Explain how increasing the pressure of gaseous reactants increases the rate of reaction.

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).

Hydrogen gas and nitrogen gas react together to form ammonia. A chemist increases the pressure of the reaction mixture.

Model answer (4 marks)

Increasing the pressure of the gaseous reactants brings the molecules closer together.

This reduces the average distance between them, so collisions between the molecules occur more frequently.

Because collisions are more frequent, the number of collisions that have sufficient energy and the correct orientation to form products also increases.

Thus the overall rate of the reaction rises when the pressure is increased.

Examiner tips

Use the word "increases" to show cause and effect; link pressure to collision frequency and then to rate.
Mention that higher pressure reduces intermolecular distance, leading to more frequent and successful collisions.
Keep the answer concise – 4 points, one sentence each.
Use UK spelling (e.g. "increases").

Common mistakes

Confusing pressure with temperature – students may say the reaction speeds up because of higher temperature instead of higher collision frequency.
Failing to mention that the increased collisions are more likely to be successful, not just more frequent.
Using vague phrases like "more molecules" without linking to pressure and collision frequency.

Mark scheme (4 marks)

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

Key terms in this question

pressure · rate of reaction