Chemistry - Thermodynamics - II Question with Solution | TestHub

Question Explanation: Calculate the standard boiling point using thermochemical data.

Concept: Phase transition at equilibrium ( $\Delta \mathrm{G}=0$ ).

Solution: $\Delta {\mathrm{H}}_{\text{vap }}=\Delta {\mathrm{H}}_{\mathrm{f}}^0\left[{\mathrm{PCl}}_3(\mathrm{g})\right]-\Delta {\mathrm{H}}_{\mathrm{f}}^0\left[{\mathrm{PCl}}_3(\mathrm{I})\right]=-287.0-(-319.7)=32.7\mathrm{kJ}/\mathrm{mol}=32700\mathrm{J}/\mathrm{mol}$. $\Delta {\mathrm{S}}_{\text{vap }}={\mathrm{S}}_{\mathrm{m}}^{\circ }\left[{\mathrm{PCl}}_3(\mathrm{g})\right]-{\mathrm{S}}_{\mathrm{m}}^{\circ }\left[{\mathrm{PCl}}_3(\mathrm{I})\right]=311.7-217.1=94.6\mathrm{J}/\mathrm{K}.\mathrm{mol}$.

At the boiling point, the liquid and gas phases are in equilibrium, so $\Delta \mathrm{G}=0$.

We can find the boiling point using ${\mathrm{T}}_{\mathrm{b}}=\frac{\Delta {\mathrm{H}}_{\text{vap }}}{\Delta {\mathrm{S}}_{\text{vap }}}=\frac{32700}{94.6}\approx 345.67\mathrm{K}$.

Final Answer: (345.67)