In this reaction, the forward reaction is accompanied by a decrease in the total number of moles of reactants. If the pressure of the system is increased, then according to the Le-Chatelier's principle, the equilibrium will shift in that direction where pressure decreases i.e., decrease in number of moles taken i.e., in the favour of formation of ammonia. Thus, higher the pressure, better is the yield of ammonia. If pressure is decreased, the equilibrium will shift in the direction where pressure increases i.e., increase in number of moles occur i.e., in backward direction. So, a decrease in pressure will form a dissociation of NH3 to form N2 and H2.
