Explain why increasing the concentration of hydrochloric acid increases the rate at which it reacts with zinc metal.

OCR GCSE Chemistry A: Gateway Science (J248) — C5.1 Monitoring chemical reactions · Explain · 4 marks · View as Markdown

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

A student investigating rate of reaction uses two different concentrations of hydrochloric acid with excess zinc granules. She notices that the more concentrated acid produces gas much more quickly than the dilute acid.

Model answer (4 marks)

Increasing the concentration of HCl means the H⁺ and Cl⁻ ions are closer together, so the reacting particles are more densely packed.

This increases the frequency of collisions between the acid ions and the zinc surface.

With more frequent collisions, a greater proportion of them have enough energy and the correct orientation to break the Zn–metal bonds and form ZnCl₂.

Hence the overall rate of the reaction increases.

Examiner tips

Use the word ‘concentration’ and explain how it changes particle spacing. Show the chain: concentration → particle density → collision frequency → successful collisions → rate. Include the key phrase ‘rate of reaction increases’ to link the explanation to the question.

Common mistakes

Saying ‘more acid means more acid’ without linking to particle density. Forgetting to mention collision frequency or successful collisions. Using vague terms like ‘more energy’ without explaining how concentration affects it.

Mark scheme (4 marks)

Increasing concentration means reacting particles are closer together
Particles collide more often / there is a higher frequency of collisions
There is a higher rate of successful collisions
Therefore the rate of reaction increases

Key terms in this question

concentration