Explain why increasing the concentration of hydrochloric acid increases the rate of reaction between hydrochloric acid and zinc.

AQA A-Level Chemistry (7405) — 3.1.5 Kinetics · Explain · 5 marks · View as Markdown

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

A student investigates the reaction between zinc granules and hydrochloric acid. She notices that when she uses a higher concentration of acid, hydrogen gas is produced much more quickly.

Model answer (5 marks)

Higher concentration of HCl gives more H⁺ ions in the same volume. This increases the number of collisions per second between H⁺ and Zn particles. With more collisions, a greater proportion have the activation energy, so more successful collisions occur per unit time. Consequently the reaction rate increases.

Examiner tips

Use the collision theory chain: concentration → more particles → more collisions → more successful collisions → higher rate
Show the logical sequence clearly, linking each point to the next

Common mistakes

Failing to mention that the increased collisions are between reactants, not just acid particles
Using vague terms like "more energy" without linking to activation energy
Not explaining that the rate increases because of more successful collisions per second

Mark scheme (5 marks)

Higher concentration means more particles of hydrochloric acid (hydrogen ions / HCl molecules) in the same volume
Particles collide more frequently / there are more collisions per second
More collisions between acid particles and zinc particles (collision between reactant particles)
More collisions have (at least) the activation energy / sufficient energy to react
Therefore more successful collisions per second, so the rate of reaction increases

Key terms in this question

concentration · rate of reaction