Inorganic Fertilisers

• Today, more than half of the world’s cereal production relies on inorganic fertilisers.
• Without fertiliser it would be near impossible to feed the world’s population.
• The raw materials used to produce inorganic fertilisers are water, air, natural gas, phosphate rock, potash rock and sulphur.
• There is a vast range of inorganic fertilisers; there are ammonium nitrate, ammonium sulphate, ammonium phosphate, urea and potassium nitrate fertilisers.
• At the core of fertiliser production is the synthesis of ammonia from nitrogen and hydrogen in the Haber Process.

The Haber Process

• In 1909, Fritz Haber successfully synthesised 100g of ammonia from nitrogen and hydrogen.
• Carl Bosch then scaled up his process and the first industrial plant opened in 1913, producing thirty tonnes per day.
• Modern ammonia plants work on the same principles as the original plants, but now produce in excess of one thousand five hundred tonnes per day.
• In the modern plants, the hydrogen is formed by reacting water with natural gas.
• Purified nitrogen from the air is then mixed with the hydrogen; the mixture is heated and passed over an iron catalyst forming ammonia:
  3H_2(g) + N_2(g) 2NH_3(g)     H = -92 kJ mol^-1
  
• The reaction is reversible; ammonia is being decomposed to form hydrogen and nitrogen at the same time as it is being produced; the reaction will eventually reach equilibrium.
• The equilibrium mixture depends upon the temperature and pressure chosen for the reaction.
• As can be seen from the reaction above, four moles of gaseous reactants (hydrogen and nitrogen) react to form two moles of gaseous product (ammonia). Using Le Chatelier’s Principle, which states that “a system at equilibrium, when subjected to a disturbance, responds in a way that tends to minimize the effect of the disturbance”, the maximum yield of ammonia can be obtained by subjecting the reaction to a high pressure.
• The forward direction of the reaction is exothermic, thus to obtain the maximum yield, the temperature must be low.
• However, high pressures are very expensive to achieve, and at low temperatures, the rate of the reaction will be very slow. Therefore, those reaction conditions which give the most economic production of ammonia are selected.
• A temperature of between 400-600^oC is selected; this is a compromise between the yield and the rate of the reaction; it is better to get moderate yields of ammonia rapidly than wait a long time for a higher yield.
• A pressure of between 70-200 atm. is selected; to achieve greater pressures would cost more money.
• Ammonia, due to the hydrogen bonding that occurs between its molecules, has a higher boiling point than nitrogen and hydrogen. Thus the ammonia can be liquefied and tapped off from the mixture as it is produced; this results in a greater yield of ammonia.

Useful books for revision