# Explain why the enthalpy change of neutralisation for the reaction between ethanoic acid and sodium hydroxide is less exothermic than the enthalpy change of neutralisation between hydrochloric acid and sodium hydroxide.

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

## Mark scheme (4 marks)

1. Hydrochloric acid is a strong acid and is fully/completely dissociated into ions in aqueous solution, whereas ethanoic acid is a weak acid and is only partially dissociated.
2. The net ionic equation for a strong acid–strong base neutralisation is simply H⁺(aq) + OH⁻(aq) → H₂O(l), so all the energy released comes from bond formation in water.
3. When ethanoic acid reacts with NaOH, energy must first be absorbed/used to dissociate/ionise the ethanoic acid molecules before neutralisation can occur.
4. This energy absorbed for ionisation of the weak acid partially offsets the exothermic energy released when H⁺ and OH⁻ combine to form water, making the overall enthalpy change less negative (less exothermic).

## Key terms

- [enthalpy change of neutralisation](https://www.gradenine.co.uk/glossary/enthalpy-change-of-neutralisation)

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