Explain why adding a catalyst increases the rate of a chemical reaction.

OCR GCSE Chemistry A: Gateway Science (J248) — C5.2 Controlling reactions · Explain · 4 marks · View as Markdown

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

A chemical company produces ammonia on a large scale. Engineers use an iron catalyst in the process to ensure the reaction proceeds at a useful rate.

Model answer (4 marks)

A catalyst provides an alternative reaction pathway.
The new pathway has a lower activation energy.
Therefore a greater proportion of reacting particles possess the required energy.
The catalyst is not consumed in the reaction.

Examiner tips

Use the exact wording from the mark scheme (e.g., "different reaction pathway", "lower activation energy").
Show the logical sequence: pathway → lower Ea → more particles above Ea → faster rate.
Mention that the catalyst is unchanged to satisfy the last point.
Keep each point brief but complete – 1 mark each.

Common mistakes

Confusing the catalyst with a reactant or product. Failing to state that the catalyst is not used up. Using vague terms like "makes the reaction faster" without explaining the mechanism.

Mark scheme (4 marks)

A catalyst provides a different reaction pathway
This new pathway has a lower activation energy
A greater proportion of particles now have at least the activation energy
The catalyst is not used up during the reaction

Key terms in this question

catalyst