Explain why increasing the pressure of gaseous reactants increases 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).

In the industrial manufacture of ammonia, nitrogen gas and hydrogen gas are reacted together at high pressure. A student suggests that increasing the pressure of the gaseous reactants will increase the rate of this reaction.

Model answer (4 marks)

Increasing the pressure of the gaseous reactants brings the gas particles closer together.
This reduces the average distance between molecules, so the collision frequency rises.
With more frequent collisions, a greater proportion of collisions have the correct orientation and energy to overcome the activation energy.
Consequently, the overall rate of the reaction increases.

Examiner tips

Use the word ‘increase’ to show cause and effect; link pressure to collision frequency and then to reaction rate.
Mention that higher pressure reduces the mean free path, leading to more collisions per unit time.
Show the logical chain: pressure → closer particles → more collisions → more successful collisions → higher rate.
Use the term ‘collision frequency’ and ‘activation energy’ where appropriate.

Common mistakes

Failing to explain the link between pressure and collision frequency.
Using vague terms like ‘more energy’ without mentioning activation energy.
Omitting the step that higher collision frequency leads to more successful collisions.

Mark scheme (4 marks)

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

Key terms in this question

pressure · rate of reaction