Fertilisers and the chemical industry

The chemical industry turns raw materials into useful products on a huge scale. The Haber process makes ammonia from nitrogen and hydrogen; most of it becomes fertilisers. Because the reaction is reversible, its conditions are a compromise between yield, rate and cost. NPK fertilisers supply nitrogen, phosphorus and potassium to help crops grow. Sulfuric acid is made by the Contact process and nitric acid by the Ostwald process.

Key facts

  • Haber process: N2 + 3H2 ⇌ 2NH3 (exothermic). Nitrogen comes from the air and hydrogen from natural gas.
  • Conditions: about 450 °C, about 200 atmospheres and an iron catalyst. The ammonia is cooled to a liquid and removed; unreacted gases are recycled.
  • A lower temperature would give a higher yield but a slow rate; a higher pressure would give a higher yield but costs more and is more dangerous. The conditions are a compromise.
  • NPK fertilisers provide nitrogen (N), phosphorus (P) and potassium (K). Ammonia + nitric acid → ammonium nitrate; ammonia + sulfuric acid → ammonium sulfate.
  • Potassium chloride and potassium sulfate are mined. Phosphate rock is treated with nitric, sulfuric or phosphoric acid to make soluble phosphates.
  • Contact process: 2SO2 + O2 ⇌ 2SO3 with a vanadium(V) oxide catalyst, about 450 °C and 1–2 atmospheres.
  • Too much fertiliser washes into rivers and causes eutrophication: algae grow, oxygen runs out and fish die.

Worked examples

Example 1

Describe the Haber process and explain why a temperature of about 450 °C is used.

  1. Nitrogen and hydrogen are compressed to about 200 atmospheres and passed over an iron catalyst at about 450 °C.
  2. The forward reaction is exothermic, so a lower temperature would give more ammonia at equilibrium.
  3. But at a low temperature the reaction is too slow. About 450 °C gives a reasonable yield at a reasonable rate.
  4. The ammonia is cooled until it liquefies and is removed; the unreacted gases are recycled.

Answer: iron catalyst, 200 atm, 450 °C; a compromise between yield and rate

Example 2

Calculate the percentage by mass of nitrogen in ammonium nitrate, NH4NO3. (Aᵣ: N = 14, H = 1, O = 16)

  1. Find the Mᵣ. Mᵣ = 2 × 14 + 4 × 1 + 3 × 16 = 80
  2. There are two nitrogen atoms. 2 × 14 = 28
  3. Find the percentage. 28 ÷ 80 × 100 = 35%

Answer: 35%

Example 3

Why is a pressure of about 200 atmospheres used in the Haber process, rather than a much higher one?

  1. There are 4 molecules of gas on the left and 2 on the right.
  2. Higher pressure shifts the equilibrium to the right: more ammonia, and a faster rate.
  3. But very high pressures need stronger, more expensive equipment and use more energy, and are more dangerous. 200 atmospheres is a compromise.

Answer: higher pressure gives a higher yield, but costs more and is more dangerous