A student investigates how changing the concentration of hydrochloric acid affects the rate of reaction with marble chips. Describe how the rate of reaction changes as the concentration of hydrochloric acid increases, and explain why this change occurs in terms of particles.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Rate of reaction increases as the concentration of hydrochloric acid increases.
Higher concentration means more H⁺ ions are present in the same volume.
The greater number of H⁺ ions leads to more frequent collisions between reactant particles.
More collisions mean a higher proportion of them have the required activation energy, so more successful collisions occur.
Therefore the rate of reaction increases because products are formed more quickly and more often.
Higher concentration means more H⁺ ions are present in the same volume.
The greater number of H⁺ ions leads to more frequent collisions between reactant particles.
More collisions mean a higher proportion of them have the required activation energy, so more successful collisions occur.
Therefore the rate of reaction increases because products are formed more quickly and more often.
Examiner tips
- Use the word ‘rate’ and ‘concentration’ early; link concentration to particle number; explain collision frequency and activation energy; finish with a clear causal statement.
- Show the logical chain: concentration → particles → collisions → activation energy → rate.
Common mistakes
- Confusing concentration with volume; not mentioning H⁺ ions; omitting the activation energy step; writing ‘more collisions’ without linking to successful collisions.
Mark scheme (5 marks)
- Rate of reaction increases as concentration of hydrochloric acid increases
- Higher concentration means more particles (of hydrochloric acid / hydrogen ions) in the same volume
- Particles collide more frequently / there are more frequent collisions
- More collisions have (sufficient / the required) activation energy / more successful collisions
- Therefore the rate of reaction increases because products are formed more quickly / more often
Key terms in this question
concentration · rate of reaction
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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