Reviewed CSCA Chemistry question · Hard

In the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g) is exothermic. The process uses an iron catalyst and a temperature of about 450°C. Which of the following best explains why a pressure of about 200 atm is used instead of a much higher pressure?

Higher pressure would shift the equilibrium to the left, decreasing yield.
Higher pressure would increase the rate of the reverse reaction more than the forward reaction.
Higher pressure would require more energy and stronger equipment, and the increase in yield becomes less significant at very high pressures.
Higher pressure would cause the catalyst to become inactive.

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Correct answer

C. Higher pressure would require more energy and stronger equipment, and the increase in yield becomes less significant at very high pressures.

Principle or equation

According to Le Chatelier's principle, high pressure favors the side with fewer moles of gas (products), increasing yield. However, very high pressures are costly and dangerous, and the yield improvement diminishes as pressure increases.

Why this answer is correct

The Haber process uses a compromise pressure of about 200 atm. Higher pressures would indeed increase the equilibrium yield of ammonia, but the additional yield is small while the costs of compression and equipment strength rise sharply. Thus, the choice balances economic and safety considerations.

Example

At 200 atm, the yield is about 30%, while at 1000 atm it might be around 50%, but the cost is much higher.

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