Describe how increasing the surface area of a solid reactant affects the rate of reaction. Use ideas about particles in your answer.

OCR GCSE Chemistry A: Gateway Science (J248) — C5.1 Monitoring chemical reactions · Describe · 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 marble chips affects the rate of reaction with hydrochloric acid. She uses large marble chips in one experiment and crushed marble powder in another, keeping all other conditions the same.

Model answer (4 marks)

Increasing the surface area of a solid reactant exposes more particles to the surface.
This means that, in a given time, more reactant particles can collide with the acid.
The number of collisions per unit time therefore rises.
With more collisions, the number of successful collisions also rises, so the rate of reaction increases and the reaction proceeds faster.

Examiner tips

Use the word "exposes" to link surface area with particle exposure. Show the chain: surface area → more surface particles → more collisions → more successful collisions → faster rate. Include the word "rate" and "faster" to demonstrate understanding of the outcome.

Common mistakes

Saying the reaction is slower or that surface area has no effect. Failing to mention collisions or successful collisions. Using vague terms like "more reactants" without linking to surface area.

Mark scheme (4 marks)

Increasing surface area means more (reacting) particles are exposed at the surface
Collisions between reactant particles happen more frequently / there are more frequent collisions
There are more successful collisions (per unit time)
Therefore the rate of reaction increases / the reaction is faster

Key terms in this question

surface area · rate of reaction