Explain how increasing the concentration of hydrochloric acid increases the rate of its reaction with marble chips.

AQA GCSE Chemistry (8462) — 4.6.1 Rate of reaction · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Increasing the concentration of HCl means more H⁺ and Cl⁻ ions are present in the solution, so the particles are closer together.
This increases the frequency of collisions between the ions and the calcium carbonate in the marble.
With more collisions, a greater proportion are successful, giving a higher rate of reaction.
Thus the reaction rate increases as the HCl concentration rises.

Examiner tips

  • Use the word ‘concentration’ and link it to ‘more particles’. Show the chain: more particles → more collisions → more successful collisions → higher rate. Use the term ‘rate of reaction’ and ‘successful collisions’ as key phrases.
  • Avoid vague statements like ‘more acid’ – specify ions and collisions.

Common mistakes

  • Failing to mention ions or collisions. Using ‘more acid’ without explaining the effect on particle density. Not linking the increased collisions to a higher rate of reaction.

Mark scheme (4 marks)

  1. Increasing concentration means particles are closer together (in solution)
  2. Collisions between reacting particles are more frequent
  3. There is a higher rate of successful collisions
  4. Therefore the rate of reaction increases

Key terms in this question

concentration

Related

More Rate of reaction questions

▶ Try answering this question with AI marking (free) →