In the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g), the reaction is exothermic. The process uses an iron catalyst and a temperature of about 450°C. Which of the following best explains why a temperature higher than the optimum is not used?
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Correct answer
B. The equilibrium yield of ammonia decreases with increasing temperature.
Principle or equation
For an exothermic reaction, increasing temperature shifts the equilibrium toward reactants, decreasing the yield of product. The optimum temperature balances rate and yield.
Why this answer is correct
The forward reaction is exothermic, so according to Le Chatelier's principle, higher temperature favors the reverse reaction, reducing the equilibrium concentration of ammonia. Although higher temperature increases the rate, the lower yield makes it uneconomical. The catalyst remains active at higher temperatures, and the reverse reaction rate is not 'too fast to measure' in a practical sense. Reactants do not decompose at typical industrial temperatures.
Example
At 400°C, the equilibrium yield of NH3 is about 25% at 200 atm, but at 500°C it drops to about 10% under the same pressure. Thus a compromise temperature is used.
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