AQA GCSE Chemistry Topic 10: Using resources - The Haber process and the use of NPK fertilisers (chemistry only)

Used to manufacture ammonia, which is used to produce nitrogen-based fertilisers.
Raw materials: nitrogen and hydrogen.
Nitrogen is obtained from the air.
Hydrogen may be obtained from natural gas or other sources.
Purified gases are passed over a catalyst of iron at approximately 450 °C and a high pressure of about 200 atmospheres.
Some hydrogen and nitrogen react to form ammonia.
The reaction is reversible, so ammonia breaks down into nitrogen and hydrogen:
nitrogen + hydrogen \rightleftharpoons ammonia
On cooling, the ammonia liquefies and is removed.
Remaining nitrogen and hydrogen are recycled.
Reaction conditions and compromise:
- The Haber Process is in dynamic equilibrium, meaning the forward and backward reactions keep going once equilibrium is reached.
- Chemical equation: N2 + 3H2 \rightleftharpoons 2NH_3 . There are fewer moles of gas on the product side (2) compared to the reactant side (4).
  - Increasing pressure shifts the equilibrium to the right, producing more ammonia.
- The forward reaction is exothermic, so a low temperature would favor the forward reaction, meaning more ammonia would be produced.
- However, the actual conditions used are not low temperature and very high pressure because:
  - A low temperature leads to a reaction rate that is too slow.
  - A very high pressure requires too much energy.

Compounds of nitrogen, phosphorus, and potassium are used as fertilisers to improve agricultural productivity.
NPK fertilisers contain compounds of all three elements.
Industrial production of NPK fertilisers can be achieved using a variety of raw materials in several integrated processes.
NPK fertilisers are formulations of various salts containing appropriate percentages of the elements.
Ammonia can be used to manufacture ammonium salts and nitric acid.
Potassium chloride, potassium sulfate, and phosphate rock are obtained by mining, but phosphate rock cannot be used directly as a fertiliser.
Phosphate rock is treated with nitric acid or sulfuric acid to produce soluble salts that can be used as NPK fertilisers.

The Earth's crust is rich in minerals that serve as essential raw materials for creating fertilizers.
Phosphate rocks are a source of potassium chloride and potassium sulfate, which provide potassium.
Since the rock is insoluble in water, it is generally reacted with acid to create useful water-soluble compounds.
- Reacting the rock with nitric acid yields phosphoric acid and calcium nitrate.
  - The phosphoric acid is neutralised with ammonia, producing ammonium phosphate.
- Reacting the rock with sulfuric acid produces a mixture of calcium phosphate and calcium sulfate, known as single superphosphate.
- Reacting the rock with phosphoric acid results in calcium phosphate, referred to as triple superphosphate.