Explain how a catalyst increases the rate of a chemical reaction, with reference to activation energy and the Maxwell–Boltzmann distribution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A catalyst provides an alternative reaction pathway for the reaction.
The alternative pathway has a lower activation energy.
Because the activation energy is lower, a greater proportion of the reacting molecules have kinetic energy equal to or greater than the activation energy, as shown by the Maxwell–Boltzmann distribution.
Consequently, the frequency of successful or effective collisions increases, so the rate of reaction increases. The catalyst is not consumed and remains unchanged at the end of the reaction.
The alternative pathway has a lower activation energy.
Because the activation energy is lower, a greater proportion of the reacting molecules have kinetic energy equal to or greater than the activation energy, as shown by the Maxwell–Boltzmann distribution.
Consequently, the frequency of successful or effective collisions increases, so the rate of reaction increases. The catalyst is not consumed and remains unchanged at the end of the reaction.
Examiner tips
- Use the word ‘alternative pathway’ to show understanding of mechanism. Mention the lower activation energy and link it to the Maxwell–Boltzmann distribution. State that the catalyst is unchanged to cover the final point. Keep the answer concise and use correct terminology.
Common mistakes
- Confusing a catalyst with a reactant or product. Failing to mention the Maxwell–Boltzmann distribution. Omitting that the catalyst is not consumed.
Mark scheme (4 marks)
- A catalyst provides an alternative reaction pathway (mechanism) for the reaction.
- This alternative pathway has a lower activation energy.
- A greater proportion/fraction of molecules have energy greater than or equal to the (lower) activation energy.
- Therefore the frequency of successful/effective collisions increases, so the rate of reaction increases. The catalyst is not consumed/remains unchanged at the end of the reaction.
Key terms in this question
catalyst · activation energy · Maxwell–Boltzmann distribution
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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