Question

Consider the following reactions:
(i) $PC{l}_5\left(g\right)\rightleftharpoons PC{l}_3\left(g\right)+C{l}_2\left(g\right)$
(ii) $N_2{O}_4\left(g\right)\rightleftharpoons 2N{O}_2\left(g\right)$
The addition of an inert gas at constant volume:

• A. Will increase the dissociation of $PC{l}_5$ as well as $N_2{O}_4$
• B. Will reduce the dissociation of $PC{l}_5$ as well as $N_2{O}_4$
• C. Will increase the dissociation of $PC{l}_5$ and step up the formation of $N{O}_2$
• D. Will not disturb the equilibrium of both the reactions

Answer 1

The correct option is D Will not disturb the equilibrium of both the reactions

When an inert gas is added to the system in equilibrium at constant volume, the total pressure will increase. But the concentrations of the products and reactants (i.e. ratio of their moles to the volume of the container) will not change.
Hence, when an inert gas is added to the system in equilibrium at constant volume there will be no effect on the equilibrium.


Theory:

Addition of inert gas :
Inert gas molecules do not interact with reactant or product molecules. So addition of inert gas increases the total no. of moles in the system.
Effect of adding inert gas at equilibrium:
(i) For reactions in which $n_p=n_r$, there is no effect of adding an inert gas at constant volume or at constant pressure on the equilibrium as partial pressure of reactant and products do not change on addition of inert gas .

(ii) For reaction in which $n_p>n_r$ (e.g.$PC{l}_5\left(g\right)\rightleftharpoons PC{l}_3\left(g\right)+C{l}_2\left(g\right)$, there is no effect of adding inert gas on the equilibrium at constant volume but at constant pressure reaction will go in that direction, which has higher number of stoichiometric moles (gaseous) i.e in this case forward direction.