A student investigates the reaction between marble chips (calcium carbonate) and dilute hydrochloric acid. She carries out two experiments using the same mass of marble chips and the same volume of acid, but in one experiment she uses large chips and in the other she uses small chips of the same total mass. Describe and explain the difference in the rate of reaction observed between the two experiments.

Cambridge International IGCSE Chemistry (0620) — 6.2 Rate of reaction · Describe and explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The small chips react faster than the large chips.

The small chips have a greater surface area for the same mass.

A larger surface area exposes more reactant particles to the acid.

More particles are available for collisions, so collisions are more frequent and the rate increases.

Examiner tips

  • Use the word ‘faster’ or ‘greater rate’ to match the mark scheme.
  • Explain that surface area is the key factor and link it to collision frequency.

Common mistakes

  • Saying the reaction is ‘more vigorous’ without linking to surface area.
  • Confusing mass with surface area – stating that larger mass gives a higher rate.

Mark scheme (4 marks)

  1. The small chips react faster / at a greater rate than the large chips
  2. Small chips have a greater/larger surface area (than large chips of the same mass)
  3. More reactant particles are exposed / available for collisions at the surface
  4. More (frequent/successful) collisions occur per unit time, so the rate increases

Key terms in this question

rate of reaction

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