A student uses a reaction profile to compare two reactions: an uncatalysed decomposition of hydrogen peroxide and the same reaction using manganese(IV) oxide as a catalyst. Explain how the reaction profile would differ between the two reactions, and use your answer to explain why the catalysed reaction is faster.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrogen peroxide decomposes to form water and oxygen. This reaction can be carried out with or without a catalyst. A reaction profile shows the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
Model answer (5 marks)
The catalysed reaction has a lower activation energy, so the peak of the profile is lower.
The catalyst offers a different (alternative) pathway, so the shape of the curve is different.
The overall energy change between reactants and products is unchanged.
Because the activation energy is lower, a larger proportion of molecules have enough energy to reach the transition state.
Thus there are more successful collisions per unit time, so the rate of reaction increases.
The catalyst offers a different (alternative) pathway, so the shape of the curve is different.
The overall energy change between reactants and products is unchanged.
Because the activation energy is lower, a larger proportion of molecules have enough energy to reach the transition state.
Thus there are more successful collisions per unit time, so the rate of reaction increases.
Examiner tips
- Use the word "lower" for activation energy; mention the peak is lower. State that the overall ΔE is unchanged. Explain that more molecules reach the transition state. Link this to a higher rate.
- common_mistakes
- :
- Saying the catalyst changes the overall ΔE. Forgetting to mention the alternative pathway. Using vague terms like "faster" without explaining the kinetic reason.
Mark scheme (5 marks)
- The activation energy is lower in the catalysed reaction
- The catalyst provides a different (alternative) reaction pathway
- The overall energy change (difference in energy between reactants and products) remains the same in both profiles
- A larger proportion of particles have energy equal to or greater than the (lower) activation energy in the catalysed reaction
- Therefore there are more successful collisions per unit time, so the rate of reaction increases
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
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- Decode the mark scheme abbreviations →
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