Question

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. $2H\left(g\right)\to H_2\left(g\right)$

Answer 1

The correct option is D $2H\left(g\right)\to H_2\left(g\right)$

Entropy is a measure of randomness.

a) During evaporation, the state of water is changing from liquid to gas, thus the entropy increases.

b) When a gas expands at constant temperature, the constituent particles to move more freely and hence the entropy increases.

c) During sublimation, the physical state is changing from solid to gas, thus the randomness increases and accordingly entropy increases.

d) In the reaction, $2H\left(g\right)$ $⟶$$H_2\left(g\right)$,
two atoms of $H$ undergoes bond formation to give $H_2$ molecule which result in decrease in randomness. Hence, entropy change is negative.