The formation of ammonia is an equilibrium reaction; N2(g) + 3 H2(g) <--> 2 NH3(g) the enthalpy of this reaction is negative. What can be done to increase the yield of products?
Using le Chatelier's principle that when a change occurs in a system at equilibrium the system will counteract the change to establish a new equilibrium we can answer this question. If the pressure is increased the equilibrium will shift to the right because the system will favour the side with fewer moles of gas. As the enthalpy is negative the forward reaction is exothermic so although increasing the temperature would increase rate of reaction it would also favour the reverse (endothermic) reaction so a lower temperature would favour the products. Removing the product as it is formed (decreasing the concentration of product) would favour the forward reaction as well.