Ammonia

Ammonia (NH3) links industrial chemistry (the Haber process), acid-base chemistry (a weak alkali), and agriculture (fertilisers). It is the only common alkaline gas.

Key facts

Property	Value
Formula	NH3
Mr	14 + 3(1) = 17
Bonding	Covalent molecule — 3 N-H bonds and 1 lone pair (pyramidal shape)
Key property	Pungent gas, less dense than air, very soluble, weak alkali
Gas test	Turns damp red litmus paper blue
Main uses	Making nitric acid and ammonium fertilisers

Where ammonia appears in 0620

• Reversible reactions and equilibrium: the Haber process and its conditions.
• Fertilisers: reacting ammonia with acids to make ammonium salts.
• Acids, bases and indicators: the reference weak alkali.
• Tests for ions and gases: the ammonia gas test and the ammonium-ion test.

The Haber process

N2(g) + 3H2(g) ⇌ 2NH3(g) (exothermic, delta H = -92 kJ/mol)

Condition	Value	Reason
Temperature	450 C	Compromise: a lower temperature raises yield but lowers rate
Pressure	200 atm	Favours the ammonia side (fewer gas molecules), raising yield
Catalyst	Iron	Speeds up the reaction without changing the equilibrium position

Nitrogen is from the fractional distillation of liquid air; hydrogen is from methane. See equilibrium and Le Chatelier.

Key reactions

With acids (making fertilisers)

NH3(g) + HCl(g) -> NH4Cl(s) (white smoke of ammonium chloride)

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

2NH3(aq) + H2SO4(aq) -> (NH4)2SO4(aq)

These make ammonium salts used as fertilisers.

Dissolved in water (a weak alkali)

NH3(g) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

Only partly ionised, giving pH about 10-11. In cation tests, aqueous ammonia behaves like NaOH but redissolves the copper(II) hydroxide to a deep blue solution and the zinc hydroxide to a colourless one.

The ammonium-ion test

NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l)

Warm any ammonium salt with NaOH; the ammonia released turns damp red litmus blue.

Uses of ammonia

• Making nitric acid: ammonia is oxidised over a platinum catalyst (the Ostwald process).
• Making ammonium salt fertilisers such as ammonium nitrate and ammonium sulfate.
• In household cleaning products, as a solution.

The Haber process is carried out at a pressure of about 200 atm. Explain, in terms of the equilibrium, why a high pressure increases the yield of ammonia, and give one reason why an even higher pressure is not used. (3 marks)

Mark scheme

• There are 4 moles of gas on the left (N2 + 3H2) and only 2 moles on the right (2NH3) [1]
• Increasing the pressure shifts the equilibrium towards the side with fewer gas molecules (the ammonia side), so the yield increases [1]
• An even higher pressure is not used because the plant and pipework would be too expensive and dangerous to build and operate [1]

Examiner note: state the mole numbers (4 becoming 2) to secure the equilibrium mark — “high pressure pushes it right” without that comparison is not enough. The limit on pressure is cost and safety, not the chemistry.

Common exam mistakes

• Testing ammonia with damp blue litmus. Ammonia is alkaline, so it turns damp red litmus blue; blue litmus would stay blue and prove nothing.
• Saying the Haber process goes to completion. It is reversible and reaches equilibrium (roughly 15% ammonia here); the unreacted nitrogen and hydrogen are recycled.
• Calling ammonia a strong alkali. It is a weak alkali — only partly ionised (NH3 + H2O ⇌ NH4+ + OH-).
• Getting the ratio wrong for ammonium sulfate. Two ammonia molecules are needed: 2NH3 + H2SO4 -> (NH4)2SO4.