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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When sodium hydroxide dissolves in water, the temperature of the solution rises noticeably.
Model answer (4 marks)
1. Dissolving NaOH(s) requires breaking the ionic lattice, which consumes energy (endothermic step). 2. Water molecules then hydrate the Na⁺ and OH⁻ ions, releasing energy (exothermic step). 3. The magnitude of the enthalpy of hydration exceeds that of the lattice enthalpy, so more energy is released than absorbed. 4. Consequently the overall enthalpy change of solution is negative, making the process exothermic and raising the temperature of the solution.
Examiner tips
- Use the word ‘exothermic’ and ‘endothermic’ to show understanding of energy flow.
- Show the comparison of magnitudes – lattice enthalpy vs hydration enthalpy – to justify the sign of ΔH.
- Link the negative ΔH to the observed temperature rise.
- Keep the answer concise, each point worth one mark.
Common mistakes
- Confusing lattice enthalpy with hydration enthalpy, or vice versa.
- Failing to state that the hydration step releases more energy than the lattice step absorbs.
- Using vague terms like ‘heat’ without specifying exothermic or endothermic.
Mark scheme (4 marks)
- Dissolving involves breaking the ionic lattice, which requires energy input (endothermic step).
- Water molecules hydrate the ions (Na⁺ and OH⁻), releasing energy (exothermic step).
- The enthalpy of hydration is greater in magnitude than the lattice enthalpy (more energy released in hydration than absorbed in breaking the lattice).
- 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 in this question
lattice enthalpy · enthalpy of hydration · exothermic
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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