Section 1 of 5Factors in the Haber Process
The Haber process makes ammonia from nitrogen and hydrogen: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The reaction is reversible, so the conditions must be chosen with care.
Temperature
- Temperature The forward reaction is exothermic, so a lower temperature shifts the equilibrium towards ammonia and gives a higher yield.
Rate
- Rate Lower temperatures slow the rate of reaction, so about 450 °C is used to produce ammonia at a reasonable speed.
Pressure
- Pressure Four molecules of reactant gas form two molecules of product gas, so high pressure shifts the equilibrium towards ammonia.
Catalyst
- Catalyst An iron catalyst speeds up the reaction so that equilibrium is reached sooner, but it does not change the yield.
Section 2 of 5Effects on Yield, Rate and Cost
Each condition affects the yield and the rate in different ways, so the final choice is a compromise.
1
Raising the temperature lowers the equilibrium yield but increases the rate, so a moderate temperature is chosen.
2
Raising the pressure increases the yield and the rate, but it requires stronger, more expensive equipment and more energy.
3
Adding a catalyst increases the rate without changing the yield, and iron is cheap, so it adds little cost.
4
Unreacted nitrogen and hydrogen are separated from the ammonia, cooled and recycled back into the reaction chamber.
| Condition | Effect on yield | Effect on rate | Cost or problem |
|---|---|---|---|
| High temperature | Lowers yield | Increases rate | Uses more energy |
| Low temperature | Increases yield | Decreases rate | Slow, uneconomic production |
| High pressure | Increases yield | Increases rate | Expensive equipment and compression energy |
| Catalyst | No effect on yield | Increases rate | Iron is cheap and needs no extra energy |
Section 3 of 5Making the Compromise
Making the Compromise
Temperature
- Temperature Around 450 °C balances a reasonable yield against an acceptable reaction rate.
Pressure
- Pressure About 200 atmospheres is high enough to improve the yield without making the plant too costly to build and run.
Recycling
- Recycling Recycling unused gases saves raw materials and increases the overall efficiency of the process.
Overall
- Overall The final conditions reflect chemistry, economics and safety, rather than the single best possible yield.
Section 4 of 5Worked Example
Question Calculate the total number of moles of gas on each side of N₂(g) + 3H₂(g) ⇌ 2NH₃(g), and state which side high pressure favours.
- Reactant side1 mol N₂ + 3 mol H₂ = 4 mol of gas
- Product side2 mol NH₃ = 2 mol of gas
- Going from 4 mol to 2 mol reduces the total number of gas molecules
- High pressure favours the side with fewer gas molecules, which is the product side
Answer 4 mol of reactant gas become 2 mol of product gas, so high pressure increases the yield of ammonia.
Section 5 of 5Exam Checklist & Tips
10.4.3 – Key Points for the Exam
- Haber process The industrial method of making ammonia from nitrogen and hydrogen, using an iron catalyst.
- Dynamic equilibrium A state in which forward and backward reactions occur at the same rate in a closed system.
- Yield The amount of desired product obtained, often expressed as a percentage of the maximum possible.
- Exothermic A reaction that transfers energy to the surroundings, so the temperature of the surroundings rises.
- Recycling Returning unreacted substances to the reactor to reduce waste and raw material costs.
Common Mistakes
- Saying a catalyst increases the yield of ammonia; a catalyst changes only how quickly equilibrium is reached.
- Assuming the highest temperature gives the best result, when a lower temperature gives a higher yield but a slower rate.
- Forgetting that the forward reaction is exothermic when describing the effect of temperature on yield.
- Stating that pressure only makes the reaction faster; it also shifts the position of equilibrium.