# Explain why the decomposition of calcium carbonate, CaCO₃(s) → CaO(s) + CO₂(g), is non-spontaneous at 298 K but becomes spontaneous at temperatures above approximately 840 °C.

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

## Mark scheme (4 marks)

1. The reaction is endothermic (ΔH > 0), so the enthalpy change opposes spontaneity.
2. Entropy increases (ΔS > 0) because a gas is produced from all-solid reactants, increasing the number/disorder of particles.
3. At low temperatures, the TΔS term is small, so ΔG (= ΔH − TΔS) remains positive, meaning the reaction is non-spontaneous.
4. At high temperatures, TΔS exceeds ΔH, making ΔG negative and the reaction spontaneous; the crossover occurs when T = ΔH/ΔS.

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