The Haber process combines nitrogen and hydrogen over an iron catalyst to make ammonia: N2(g) + 3H2(g) ⇌ 2NH3(g), at about 450–500 °C and about 200 atmospheres.
The Haber process is the industrial method for manufacturing ammonia by combining nitrogen and hydrogen. Ammonia is the starting point for nitrogenous fertilisers, so the process is economically vital, and the syllabus expects you to know the balanced equation, the catalyst, the conditions and the raw materials. It is a set-piece question in the Manufactured Substances chapter of 4541.
The balanced equation
Nitrogen and hydrogen combine in a fixed ratio to form ammonia, and the reaction is reversible. With correct formulae and state symbols:
N2(g) + 3H2(g) ⇌ 2NH3(g)
One volume of nitrogen reacts with three volumes of hydrogen, a 1:3 ratio, to give two volumes of ammonia. The nitrogen is obtained from the fractional distillation of liquefied air, and the hydrogen is obtained from natural gas (methane) or other hydrocarbons. Because the reaction reaches equilibrium, only part of the nitrogen and hydrogen is converted on each pass, so the unreacted gases are recycled.
Conditions required
The operating conditions are an iron catalyst, a temperature of about 450–500 °C, and a pressure of about 200 atmospheres. The iron catalyst speeds up the reaction so that equilibrium is reached faster without being used up. The moderate temperature is a compromise: a lower temperature would give more ammonia at equilibrium but far too slowly, while the high pressure favours the side with fewer gas molecules, which is the ammonia side, and so increases the yield.
What is produced and its uses
The ammonia formed is a colourless gas with a sharp, pungent smell; it is cooled and liquefied so it can be separated from the unreacted nitrogen and hydrogen, which are returned to the reactor. Most ammonia is used to make nitrogenous fertilisers. Neutralising ammonia with an acid gives ammonium salts used as fertilisers:
NH3(aq) + HNO3(aq) → NH4NO3(aq)
2NH3(aq) + H2SO4(aq) → (NH4)2SO4(aq)
Ammonia is also used to make nitric acid and various cleaning agents.
Where it appears in the SPM exam
In 4541/1 you identify the catalyst, the ratio of gases, or the correct equation. In 4541/2 the standard question asks you to write the balanced equation with state symbols, name the iron catalyst, state the temperature and pressure, and give the sources of nitrogen and hydrogen. You are often asked to explain why the chosen conditions are used, and to name uses of ammonia, especially fertilisers such as ammonium nitrate, ammonium sulfate and urea.
How we teach it
Our teachers make sure you get the 1:3 ratio right, name iron (not vanadium(V) oxide, which belongs to the Contact process) as the catalyst, and remember that the reaction is reversible. Students most often lose marks by confusing the two industrial processes, by leaving out the state symbols, or by forgetting where the raw gases come from. Learning the two processes side by side, each with its own catalyst and product, keeps them clearly separated in your memory.
Quick summary
The Haber process makes ammonia by combining nitrogen and hydrogen in a 1:3 ratio over an iron catalyst at about 450–500 °C and about 200 atmospheres, in a reversible reaction. Learn the equation with state symbols, the catalyst and conditions, the sources of the gases, and the fertiliser uses of ammonia, and this dependable industrial topic is fully covered.
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