# Explain why the enthalpy change of solution measured when sodium hydroxide, NaOH(s), dissolves in water is exothermic, using the concepts of lattice enthalpy and enthalpy of hydration.

> IB DP Chemistry Standard Level (2023 syllabus) — R1.1 Measuring enthalpy changes · Explain · 4 marks

> When sodium hydroxide dissolves in water, the temperature of the solution rises noticeably.

## Mark scheme (4 marks)

1. Dissolving involves breaking the ionic lattice, which requires energy input (endothermic step).
2. Water molecules hydrate the ions (Na⁺ and OH⁻), releasing energy (exothermic step).
3. The enthalpy of hydration is greater in magnitude than the lattice enthalpy (more energy released in hydration than absorbed in breaking the lattice).
4. Because more energy is released than absorbed overall, the enthalpy change of solution is negative / exothermic, causing the temperature of the solution to rise.

## Key terms

- [lattice enthalpy](https://www.gradenine.co.uk/glossary/lattice-enthalpy)
- [enthalpy of hydration](https://www.gradenine.co.uk/glossary/enthalpy-of-hydration)
- [exothermic](https://www.gradenine.co.uk/glossary/exothermic)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](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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