The Haber process and NPK fertilisers (HT)
Plants need three main nutrients in large quantities: nitrogen (N), phosphorus (P), potassium (K). The Haber process makes ammonia (NH₃), which is the starting point for nitrogen-containing fertilisers.
The Haber process
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH negative (exothermic forward).
Sources of reactants
- Nitrogen from the air (78%).
- Hydrogen from natural gas (CH₄ + steam → CO + 3H₂).
Conditions
- Temperature: ~450 °C (compromise — see C6.5).
- Pressure: ~200 atm (favours fewer moles of gas → ammonia side).
- Catalyst: iron — speeds up reaction without changing equilibrium position.
Recycle
Unreacted N₂ and H₂ are cooled to liquefy out the NH₃, then recycled back into the reactor.
Why these conditions
Le Chatelier's principle applied:
- High pressure favours forward reaction (1 + 3 → 2 moles of gas; fewer moles on right).
- Low temperature favours forward (exothermic forward).
- BUT low T means slow rate. Use moderate T (450 °C) as a compromise: reasonable yield + reasonable rate.
- Iron catalyst speeds up reaction so it gets to equilibrium faster (doesn't change position).
NPK fertilisers
NPK fertilisers contain N, P and K in soluble compounds.
Nitrogen sources
- Ammonium nitrate (NH₄NO₃) — made from ammonia + nitric acid (HNO₃ from oxidation of NH₃ over Pt catalyst).
- Ammonium sulfate ((NH₄)₂SO₄) — from ammonia + sulfuric acid.
Phosphorus sources
- Mined as phosphate rock (Ca₃(PO₄)₂).
- Treated to give:
- Single superphosphate (Ca(H₂PO₄)₂ + CaSO₄) from phosphate rock + H₂SO₄.
- Triple superphosphate (Ca(H₂PO₄)₂) from phosphate rock + H₃PO₄.
- Ammonium phosphate ((NH₄)₃PO₄) — supplies both N and P.
Potassium sources
- Mined as potassium chloride (KCl) or potassium sulfate (K₂SO₄) — used directly.
Industrial vs lab scale
In the school lab, you can prepare ammonium sulfate by titrating NH₃ solution with H₂SO₄. Industrial scale uses giant reactors with continuous flow, recovery and recycling for high economy.
Why fertilisers matter
- Boost crop yields → feed growing population.
- BUT excess can pollute waterways (eutrophication: algae bloom → low O₂ → fish die).
⚠Common mistakes
- Forgetting the iron catalyst in Haber.
- Confusing 200 atm/450 °C with other equilibria.
- Saying "high T gives more product" — it reduces yield (forward is exothermic).
- Forgetting that catalyst doesn't shift equilibrium — only speeds reaching it.
Links
Builds on C6.5 (Le Chatelier), C6.3 (catalysts). Final HT topic — connects all of equilibrium and energy chemistry.
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