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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
1. Heat from the reaction mixture is lost to the surroundings during the experiment.
2. The polystyrene cup absorbs some of the heat released by the neutralisation.
3. Both of these factors give a lower temperature rise than the true value, so the calculated enthalpy change is less exothermic.
4. The calculation assumes the solution’s specific heat capacity is 4.18 J g⁻¹ K⁻¹ (that of pure water), which may not be accurate for an ionic solution, adding further error.
2. The polystyrene cup absorbs some of the heat released by the neutralisation.
3. Both of these factors give a lower temperature rise than the true value, so the calculated enthalpy change is less exothermic.
4. The calculation assumes the solution’s specific heat capacity is 4.18 J g⁻¹ K⁻¹ (that of pure water), which may not be accurate for an ionic solution, adding further error.
Examiner tips
- Use the command word ‘Explain’ – give reasons, not just facts. Mention heat loss, cup absorption and the specific‑heat assumption. Keep each point concise and directly linked to the marks scheme.
Common mistakes
- Assuming the measured temperature rise equals the true rise without noting heat loss. Using the wrong specific heat value or not recognising it’s an assumption. Omitting the role of the polystyrene cup as a heat sink.
Mark scheme (4 marks)
- Heat is lost from the reaction mixture to the surroundings during the experiment.
- The polystyrene cup itself absorbs some heat from the reaction mixture.
- Both of these factors result in a lower measured temperature rise than the true value.
- 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 in this question
enthalpy change of neutralisation · polystyrene cup calorimeter
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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