Question

Standard entropies of $X_2 ,Y_2$ and $XY_3$ are $60 , 40 $ and $ 50 \, JK^{-1} mol^{-1}$ respectively. For the reaction $ \frac{1}{2} X_2 + \frac{3}{2}Y_2 \rightleftharpoons XY_3, \Delta H = -30\, kJ$ to be at equilibrium, the temperature should be

• A. 500 K
• B. 750 K
• C. 1000 K
• D. 1250 K

Answer 1

Answer: (B)

$ \frac{1}{2} X_2 + \frac{3}{2} Y_2 \rightleftharpoons XY_3$
$\Delta S^{\circ} = \sum S^{\circ}_P - \sum S_R^{\circ} $
$ = 50 - (30 + 60)$
$ \Delta S^{\circ} = -40 \, JK^{-1} \, mol^{-1}$
$T = \frac{\Delta H^{\circ}}{\Delta S^{\circ}} = \frac{-30 \times 10^3 \, J \, mol^{-1}}{-40 \, JK^{-1} \, mol^{-1}} = 750 K$