The forward reaction produces fewer moles of gas: 4 moles of gas reactants → 2 moles of gas product. By Le Chatelier's principle:
- High pressure favours NH₃ (shifts equilibrium toward fewer gas moles). But very high pressures are expensive to maintain and dangerous. ~200 atm is the economic sweet spot.
- Low temperature favours NH₃ (forward reaction is exothermic; cooling shifts equilibrium to the right). But low temperatures make the rate too slow to be economically viable.
- ~450°C is a compromise: the rate is fast enough for commercial production, but the yield of ammonia at equilibrium is only about 15–25%. The unreacted gases are recycled, so overall conversion is still high.
- The iron catalyst speeds up the rate at which equilibrium is reached but does not change the position of equilibrium. It allows a lower temperature to be used, which improves the economics.
Quick Check: Why is a very high temperature not used in the Haber process even though it would speed up the reaction?
A very high temperature would speed up the rate but would shift the equilibrium to the left (away from ammonia) because the forward reaction is exothermic. This would reduce the yield of ammonia, making the process less economical.
This how it works covers Why These Conditions? (Le Chatelier's Principle) within The Haber Process & Fertilisers for GCSE Chemistry. Revise The Haber Process & Fertilisers in Rates of Reaction for GCSE Chemistry with 6 exam-style questions and 6 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 5 of 13 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for The Haber Process & Fertilisers
In the Haber process, where is the nitrogen obtained from?
State the equation for the Haber process and give the conditions used. Include the catalyst.