Lesson 4 of 4
The Haber process
Balance equilibrium yield, reaction speed and operating cost.
A-Level 9476 (2026-2027)
I can explain industrial Haber-process conditions.Syllabus
Syllabus 9476, 9(g). Explain industrial Haber-process conditions.
A useful ammonia process needs both yield and speed
The operating conditions are an economic compromise supported by equilibrium and kinetics.
N2(g) + 3H2(g) ⇌ 2NH3(g) is exothermic, with ΔH about -92 kJ for the equation as written. Lower temperature favours the equilibrium amount of ammonia; higher pressure favours the side with fewer gaseous particles. Neither statement alone specifies the best operating plant.
| Choice | Benefit | Compromise or limit |
|---|---|---|
| Moderately high temperature | Gives a useful rate for breaking and rearranging strong bonds. | A higher temperature lowers the equilibrium ammonia yield; a very low temperature is too slow. |
| High pressure | Favours two product gas moles over four reactant gas moles and increases collision frequency. | Compression and pressure-resistant equipment have costs; unlimited pressure is not economical. |
| Iron catalyst | Provides a lower-barrier surface mechanism, allowing a useful rate. | Does not change K or the equilibrium yield at a fixed temperature. |
| Cool the outlet mixture | Condense and remove ammonia. | Unreacted nitrogen and hydrogen are separated from the liquid product and recycled. |
- Feed nitrogen and hydrogen
Use an appropriate 1:3 stoichiometric supply and pass the gases over the catalyst under controlled conditions.
- Allow partial conversion
Each pass gives a mixture limited by both equilibrium and residence time.
- Condense ammonia and recycle gases
Removing product and returning unreacted gases increases total resource use efficiency without claiming complete conversion in one pass.
The quoted conditions are representative, not a universal specification for every plant. In a data question, compare the actual equilibrium-yield curve, reaction-rate information, energy use and equipment constraints supplied. Keep the explanations separate: temperature affects both K and rate; a catalyst affects rate; recycling improves overall conversion.
Check your understanding
Why is the Haber process run at about 450 degrees C, although a lower temperature gives a higher equilibrium yield?
Show the answer
At lower temperatures equilibrium is reached too slowly to be economic.
The forward reaction is exothermic, so cooling raises the yield but slows the rate. About 450 degrees C with an iron catalyst is a compromise between yield and rate.