Entropy and spontaneous changeEdexcel International A Level Chemistry: Mind map

Exam questions on Entropy and spontaneous change

1. A gas jar containing brown nitrogen dioxide is placed beneath a gas jar of colourless air, and the lid between them is removed. After a few minutes both jars contain a uniform pale brown mixture. There is no measurable energy change during mixing.
   Explain, in terms of molecules and energy quanta, why the entropy of the system increases when the gases mix.2 marks
2. A student predicts the sign of the entropy change of the system, ΔS, for several changes at room temperature and pressure.
   Explain why the entropy of water increases when liquid water at 100 °C changes to steam at 100 °C.2 marks
3. A student dissolves citric acid and sodium hydrogencarbonate in water. The mixture fizzes and its temperature falls from 21 °C to 12 °C. The equation for the reaction is C₆H₈O₇(aq) + 3NaHCO₃(aq) → Na₃C₆H₅O₇(aq) + 3H₂O(l) + 3CO₂(g).
   Explain why this reaction occurs spontaneously even though it is endothermic.3 marks