Chapter 30: Haber Process and NPK Fertilisers

30.1-The Haber Process

Making ammonia

• Ammonia is an important industrial product used to make fertilisers, explosives and dyes. It is manufactured using the Haber process.
• This involves a reversible reaction between nitrogen and hydrogen:
  • N2(g) + 3H2(g) ⇌ 2NH3(g)
  • The reaction can reach a dynamic equilibrium.

In the Haber process:

1. Nitrogen (extracted from the air) and hydrogen (obtained from natural gas) are pumped through pipes
2. The pressure of the mixture of gases is increased to 200 atmospheres
3. The pressurised gases are heated to 450°C and passed through a tank containing an iron catalyst
4. The reaction mixture is cooled so that ammonia liquefies and can be removed
5. Unreacted nitrogen and hydrogen are recycled

Practice Problems:

• What are the industrial conditions in the Haber process and why
  • Pressure of 200 atmospheres
  • 450 degrees
• Catalyst used?
  • Iron
• Used to make?
  • Fertilizers, explosives and dyes

30.2-NPK Fertilisers

What are fertilisers?

• Fertilisers provide mineral ions needed for healthy growth in plants.
• As plants grow, they absorb mineral ions from the water in the soil through their root hair cells.
• Over time, the concentration of these ions decreases, so farmers and gardeners add fertilisers to the soil.

Nitrogen, Phosphorous and Potassium

• Fertilisers are formulations which may contain nitrogen, phosphorus and potassium compounds to promote plant growth.
• Fertilisers that supply all three elements are often called NPK fertilisers, after the chemical symbols for these three elements.
• Fertiliser compounds must be soluble in water so they can be absorbed by the root hair cells:
  • ammonium ions, NH4+, and nitrate ions, NO3-, are sources of soluble nitrogen
  • phosphate ions, PO43-, are a source of soluble phosphorus
• all common potassium compounds dissolve in water to produce potassium ions, K+

Practice Problems

• Suggest a source of potassium in an NPK fertilizer?
  • Potassium chloride and potassium sulphate can be mined and used as a source of potassium
• Explain the composition of the gases that leave the reactor?
  • Nitrogen and hydrogen are the reactants, ammonia is the useful product
  • The reaction is reversible so all three gases are present in the mixture
• Explain how ammonia is separated from the mixture of gases?
  • The mixture is cooled to -33.3 °C
  • So that ammonia condenses to a liquid / liquifies (and can be separated)
  • Whilst hydrogen and nitrogen remain as gases