Explain why increasing the concentration of a dissolved reactant increases the rate of a chemical reaction.

Cambridge International IGCSE Chemistry (0620) — 6.2 Rate of reaction · 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 dissolved reactant increases the number of reactant particles in a given volume.

This raises the frequency of collisions between reactant molecules.

With more collisions per unit time, the probability of successful, effective collisions rises.

Consequently, the rate of the chemical reaction increases.

Examiner tips

Use the word "increases" to link concentration and collision frequency. Mention "effective collisions" to show understanding of reaction mechanism. Keep the answer concise and in the order of the mark scheme points.

Common mistakes

Failing to mention that more collisions lead to more effective collisions. Using vague terms like "more reactants" without linking to collision frequency. Writing a long paragraph that repeats the same idea without structure.

Mark scheme (4 marks)

Higher concentration means more particles (of reactant) in the same volume
Particles collide more frequently / there are more collisions per unit time
More collisions result in successful / effective collisions
Therefore the rate of reaction increases

Key terms in this question

concentration