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

For which one of the following reactions, the entropy change is positive?
  1. $\mathrm{H}_2(g)+\frac{1}{2} \mathrm{O}_2(g) \longrightarrow \mathrm{H}_2 \mathrm{O}(l)$
  2. $\mathrm{Na}^{+}(g)+\mathrm{Cl}^{-}(g) \longrightarrow \mathrm{NaCl}(s)$
  3. $\mathrm{NaCl}(I) \longrightarrow \mathrm{NaCl}(\mathrm{s})$
  4. $\mathrm{H}_2 \mathrm{O}(l) \longrightarrow \mathrm{H}_2 \mathrm{O}(\mathrm{g})$

Solution

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. $ \mathrm{H}_2 \mathrm{O}(l) \longrightarrow \mathrm{H}_2 \mathrm{O}(g) ; \quad \Delta H=+40.8 \mathrm{~kJ} \mathrm{~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

Asked in: AP EAMCET 2013

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