Explain why two reactions with identical activation energies can have different rates at the same temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The rate of a chemical reaction depends on both the frequency of collisions between reactant particles and the proportion of those collisions that have sufficient energy to result in a reaction.
Model answer (4 marks)
The rate depends on the number of successful collisions per unit time, not just the proportion of collisions that have enough energy. If two reactions have the same activation energy, the same fraction of collisions will possess energy ≥ Ea at a given temperature. However, the overall rate can differ because the two reactions may have different collision frequencies. Factors such as reactant concentration, molecular size or mass, and the nature of the reacting species can change how often reactant molecules meet. A higher collision frequency gives more opportunities for successful collisions, so the reaction with the higher frequency will proceed faster even though the proportion of energetic collisions is identical.
Examiner tips
- Mention both the proportion of energetic collisions and the absolute collision frequency; link them to concentration and molecular size.
- Use the phrase ‘identical activation energies’ to show you understand the premise.
- Explain how collision frequency can vary (concentration, size, mass).
Common mistakes
- Confusing proportion of energetic collisions with total collision frequency.
- Forgetting to mention concentration or molecular size as factors that affect collision frequency.
- Using vague terms like ‘more collisions’ without explaining why the frequency differs.
Mark scheme (4 marks)
- Rate depends on the frequency of successful collisions, not just the proportion of particles with energy ≥ Ea.
- If activation energies are the same, the same fraction/proportion of collisions will have energy ≥ Ea at a given temperature.
- The reactions can differ in the frequency of collisions between reactant molecules, for example due to different concentrations or different molecular sizes/masses.
- A higher collision frequency means more successful collisions per unit time even if the proportion with energy ≥ Ea is the same, so the rate is higher.
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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