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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Hydrochloric acid is a strong acid and is fully dissociated into H⁺ and Cl⁻ ions in aqueous solution, whereas ethanoic acid is a weak acid and is only partially dissociated.
The net ionic equation for a strong acid–strong base neutralisation is H⁺(aq)+OH⁻(aq)→H₂O(l), so all the energy released comes from bond formation in water.
When ethanoic acid reacts with NaOH, energy must first be absorbed to dissociate the CH₃COOH molecules before H⁺ can combine with OH⁻.
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).
The net ionic equation for a strong acid–strong base neutralisation is H⁺(aq)+OH⁻(aq)→H₂O(l), so all the energy released comes from bond formation in water.
When ethanoic acid reacts with NaOH, energy must first be absorbed to dissociate the CH₃COOH molecules before H⁺ can combine with OH⁻.
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).
Examiner tips
- Use the terms ‘strong acid’ and ‘weak acid’ to show understanding of dissociation. Show the net ionic equation for the strong acid–strong base case. Explain that ionisation of the weak acid requires energy. State that this energy reduces the magnitude of the exothermic enthalpy change.
Common mistakes
- Confusing the enthalpy change of neutralisation with the enthalpy of ionisation. Forgetting to mention that the weak acid is only partially dissociated. Using the wrong net ionic equation for the weak acid–strong base reaction.
Mark scheme (4 marks)
- 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.
- 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.
- When ethanoic acid reacts with NaOH, energy must first be absorbed/used to dissociate/ionise the ethanoic acid molecules before neutralisation can occur.
- 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 in this question
enthalpy change of neutralisation
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Measuring enthalpy changes questions
- Explain why Hess's law is used to determine the standard enthalpy change of form…
- Explain why the experimentally determined enthalpy change of combustion of ethan…
- Explain why the standard enthalpy change of combustion of propan-1-ol is more ne…
- A student dissolves 4.00 g of anhydrous copper(II) sulfate in 50.0 cm³ of water …