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

Eduqas GCSE Chemistry — C6 Rate and extent of chemical change · Explain · 4 marks · View as Markdown

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

A student is investigating how the concentration of hydrochloric acid affects how quickly it reacts with marble chips.

Model answer (4 marks)

Higher concentration means more particles of the reactant in the same volume.
Particles collide more frequently, giving more collisions per unit time.
More collisions have sufficient kinetic energy to overcome the activation energy.
Therefore the rate of reaction increases.

Examiner tips

  • Use the word "concentration" and link it to "more particles". Show the chain: more particles → more collisions → more effective collisions → higher rate. Use the term "activation energy" to explain why energy matters.
  • Include the causal sequence in the correct order to hit all four marks.

Common mistakes

  • Confusing concentration with temperature. Not mentioning activation energy or effective collisions. Writing a generic statement like "more reactant = faster" without explaining the mechanism.

Mark scheme (4 marks)

  1. Higher concentration means more particles (of reactant) in the same volume
  2. Particles collide more frequently / there are more collisions per unit time
  3. More collisions have sufficient / enough energy to overcome the activation energy
  4. Therefore the rate of reaction increases

Key terms in this question

concentration

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