Formation of NH3 takes place according to Le-Chatelier's principle. N2(g)+3H2(g)⟶2NH3(g) Number of moles of reactant =1+3=4 Number of moles of product =2 As number of moles of product is less than number of moles of reactants. So, increase in pressure will favour forward direction. As NH3 is a gas, so decrease in temperature will favour forward direction. Hence, high pressure and low temperature are favourable conditions for formation of NH3.