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:
- Nitrogen (extracted from the air) and hydrogen (obtained from natural gas) are pumped through pipes
- The pressure of the mixture of gases is increased to 200 atmospheres
- The pressurised gases are heated to 450°C and passed through a tank containing an iron catalyst
- The reaction mixture is cooled so that ammonia liquefies and can be removed
- 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?
- 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
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