In general, an increase in pressure (decrease in volume) favours the net reaction that decreases the total number of moles of gases (the reverse reaction, in this case), and a decreases in pressure (increase in volume) favours the net reaction that increases the total number of moles of gases (here, the forward reaction). For reactions in which there is no change in the number of moles of gases, a pressure, (or volume) change has no effect on the position of equilibrium.