Explain why decreasing the surface area of a solid reactant decreases the rate of reaction.

Edexcel GCSE Chemistry (1CH0) — 7.1 Rates of reaction · Explain · 4 marks · View as Markdown

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

A student is investigating how the size of zinc pieces affects the rate of reaction between zinc and sulfuric acid. The student notices that large pieces of zinc react much more slowly than small pieces of zinc of the same total mass.

Model answer (4 marks)

Decreasing the surface area of a solid reactant reduces the number of zinc atoms that are exposed to the acid.

Only atoms on the surface can collide with acid molecules, so a smaller surface area means fewer zinc atoms are available for collisions.

Fewer collisions between zinc and acid particles occur per unit time.

With fewer collisions, there are fewer successful collisions that lead to reaction, so the overall rate of reaction decreases.

Examiner tips

Use the word ‘surface area’ and explain that only surface atoms react.
Show the chain: less surface area → fewer exposed atoms → fewer collisions → lower rate.
Mention that the rate is proportional to the number of successful collisions per unit time.

Common mistakes

Confusing surface area with total mass or volume.
Saying the reaction is slower because the acid is weaker, not because of surface area.
Using vague terms like ‘less reaction’ instead of ‘fewer successful collisions’.

Mark scheme (4 marks)

Decreasing surface area means fewer exposed reacting particles (on the surface of the zinc)
This means collisions between zinc and acid particles are less frequent
There are fewer successful collisions per unit time
Therefore the rate of reaction decreases

Key terms in this question

surface area · rate of reaction