Ammonium Nitrate

Ammonium nitrate (NH4NO3) is a widely used nitrogen fertiliser and the compound that connects the Haber process to agricultural and environmental chemistry.

Key facts

Property	Value
Formula	NH4NO3
Mr	14 + 4(1) + 14 + 3(16) = 80
Bonding	Ionic — NH4+ and NO3- ions
Nitrogen content	2(14)/80 = 35% by mass
Key property	Very soluble; dissolving is endothermic (cold packs)
Main use	Nitrogen fertiliser

Where ammonium nitrate appears in 0620

• Fertilisers: production from ammonia and nitric acid, and the role of nitrogen in plants.
• Air quality and climate change / water pollution: eutrophication caused by fertiliser runoff.
• Preparation of salts: an ammonium salt made by neutralisation.
• Tests for ions: the ammonium-ion test (warm with NaOH -> ammonia).

Production

NH3(g) + HNO3(aq) -> NH4NO3(aq)

This is a neutralisation: ammonia (a base) reacts with nitric acid. Evaporating the solution gives white crystals. Industrially the ammonia is from the Haber process and the acid from the Ostwald process, so the whole route begins with nitrogen from the air.

As a fertiliser

Plants need nitrogen for proteins and chlorophyll. Ammonium nitrate is a straight nitrogen fertiliser (not an NPK fertiliser — those are blends that also contain phosphorus and potassium). Its value is that it carries nitrogen in two absorbable forms:

• NH4+ (ammonium)
• NO3- (nitrate)

Its 35% nitrogen by mass is a common calculation: mass of nitrogen (2 x 14 = 28) divided by Mr (80), then x100.

Environmental concern: eutrophication

Excess fertiliser washed into rivers and lakes (runoff) causes:

1. Nitrate levels in the water rise.
2. Algae grow rapidly (algal bloom) and cover the surface.
3. Light is blocked, so aquatic plants die.
4. Bacteria decompose the dead plants and algae.
5. The bacteria use up dissolved oxygen.
6. Fish and other organisms die from lack of oxygen.

Identifying ammonium nitrate

• Ammonium ion (NH4+): warm with aqueous sodium hydroxide. Ammonia gas is released, turning damp red litmus paper blue: NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l)
• Nitrate ion (NO3-): warm with sodium hydroxide and aluminium foil, which reduces nitrate to ammonia. Note that in ammonium nitrate the ammonium ion already gives off ammonia with NaOH alone, so this test cannot be used on the salt by itself.

Heating ammonium nitrate decomposes it (NH4NO3 -> N2O + 2H2O at moderate temperatures, becoming explosive when heated strongly), which is why it is stored and transported with care. This decomposition is context, not a required 0620 equation.

Explain how the overuse of ammonium nitrate fertiliser can lead to the death of fish in a nearby river. (4 marks)

Mark scheme

• Excess fertiliser is washed off the fields into the river by rain (runoff / leaching) [1]
• Nitrates cause rapid growth of algae, forming an algal bloom that blocks light so water plants die [1]
• Bacteria decompose the dead plant material and algae [1]
• The bacteria use up the dissolved oxygen in the water, so the fish die from lack of oxygen [1]

Examiner note: the mark for the killing step is the oxygen, not the algae. Answers that stop at “algae kill the fish” or “the water is poisoned” miss the point — it is the bacterial decomposition removing dissolved oxygen that is fatal.

Common exam mistakes

• Calling ammonium nitrate an NPK fertiliser. It supplies nitrogen only; NPK fertilisers are mixtures that also contain phosphorus and potassium.
• Stating that dissolving is exothermic. Ammonium nitrate dissolving is endothermic (the temperature falls, hence its use in cold packs) — the opposite of sodium hydroxide.
• In eutrophication, writing that the algae or fertiliser directly poison the fish. The oxygen is removed by bacteria decomposing dead material; that is what kills the fish.
• Miscounting nitrogen when finding the percentage: NH4NO3 contains two nitrogen atoms, so mass of N is 28, giving 35%, not 17.5%.