The Haber Process T H
The Haber process manufactures ammonia (NH3), the starting point for nitrogen fertilisers. It’s a classic reversible reaction run at carefully chosen compromise conditions — and explaining why those conditions are used is Higher-tier gold dust.
N₂ comes from the air; H₂ from natural gas (methane).
- Raw materials: nitrogen from the air; hydrogen from natural gas (methane).
- Conditions: iron catalyst, about 450 °C, about 200 atmospheres.
- The reaction is reversible: N2(g) + 3H2(g) ⇌ 2NH3(g).
- Separation: on cooling, the ammonia liquefies (it has a higher boiling point) and is removed; the unreacted N2 and H2 are recycled.
The forward reaction is exothermic and goes from 4 gas molecules to 2. So:
- Temperature (450 °C). A lower temperature would give a higher yield (equilibrium shifts to the exothermic forward reaction) but the rate would be too slow. 450 °C is a compromise — a reasonable yield produced at a reasonable rate.
- Pressure (200 atm). A higher pressure gives a higher yield (equilibrium shifts to the side with fewer gas molecules) but very high pressures are expensive and dangerous. 200 atm is a compromise between yield and cost/safety.
- Iron catalyst. Speeds up the reaction so equilibrium is reached faster, but does not change the position of equilibrium (the yield). It lets a good rate be achieved at the lower, cheaper temperature.
Find the compromise — rate vs yield
Change the temperature and pressure and watch the yield of ammonia and the rate of reaction respond. There is no setting that wins on both — that is why the industrial conditions are a compromise.
N2(g) + 3H2(g) ⇌ 2NH3(g) · forward is exothermic, 4 gas molecules → 2
- Forgetting it’s reversible. Use the ⇌ sign — not all the N2 and H2 turn into ammonia in one pass, which is why the gases are recycled.
- Why ammonia is removed by cooling. Ammonia has a higher boiling point than N2 and H2, so it condenses to a liquid while they stay gases.
- The catalyst doesn’t change the yield — it only makes equilibrium arrive faster.
🧪 Exam-style questions
Name the source of the nitrogen and the source of the hydrogen used in the Haber process.
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- Nitrogen comes from the air. 1 mark
- Hydrogen comes from natural gas (methane). 1 mark
Which conditions are used in the Haber process? Tick (✓) one box.
The reaction mixture is cooled so that only the ammonia condenses. Why does only the ammonia condense? Tick (✓) one box.
What happens to the unreacted nitrogen and hydrogen? Tick (✓) one box.
A lower temperature would increase the yield of ammonia. Explain why a temperature of about 450 °C is used in the Haber process instead.
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- The forward reaction is exothermic, so a lower temperature shifts the equilibrium to the right, giving a higher yield of ammonia. 1 mark
- But at a lower temperature the rate of reaction is too slow, so equilibrium is reached too slowly to be economic. 1 mark
- So 450 °C is a compromise — a reasonable yield obtained at a reasonable rate. 1 mark
What is the effect of the iron catalyst on the yield of ammonia? Tick (✓) one box.
NPK Fertilisers T
The ammonia from the Haber process feeds into fertilisers — the products that keep farm soils productive. NPK fertilisers supply the three elements plants need most: nitrogen (N), phosphorus (P) and potassium (K).
- They contain compounds of nitrogen, phosphorus and potassium, used to improve agricultural productivity.
- An NPK fertiliser is a formulation — a mixture of salts blended to give the right percentage of each element.
- The compounds must be water-soluble so plants can absorb the ions through their roots: ammonium (NH4+) and nitrate (NO3–) for nitrogen, phosphate (PO43–) for phosphorus, and potassium ions (K+).
Ammonia is the key building block:
- Ammonia is an alkali, so it neutralises acids to make ammonium salts (C4). With nitric acid it makes the important fertiliser ammonium nitrate:
NH3 + HNO3 → NH4NO3
- Ammonia can also be oxidised to make nitric acid (the source of the nitrate).
- Lab vs industrial: in the lab, a chemist makes a pure batch by titrating ammonia with acid (C4); industrially it’s a large-scale, continuous integrated process producing huge quantities.
Potassium chloride, potassium sulfate and phosphate rock are obtained by mining. Phosphate rock is insoluble, so it can’t be used directly — it’s treated with acid to make soluble salts:
| Treat phosphate rock with… | Products |
|---|---|
| nitric acid | phosphoric acid + calcium nitrate |
| sulfuric acid | single superphosphate (calcium phosphate + calcium sulfate) |
| phosphoric acid | triple superphosphate (calcium phosphate) |
- An NPK fertiliser is a formulation (a mixture) — not a single compound.
- Fertiliser compounds must be soluble, or the plant can’t take them up — which is exactly why insoluble phosphate rock has to be reacted with acid first.
- Lab vs industry: the lab method (titration) makes small, pure batches; industry runs a continuous, large-scale process.
🧪 Exam-style questions
State the three elements that the letters N, P and K stand for in an NPK fertiliser.
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Nitrogen (N), phosphorus (P) and potassium (K) — the three elements plants need most. 1 mark
Ammonia reacts with nitric acid to make an important fertiliser. Name this fertiliser.
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Ammonium nitrate. 1 mark (NH3 + HNO3 → NH4NO3, a key nitrogen fertiliser.)
Why must the compounds in a fertiliser be soluble in water? Tick (✓) one box.
Phosphate rock is reacted with sulfuric acid. What is the product called? Tick (✓) one box.