In the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -92 kJ/mol. 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?

Correct answer

B. The equilibrium yield of ammonia decreases because the reaction is exothermic

Principle or equation

For an exothermic reaction, increasing temperature shifts the equilibrium toward reactants according to Le Chatelier's principle, decreasing the yield of product.

Why this answer is correct

The Haber process is exothermic (ΔH negative). According to Le Chatelier's principle, increasing temperature favors the endothermic direction, which is the reverse reaction, so the equilibrium yield of ammonia decreases. Although higher temperature increases the rate, the lower yield is not economical. The catalyst is active at 450°C and does not become less active at slightly higher temperatures. The reverse rate does increase, but the key reason is the equilibrium shift. Pressure is controlled separately and does not become too high solely because of temperature.

Example

For the exothermic reaction 2SO2 + O2 ⇌ 2SO3, increasing temperature lowers the equilibrium concentration of SO3.