# Explain why the dissolution of ammonium nitrate in water, a process that is endothermic, is nonetheless spontaneous at room temperature.

> IB DP Chemistry Higher Level (2023 syllabus) — R1.4 Entropy and spontaneity (HL only) · Explain · 4 marks

> When ammonium nitrate dissolves in water, the temperature of the solution decreases noticeably, yet the process proceeds without any external energy input.

## Mark scheme (4 marks)

1. A spontaneous process requires the total Gibbs energy change (ΔG) to be negative.
2. The dissolution results in a large increase in entropy (positive ΔS) because ions and water molecules become more disordered when the ionic lattice disperses into solution.
3. Using ΔG = ΔH − TΔS: because ΔH is positive (endothermic) but TΔS is also positive and sufficiently large, the TΔS term dominates.
4. Therefore, the entropy-driven decrease in Gibbs energy provides the thermodynamic driving force, making the process spontaneous even though it is endothermic.

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