17.3 - Making a Fertiliser Salt

Making a fertiliser salt from ammonia

• Ammonia dissolves in water to form an alkaline solution.
• Reacting ammonia with an acid neutralises it, forming an ammonium salt:
  • Ammonia + acid → ammonium salt
• The salt contains the ammonium ion (NH₄⁺).
• The acid used determines which salt is made.

Nitric acid gives ammonium nitrate

Ammonia + nitric acid → ammonium nitrate

NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)

Ammonium nitrate contains nitrogen from both the ammonia and the nitric acid, so it delivers a high percentage of nitrogen by mass.

Sulfuric acid gives ammonium sulfate

Ammonia + sulfuric acid → ammonium sulfate

2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

Two ammonia molecules react per molecule of sulfuric acid because sulfuric acid provides two hydrogen ions.

Why ammonium salts work as fertilisers

Ammonium salts are very soluble in water, so they dissolve in soil water and are easily absorbed by plant roots.

Nitric acid is made from ammonia

Nitric acid is made by reacting ammonia with oxygen and water, so ammonia is the starting material for both the ammonium and nitrate ions in ammonium nitrate.