Topic 4 of 4
The Haber process
Balance equilibrium yield, reaction speed and operating cost.
A-Level 9476 (2026-2027)
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.