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Question: "The Haber process operates at 450°C and 200 atmospheres. Explain why these conditions are used." (6 marks)
The forward reaction in the Haber process is exothermic. [1] According to Le Chatelier's Principle, lower temperatures favour the exothermic forward direction, giving a higher yield of ammonia. [1] However, at very low temperatures the reaction is too slow to be economically viable, so 450°C is a compromise between yield and rate. [1] Looking at the equation, the left side has 4 moles of gas molecules (1 N₂ + 3 H₂) and the right side has only 2 moles (2 NH₃). [1] Increasing pressure shifts the equilibrium to the side with fewer gas molecules — the right side — increasing the yield of ammonia. [1] However, very high pressures require expensive equipment and are dangerous to maintain, so 200 atm is an economic compromise between yield and cost. [1]
Examiner note: This 6-mark question demands three paired arguments — one for temperature (Le Chatelier direction + rate compromise) and one for pressure (gas molecule count + economic compromise). A common error is writing "higher pressure gives more ammonia" without explaining the mechanism via gas molecule counts. The word "compromise" is key at both points and will be expected by the examiner for full marks.
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