In the early twentieth century, Fritz Haber developed a method for producing ammonia, as shown by the equation: $$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$$ (a) Ammonia is used as a cleaning agent - HSC - SSCE Chemistry - Question 24 - 2001 - Paper 1

Liquefying ammonia affects the yield of the reaction by increasing the concentration of ammonia in the liquid phase, which allows for greater production of ammonia through Le Chatelier's principle. When ammonia is liquefied, it leads to a higher partial pressure of ammonia in the equilibrium system, favoring the formation of products (NH₃) as the system shifts to relieve the pressure.

Monitoring temperature and pressure in the reaction vessel is essential for several reasons. Firstly, the reaction is exothermic; controlling the temperature helps to optimize the yield of ammonia while minimizing by-products. Secondly, the pressure affects the equilibrium position according to Le Chatelier's principle. Higher pressure generally shifts the equilibrium towards the formation of ammonia, therefore maximizing output. Additionally, monitoring these conditions ensures safety, as deviations can lead to unsafe reactions.