10.4 The Haber process and the use of NPK fertilisers (Triple)

• The purified H2 and N2 gases are passed over an iron catalyst at a high temperature (about 450 °C) and a high pressure (about 200 atm) • The iron catalyst speeds up the rate of reaction, so that a lower temperature could be used in the process. • Some of the hydrogen and nitrogen react to form ammonia. N2+ 3 H2 ⇌ 2 NH3 • The reaction is reversible so ammonia breaks down again into nitrogen and hydrogen. • On cooling, the ammonia liquefies and is removed. The remaining hydrogen and nitrogen are recycled. This means almost no material is wasted. • Ammonia is used for production of nitrogen-containing fertilisers.

The conditions are a compromise between rate and the yield: ● The reaction is exothermic. An optimum temperature of 450 °C is used. Using a lower temperature would give a higher yield, but the rate of NH3 production would be too slow. ● A pressure of 200 atm is used. Using a higher pressure would give a higher yield, but would be too expensive, because of the cost of energy to produce the high pressure.

Compounds of nitrogen, phosphorus and potassium are used as fertilisers to improve agricultural productivity. NPK fertilisers contain compounds of all three elements.

Ammonia can be used to manufacture ammonium salts. The ammonium sulfate, phosphate, and nitrate can be produced by reaction of ammonia with the requisite acid. 2 NH3+ H2SO4→ (NH4)2SO4 2 NH3+ H3PO4 → (NH4)3PO4 NH3+ HNO3→ NH4NO3

● Phosphate rock is reacted with nitric acid to produce phosphoric acid and calcium nitrate. ● Phosphate rock can be reacted with sulfuric acid to produce a mixture of calcium phosphate and calcium sulfate ● Phosphate rock can be reacted with phosphoric acid to produce calcium phosphate.