Thermodynamics and Thermochemistry 1 Question 22
25. For the process, $\mathrm{H}{2} \mathrm{O}(l) \longrightarrow \mathrm{H}{2} \mathrm{O}(g)$ at $T=100^{\circ} \mathrm{C}$ and 1 atmosphere pressure, the correct choice is
(a) $\Delta S_{\text {system }}>0$ and $\Delta S_{\text {surrounding }}>0$
(2014 Adv.)
(b) $\Delta S_{\text {system }}>0$ and $\Delta S_{\text {surrounding }}<0$
(c) $\Delta S_{\text {system }}<0$ and $\Delta S_{\text {surrounding }}>0$
(d) $\Delta S_{\text {system }}<0$ and $\Delta S_{\text {surrounding }}<0$
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Solution:
- PLAN This problem is based on assumption that total entropy change of universe is zero.
At $100^{\circ} \mathrm{C}$ and 1 atmosphere pressure,
$$ \begin{aligned} & \qquad \mathrm{H}{2} \mathrm{O}(l) \rightleftharpoons \mathrm{H}{2} \mathrm{O}(g) \text { is at equilibrium. } \ & \text { For equilibrium, } \quad \Delta S_{\text {total }}=0 \ & \text { and } \Delta S_{\text {system }}+\Delta S_{\text {surrounding }}=0 \end{aligned} $$
As we know during conversion of liquid to gas entropy of system increases, in a similar manner entropy of surrounding decreases.
$\therefore \quad \Delta S_{\text {system }}>0$ and $\Delta S_{\text {surrounding }}<0$
