Question

For the reaction : $\left(\text{PCl}\right)_{\text{5}} \left(\text{g}\right) \rightleftharpoons \left(\text{PCl}\right)_{\text{3}} \left(\text{g}\right) \left(\text{+Cl}\right)_{\text{2}} \left(\text{g}\right) .$ The forward reaction at constant temperature is favoured by

• A. Introducing an inert gas at constant volume
• B. Introducing chlorine gas at constant volume
• C. Introducing an inert gas at constant pressure
• D. None of these

Answer 1

Answer: (C)

$ & \left(\text{PCl}\right)_{5} \left(\text{g}\right) & \rightleftharpoons & \left(\text{PCl}\right)_{3} \left(\text{g}\right) & + & \left(\text{Cl}\right)_{2} \left(\text{g}\right) \\ \text{mole at equilibrium} & \text{a} - \text{x} & & \text{x} & & \text{x}$
$\left(\text{K}\right)_{\text{C}} = \frac{\left(\text{x}\right)^{2}}{\left(\text{a} - \text{x}\right) \text{V}}$
Addition inert gas at constant V has no effect on reactions having $\Delta \text{n} = 0 \text{ or } \Delta \text{n} \neq 0$ . But addition of inert gas at constant P has effect on reactions having $\Delta \text{n} \neq 0$ ; and no effect if $\Delta \text{n} = 0$ . The given reaction has $\Delta \text{n} \neq 0 :$ on addition of inert gas at constant P, volume increases. To have K_C constant, x must increase.