The Haber process: raw materials and equation
The Haber process makes ammonia (NH3).
- Nitrogen is taken from the air (air is 78% nitrogen).
- Hydrogen is usually made from natural gas (methane) reacting with steam.
The gases are purified and mixed in the ratio 1 N2 : 3 H2.
nitrogen + hydrogen ⇌ ammonia
N2(g) + 3H2(g) ⇌ 2NH3(g)
The ⇌ sign means the reaction is reversible: ammonia can break back into nitrogen and hydrogen. The forward reaction is exothermic (it gives out heat).
Choosing the conditions: a compromise
Pressure: about 200 atmospheres
On the left there are 4 molecules of gas; on the right only 2. Higher pressure pushes the equilibrium to the side with fewer gas molecules, so the yield of ammonia goes up. Higher pressure also makes particles collide more often, so the rate goes up. But very high pressure needs thick, costly pipes and more energy, and is more dangerous. So about 200 atm is used.
Temperature: about 450 °C
The forward reaction is exothermic. A lower temperature gives a higher yield, but the reaction becomes too slow. A higher temperature is faster but gives a lower yield. About 450 °C is a compromise: a reasonable yield made quickly.
Catalyst: iron
An iron catalyst speeds up both the forward and backward reactions. It does not change the yield, but equilibrium is reached much faster.
Removing ammonia and recycling gases
Only about 15–30% of the gases turn into ammonia in one pass. The hot mixture leaving the reactor is cooled. Ammonia has a much higher boiling point than nitrogen and hydrogen, so it condenses into a liquid and is removed. The unreacted nitrogen and hydrogen are recycled back to the reactor. In the end about 97% of the gas is turned into ammonia.
Production and uses of NPK fertilisers
Plants need three main elements to grow well: N nitrogen (leaves and growth), P phosphorus (roots) and K potassium (flowers, fruit and health). NPK fertilisers are mixtures of salts that supply these in the right amounts. They improve crop yields.
- Nitrogen: ammonia is reacted with acids. Ammonia + nitric acid → ammonium nitrate. Ammonia + sulfuric acid → ammonium sulfate. (The nitric acid itself is made from ammonia.)
- Phosphorus: phosphate rock is mined. It does not dissolve, so it is reacted with acid. With nitric acid it gives phosphoric acid and calcium nitrate; with sulfuric acid it gives single superphosphate; with phosphoric acid it gives triple superphosphate. Phosphoric acid + ammonia → ammonium phosphate.
- Potassium: potassium chloride and potassium sulfate are mined and can be used directly.
In industry this is done on a huge scale in continuous plants. In a school lab you can make ammonium sulfate by titrating ammonia solution with sulfuric acid and then crystallising.
Try it: read a fertiliser bag
Look at a fertiliser packet (garden shop or a farmer's bag). Find the three numbers, for example 19:19:19 or 10:26:26. They are the N, P and K percentages. Which nutrient is highest? Which crop or stage of growth might it be for?
Key formulas and definitions
- N₂ + 3H₂ ⇌ 2NH₃ (forward reaction exothermic)
- Conditions: ~200 atm, ~450 °C, iron catalyst
- NH₃ + HNO₃ → NH₄NO₃ (ammonium nitrate)
- 2NH₃ + H₂SO₄ → (NH₄)₂SO₄ (ammonium sulfate)
- NPK = nitrogen, phosphorus, potassium
Worked examples
1. Why does a higher pressure increase the yield of ammonia?
There are 4 molecules of gas on the left (1 N₂ + 3 H₂) and 2 on the right (2 NH₃). Increasing pressure shifts the equilibrium to the side with fewer gas molecules, which is the ammonia side.
2. How many moles of hydrogen are needed to make 10 mol of ammonia?
From N₂ + 3H₂ → 2NH₃, 3 mol H₂ make 2 mol NH₃. So for 10 mol NH₃: 10 × 3/2 = 15 mol H₂.
3. A farmer's crop has weak roots. Which nutrient is lacking, and which compound could supply it?
Phosphorus (P). Ammonium phosphate or superphosphate (made from phosphate rock and acid) would supply it.
4. Explain why 450 °C is called a compromise temperature.
The forward reaction is exothermic, so a low temperature gives a higher yield, but then the rate is too slow. A high temperature is fast but gives a low yield. 450 °C gives an acceptable yield at an acceptable rate.
Common mistakes
- Saying the catalyst increases the yield. It only speeds up reaching equilibrium; the yield stays the same.
- Thinking a higher temperature increases the yield. For this exothermic reaction it lowers the yield.
- Forgetting the ⇌ sign: the reaction is reversible, so not all of the gas changes into ammonia at once.
- Mixing up the sources: nitrogen comes from air, hydrogen from natural gas (not from water in the air).