Total entropy change and feasibilityEdexcel International A Level Chemistry: Mind map

Exam questions on Total entropy change and feasibility

1. Instant cold packs contain solid ammonium nitrate and a sealed pouch of water. When the pouch is burst, the solid dissolves: NH₄NO₃(s) + aq → NH₄⁺(aq) + NO₃⁻(aq). The enthalpy change of solution is +25.7 kJ mol⁻¹ and the entropy change of the system is +108.5 J K⁻¹ mol⁻¹. The pack is used at 298 K and becomes noticeably cold as the solid dissolves.
   Calculate the total entropy change for the dissolving at 298 K and state what it shows about feasibility.2 marks
2. In the Haber process, nitrogen and hydrogen react to make ammonia: N₂(g) + 3H₂(g) → 2NH₃(g), ΔH = −92.0 kJ mol⁻¹. Standard molar entropies at 298 K are: N₂(g) 191.6, H₂(g) 130.6 and NH₃(g) 192.3 J K⁻¹ mol⁻¹.
   Explain, with a calculation of ΔSsurroundings at 298 K, why this reaction is feasible even though ΔSsystem is negative.2 marks
3. Limestone is heated in a kiln to make quicklime: CaCO₃(s) → CaO(s) + CO₂(g), ΔH = +178 kJ mol⁻¹. Standard molar entropies at 298 K are: CaCO₃(s) 92.9, CaO(s) 39.7 and CO₂(g) 213.6 J K⁻¹ mol⁻¹. Assume that ΔH and the entropy values do not change with temperature.
   Calculate ΔSsystem and ΔSsurroundings at 298 K for this reaction, and use them to show that the decomposition is not feasible at 298 K.3 marks