2019 ThermodynamicsEntropy and Spontaneity Easy Question 1 of 2 in Entropy and Spontaneity

In which case change in entropy is negative?

A Evaporation of water
B Expansion of a gas at constant temperature
C Sublimation of solid to gas
D $2H(g)\rightarrow H_2(g)$

The answer is D.

Solution

Evaporation of water: $H_2O(l)\rightarrow H_2O(v)$, $\Delta S>0$ Expansion of a gas at constant temperature: $\Delta S>0$ Sublimation of solid to gas: $\Delta S>0$ $2H(g)\rightarrow H_2(g)$: $\Delta S<0$ (since $\Delta n_g<0$)

Concept sign of entropy change

Entropy and Spontaneity in past papers

2 questions from this chapter have appeared across 2 exam years.