A student adds manganese(IV) oxide powder to a solution of hydrogen peroxide. The student observes that gas is produced rapidly, but at the end of the reaction the mass of manganese(IV) oxide is unchanged. Explain the role of the manganese(IV) oxide in this reaction and explain, in terms of particles, why using a larger mass of manganese(IV) oxide powder would increase the rate of gas production.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Manganese(IV) oxide acts as a catalyst, providing a surface on which hydrogen peroxide decomposes into water and oxygen.
The catalyst is not consumed, so its mass remains unchanged.
A larger mass of MnO₂ gives a larger total surface area, offering more active sites for the reaction.
With more surface exposed, hydrogen peroxide molecules collide with the catalyst more often, increasing the number of successful collisions per second and thus the rate of gas production.
The catalyst is not consumed, so its mass remains unchanged.
A larger mass of MnO₂ gives a larger total surface area, offering more active sites for the reaction.
With more surface exposed, hydrogen peroxide molecules collide with the catalyst more often, increasing the number of successful collisions per second and thus the rate of gas production.
Examiner tips
- Use the word ‘catalyst’ and explain it is not consumed. Mention surface area and active sites. Show the link between more collisions and a faster rate. Keep the answer concise and to the point.
Common mistakes
- Saying the catalyst is ‘used up’ or ‘reacts’. Forgetting to mention surface area or active sites. Using vague terms like ‘more of it’ without explaining the particle‑level reason.
Mark scheme (4 marks)
- Manganese(IV) oxide acts as a catalyst
- A catalyst speeds up the rate of reaction without being used up / without changing in mass or chemical composition
- A larger mass of manganese(IV) oxide provides a greater surface area (more active sites / more surface exposed to hydrogen peroxide particles)
- More frequent collisions between hydrogen peroxide particles and the catalyst surface, so more successful collisions per second / reaction happens faster
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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