# Explain why the experimental value of the enthalpy change of neutralisation measured using a polystyrene cup calorimeter is typically less exothermic than the accepted theoretical value.

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

> A student mixes 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid with 50.0 cm³ of 1.00 mol dm⁻³ sodium hydroxide solution in a polystyrene cup and records the maximum temperature change to determine the enthalpy change of neutralisation.

## Mark scheme (4 marks)

1. Heat is lost from the reaction mixture to the surroundings during the experiment.
2. The polystyrene cup itself absorbs some heat from the reaction mixture.
3. Both of these factors result in a lower measured temperature rise than the true value.
4. The specific heat capacity of the solution is assumed to equal that of pure water (4.18 J g⁻¹ K⁻¹), which may not be accurate for an ionic solution, introducing further error.

## Key terms

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

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-experimental-value-of-0ef31d71) · Published by Druglandscape Ltd.