Question

For which one of the following reactions, the entropy change is positive?

• A. $H _{2}(g)+\frac{1}{2} O _{2}(g) \longrightarrow H _{2} O (l)$
• B. $Na ^{+}(g)+ Cl ^{-}(g) \longrightarrow NaCl ( s )$
• C. $NaCl (l) \longrightarrow NaCl ( s )$
• D. $H _{2} O (l) \longrightarrow H _{2} O (g)$

Answer 1

Answer: (D)

Evaporation of water in an open vessel is a process which takes place by itself, by absorption of heat from the surroundings, because the gaseous water molecules are more random than the liquid water molecules.

$H _{2} O (l) \longrightarrow H _{2} O (g) ; \,\Delta H=+40.8\, kJ\, mol ^{-1}$

In other words, the process is spontaneous because it is accompanied by increase of entropy, which is further a measure of randomness or disorder of the system.