# Manganese dioxide (MnO₂) is used as a catalyst in the decomposition of hydrogen peroxide solution to produce water and oxygen. A student carries out the reaction at room temperature and observes rapid gas production. The student then repeats the experiment using the same mass of manganese dioxide but ground into a fine powder instead of small lumps. Explain why the rate of reaction is faster when the manganese dioxide is in powdered form, and explain why the total volume of oxygen collected at the end of both experiments is the same.

> Edexcel A-Level Chemistry (9CH0) — Topic 4 Physical chemistry and transition elements · Explain · 5 marks

> Hydrogen peroxide decomposes according to the equation: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). Manganese dioxide acts as a catalyst in this reaction.

## Mark scheme (5 marks)

1. Powdering the manganese dioxide increases the surface area (of the catalyst / of the solid)
2. A greater surface area means more active sites / more of the catalyst is exposed to the hydrogen peroxide molecules
3. This increases the frequency of successful collisions (between hydrogen peroxide molecules and the catalyst surface), so the rate increases
4. The catalyst is not used up / the amount of hydrogen peroxide (reactant) is the same in both experiments
5. Therefore the same amount of hydrogen peroxide is decomposed / the same number of moles of reactant produces the same number of moles of oxygen, so the total volume of oxygen is the same

## Key terms

- [catalyst](https://www.gradenine.co.uk/glossary/catalyst)
- [rate of reaction](https://www.gradenine.co.uk/glossary/rate-of-reaction)
- [decomposition](https://www.gradenine.co.uk/glossary/decomposition)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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