Explain why dissolving ammonium nitrate, NH₄NO₃(s), in water produces a solution that feels cold to the touch, and state what this indicates about the sign of ΔH for the dissolving process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When ammonium nitrate is dissolved in water, the mixture becomes noticeably cold. This property is exploited in instant cold packs used in sports medicine.
Model answer (4 marks)
The dissolution of NH₄NO₃(s) is an endothermic process; energy is absorbed from the surroundings (the water/solution) by the system. The energy required to break the ionic lattice (lattice enthalpy) is larger than the energy released when the ions are hydrated, so the net ΔH is positive. Because energy is taken from the water, its temperature falls, making the solution feel cold to the touch.
Examiner tips
- Use the word ‘endothermic’ and state that ΔH is positive; link this to the temperature drop.
- Explain the lattice vs hydration enthalpy to show why energy is absorbed.
- Mention that the surroundings lose thermal energy, giving the cold sensation.
Common mistakes
- Saying the process is exothermic or giving a negative ΔH.
- Failing to explain the lattice and hydration energy difference.
- Not linking the temperature drop to energy absorption from the surroundings.
Mark scheme (4 marks)
- The dissolving process is endothermic, meaning energy is absorbed FROM the surroundings (the water/solution) BY the system.
- Energy is taken from the water/solution, reducing the thermal energy (temperature) of the surroundings, which is why the solution feels cold.
- The energy required to break apart the ionic lattice (lattice enthalpy) exceeds the energy released when ions are hydrated (hydration enthalpy), giving an overall energy input.
- ΔH for the dissolving process is positive (ΔH > 0).
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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