Haber process

The Haber process is the industrial process used to manufacture ammonia, NH₃, by combining nitrogen and hydrogen over an iron catalyst. Malay: proses Haber · Chinese: 哈柏法.

The reaction is reversible and the forward reaction is exothermic: N₂ + 3H₂ ⇌ 2NH₃. The nitrogen is obtained from the air and the hydrogen usually from natural gas. Because four molecules of gas on the left become two on the right, a high pressure pushes the equilibrium toward ammonia and raises the yield, while the iron catalyst speeds the reaction up without changing the yield. A high temperature would speed the reaction but lower the yield of an exothermic reaction, so a moderately high temperature is chosen as a compromise: fast enough to be economical, cool enough to keep a useful yield. Unreacted nitrogen and hydrogen are recycled so little is wasted.

Example. The ammonia made this way is the starting point for nitrogenous fertilisers such as ammonium nitrate and urea, and for the manufacture of nitric acid. A plant liquefies and separates the ammonia from the unreacted gases by cooling, then returns those gases to the reactor.

Confusion to avoid. Do not confuse the Haber process with the Contact process: the Haber process makes ammonia, while the Contact process makes sulfuric acid. Remember too that pressure and temperature set the yield, but the catalyst changes only the rate.

A common exam task is to justify each chosen condition in turn: why a high pressure is used, why only a moderately high temperature is used, and why an iron catalyst is added, always stating that the catalyst affects the rate but not the yield. Being able to give those reasons cleanly, in the right order, is where the marks lie.

The Haber process is the second great case study of rate and equilibrium in industry, and comparing it with the Contact process is a frequent task in SPM Chemistry.