Production & Use of Fertilisers (AQA GCSE Chemistry)

• Compounds containing nitrogen, potassium and phosphorus are used as fertilisers to increase crop yields
• NPK fertilisers are formulations containing appropriate ratios of all three elements
• From these three essential elements:
  • Nitrogen promotes healthy leaves,
  • Potassium promotes growth, healthy fruit and flowers
  • Phosphorus promotes healthy roots

• A distinct advantage of artificial fertilisers is that they can be designed for specific needs whereas in natural fertilizers, such seaweed or manure, the proportions of elements cannot be controlled
• Fertiliser compounds contain the following water soluble ions:
  • Ammonium ions, NH₄⁺ and nitrate ions, NO₃^–, which are sources of soluble nitrogen
  • Phosphate ions, PO₄^3-, which are a source of soluble phosphorus

• Most common potassium compounds dissolve in water to produce potassium ions, K⁺

• Ammonia is an alkaline substance and neutralises acids producing a salt and water
• The salt it produces contains the ammonium ion, NH₄⁺, which is a component of several fertilisers
• Ammonia also undergoes oxidation to produce nitric acid, HNO₃
• Nitric acid is used as the source of the nitrate ion, NO₃^–, which is another important ion found in fertilisers
• Ammonium nitrate, a fertiliser and one of the most important ammonium salts, is made by reacting ammonia with nitric acid:

NH₃ (aq) + HNO₃ (aq) → NH₄NO₃ (aq)

• It is prepared on large scale industrial proportions but can also be prepared in the laboratory using a different method
• In the laboratory it is prepared by titrating ammonia with sulfuric acid:

2NH₃ (aq) + H₂SO₄ (aq) → (NH₄)₂SO₄ (aq)

• The Earth’s crust also contains useful minerals which are useful raw materials for making fertilisers
• Phosphate rocks contain potassium chloride and potassium sulfate as they provide potassium
• The rock itself is insoluble in water to it is usually reacted with acid to produce useful compounds with water soluble compounds:
  • With nitric acid, phosphoric acid and calcium nitrate are produced
  • The phosphoric acid  is neutralised with ammonia forming ammonium phosphate
  • With sulfuric acid, a mixture of calcium phosphate and calcium sulfate is produced - this is know as single superphosphate
  • With phosphoric acid, calcium phosphate is produced - this is known as triple superphosphate