Question

How will the equilibrium position of a gas-phase reaction be affected if the volume of the reaction vessel changes? Are there reactions that will not have their equilibria shifted by a change in volume? Explain. Why does changing the pressure in a rigid container by adding an inert gas not shift the equilibrium position for a gas-phase reaction?

Short answer

A decrease/increase in volume will move the equilibrium to the side with the smaller/larger number of gas molecules in the gas-phase reaction.

The volume of the rigid container is fixed. So, the addition of inert gas does not shift the equilibrium.

Step-by-step solution

How will the equilibrium position of gas-phase reaction be affected when volume changes

When the volume of a reactant or product changes, the partial pressure of all reactants and products changes by the same amount. The amount of gaseous particles on either side determines the shift in equilibrium. An increase in volume will move the equilibrium to the side of the gas phase with more particles. The side with fewer gas-phase particles will benefit from a reduction in volume. A change in volume will not move the equilibrium if the number of gas-phase particles on either side of the reaction is the same.

Why does changing the pressure does not shift the equilibrium position for gas-phase reaction

Because the capacity of the container does not change when we add an unreactive gas, we do not affect the partial pressures (or concentrations) of any of the substances in equilibrium with each other. The reaction is still in equilibrium if the partial pressures (and concentrations) remain constant.