Entropy and spontaneityEdexcel A-Level Chemistry: Mind map

Exam questions on Entropy and spontaneity

1. In a demonstration, solid barium hydroxide octahydrate is mixed with solid ammonium chloride in a flask. The mixture becomes a liquid, ammonia gas is given off and the flask becomes so cold that it freezes a wet wooden block to the bench. The reaction is Ba(OH)₂·8H₂O(s) + 2NH₄Cl(s) → BaCl₂(s) + 2NH₃(g) + 10H₂O(l). It happens spontaneously at room temperature even though it is endothermic.
   Explain, in terms of disorder, why the entropy of the system increases in this reaction.2 marks
2. The Haber process for making ammonia, N₂(g) + 3H₂(g) → 2NH₃(g), is exothermic with ΔH = −92 kJ mol⁻¹. A student uses it to practise calculating the entropy change of the surroundings, and assumes that ΔH does not change with temperature.
   Explain why the entropy change of the surroundings is positive for an exothermic reaction.2 marks
3. Calcium carbonate decomposes on heating: CaCO₃(s) → CaO(s) + CO₂(g). The enthalpy change is ΔH = +178 kJ mol⁻¹. Standard entropies in J K⁻¹ mol⁻¹: CaCO₃(s) 92.9; CaO(s) 39.7; CO₂(g) 213.6.
   Calculate the entropy change of the system, ΔS system, for the decomposition.3 marks