The Haber Process
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
The Haber Process
The Haber Process is the industrial, reversible synthesis of ammonia, mainly for fertiliser production.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol (forward reaction is exothermic)
Worked Example: Identifying the Sources
- Question: which row shows the sources of nitrogen and of hydrogen for the Haber process? (A) nitrates / water (B) nitrates / methane (C) air / water (D) air / methane
- Answer: D. Nitrogen comes from the air (about 78% N₂); hydrogen comes from methane (natural gas) by steam reforming: CH₄ + H₂O → CO + 3H₂
Typical Conditions and Why They Are Used
| Condition | Value used |
|---|---|
| Temperature | 450°C |
| Pressure | 20 000 kPa / 200 atm |
| Catalyst | iron |
Temperature: a Compromise
The forward reaction is exothermic, so a lower temperature would give a higher equilibrium yield. But at low temperature the rate is too slow to be economical. 450°C is a compromise between a reasonable yield and a reasonable rate.
Pressure: High Pressure Favoured
4 mol of gas (N₂ + 3H₂) → 2 mol of gas (2NH₃). Increasing pressure shifts equilibrium towards the side with fewer gas molecules (products), increasing yield, and also increases the rate (particles closer together). Pressure is not pushed even higher because building and running very high-pressure plant is expensive and hazardous - 200 atm balances yield/rate against cost and safety.
Catalyst: Iron
Iron speeds up the rate (economically essential, since without it the reaction would be far too slow at 450°C) but has no effect on the equilibrium yield.
Recycling
Only about 15-20% of the N₂/H₂ converts to NH₃ per pass through the catalyst chamber (the reaction is reversible and does not go to completion). The mixture is cooled so ammonia liquefies (b.p. −33°C) and is removed; the unreacted N₂ and H₂ are recycled back to the start, raising the overall efficiency close to 98%.
Common Mistakes
- ❌ Saying low temperature is used because it gives the fastest rate
- ✓ low temperature gives a higher YIELD (favours the exothermic direction), but a SLOWER rate - 450°C is a compromise, not the fastest option
- ❌ Saying the iron catalyst increases the yield of ammonia
- ✓ the catalyst only increases the rate at which equilibrium is reached - the equilibrium yield is unchanged
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