In the industrial synthesis of ammonia, 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 even though it would increase the rate of reaction?
Show the answer and explanation
Correct answer
A. Higher temperature would shift the equilibrium to the left, decreasing the yield of ammonia.
Principle or equation
For an exothermic reaction, increasing temperature shifts the equilibrium toward reactants (Le Chatelier's principle), reducing 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, increasing temperature favors the endothermic reverse reaction, decreasing the equilibrium concentration of NH3. Although higher temperature increases the rate, the yield would be too low. The catalyst does not decompose at typical higher temperatures, and pressure is controlled separately.
Example
For the exothermic reaction 2SO2 + O2 ⇌ 2SO3, increasing temperature reduces SO3 yield.
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