A decrease in volume causes the reaction to move to decrease the total pressure by decreasing the number of molecules present. Since two molecules of reactant combine to form one molecule of product, the reaction will move in the forward direction.
A careful distinction must be made when deciding which direction a gaseous equilibrium will shift when adding an inert gas not taking part in the reaction. If in the above equilibrium (2), helium gas were injected at constant volume, there would be no shift in the equilibrium as the partial pressures of the two gases that are reacting would remain the same. However, if helium were injected into the same sealed vessel at constant pressure, the volume of the vessel must necessarily increase. Increasing the volume will shift the equilibrium to left toward the production of more gas molecules.
