# Hydrogen peroxide decomposes to form water and oxygen. The activation energy for this reaction is high. A student adds manganese(IV) oxide to hydrogen peroxide and observes that the reaction proceeds rapidly at room temperature. Explain, in terms of bond breaking and bond forming, why the decomposition of hydrogen peroxide is exothermic, and explain how the manganese(IV) oxide increases the rate of reaction without being used up.

> AQA A-Level Chemistry (7405) — 3.1.8 Thermodynamics (A-Level only) · Explain · 5 marks

> Hydrogen peroxide, H₂O₂, decomposes according to the equation: 2H₂O₂(l) → 2H₂O(l) + O₂(g). The total energy required to break all bonds in the reactants is less than the total energy released when all bonds form in the products.

## Mark scheme (5 marks)

1. Energy is required to break bonds in the reactants (H₂O₂)
2. Energy is released when new bonds form in the products (H₂O and O₂)
3. The energy released forming bonds is greater than the energy required to break bonds, so the overall reaction is exothermic / releases energy to the surroundings
4. Manganese(IV) oxide is a catalyst, which provides an alternative reaction pathway with a lower activation energy
5. The catalyst is not used up / is chemically unchanged at the end of the reaction, so it can continue to catalyse further reactions

## Key terms

- [exothermic](https://www.gradenine.co.uk/glossary/exothermic)
- [bond breaking](https://www.gradenine.co.uk/glossary/bond-breaking)
- [bond forming](https://www.gradenine.co.uk/glossary/bond-forming)
- [activation energy](https://www.gradenine.co.uk/glossary/activation-energy)

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/hydrogen-peroxide-decomposes-to-form-water-5fdf62c8) · Published by Druglandscape Ltd.