# A student investigates two chemical reactions, Reaction X and Reaction Y. In Reaction X, the temperature of the surroundings increases during the reaction. In Reaction Y, the temperature of the surroundings decreases during the reaction. Explain, in terms of bond breaking and bond making, why the overall energy change is different for each reaction, and state which reaction releases more energy when bonds form in the products than is required to break bonds in the reactants.

> OCR A-Level Chemistry A (H432) — 3.2 Physical chemistry · Explain · 5 marks

> Reaction X: magnesium burns in oxygen to form magnesium oxide. Reaction Y: citric acid dissolves and reacts in water, causing the solution to cool.

## Mark scheme (5 marks)

1. Reaction X is exothermic because energy is transferred to the surroundings / the temperature of the surroundings increases
2. Reaction Y is endothermic because energy is taken in from the surroundings / the temperature of the surroundings decreases
3. Bond breaking is an endothermic process (requires energy input)
4. Bond making is an exothermic process (releases energy)
5. In Reaction X (exothermic), more energy is released during bond making in the products than is required to break bonds in the reactants, so Reaction X releases more energy when bonds form

## Key terms

- [bond breaking](https://www.gradenine.co.uk/glossary/bond-breaking)
- [bond making](https://www.gradenine.co.uk/glossary/bond-making)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](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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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-investigates-two-chemical-reactions-85b9868d) · Published by Druglandscape Ltd.