it at 45 °C, keeping all other variables the same. Explain why the rate of reaction is faster at 45 °C than at 25 °C.

OCR A-Level Chemistry B: Salters (H433) — 5.2 Energy · Explain · 5 marks · View as Markdown

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

Marble chips react with hydrochloric acid to produce carbon dioxide gas. The rate of reaction can be monitored by measuring the volume of gas collected over time.

Model answer (5 marks)

At a higher temperature the particles have more kinetic energy.

This means they move faster, so they collide more often – the collision frequency increases.

In addition, a greater proportion of the particles have energy equal to or greater than the activation energy.

Therefore there is a higher rate of successful collisions.

Both the increased collision frequency and the greater proportion of particles with sufficient energy contribute to the faster rate of reaction.

Examiner tips

  • Use the key phrases: "more kinetic energy", "faster particles", "greater collision frequency", "greater proportion of particles with activation energy", "higher rate of successful collisions".
  • Show the link between temperature, kinetic energy and collision frequency before mentioning activation energy.
  • Mention both factors (frequency and energy) as the reason for the rate increase.
  • Use UK spelling (e.g., "favourable").

Common mistakes

  • Failing to mention both collision frequency and activation energy. Using vague terms like "more energy" without linking to kinetic energy or collision frequency. Mixing up temperature with pressure or concentration effects.

Mark scheme (5 marks)

  1. At a higher temperature, particles have more kinetic energy
  2. Particles move faster, so they collide more often / there is a greater frequency of collisions
  3. A greater proportion of particles have energy greater than or equal to the activation energy
  4. There is therefore a higher rate of successful collisions
  5. Both the increased collision frequency AND the greater proportion of particles with activation energy contribute to the faster rate of reaction

Key terms in this question

rate of reaction

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