Explain how increasing the concentration of a reactant in solution increases the rate of a chemical reaction, using collision theory.

IB DP Chemistry Standard Level (2023 syllabus) — R2.2 How fast? The rate of chemical change · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Increasing the concentration of a reactant in solution means there are more molecules or ions per unit volume. This raises the frequency of collisions between reactant particles. With more collisions, a larger proportion of them will have the energy required to overcome the activation energy barrier. Consequently, the rate of the chemical reaction increases.

Examiner tips

Use the phrase "frequency of collisions" and "activation energy" to show understanding of collision theory.
Show the logical chain: more concentration → more particles → more collisions → more successful collisions → higher rate.
Mention that the increase is proportional to concentration for a first‑order reaction if appropriate.

Common mistakes

Failing to mention that collisions must exceed the activation energy.
Using vague terms like "more reactants" without linking to collision frequency.
Confusing concentration with temperature effects.

Mark scheme (4 marks)

Increasing concentration means more particles (molecules/ions) per unit volume
This leads to a greater frequency of collisions between reactant particles
A greater proportion / larger number of collisions exceed the activation energy
Therefore the rate of reaction increases

Key terms in this question

concentration · collision theory

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