In the industrial synthesis of ammonia, the reaction N2(g) + 3H2(g) ⇌ 2NH3(g) is exothermic. The process uses an iron catalyst and a temperature of about 450°C, which is higher than the temperature that would give the maximum equilibrium yield. What is the primary reason for using this higher temperature?

Correct answer

B. To increase the rate at which equilibrium is reached.

Principle or equation

Although lower temperatures favor the exothermic forward reaction and increase the equilibrium yield of ammonia, the rate of reaction would be impractically slow. A higher temperature is used to achieve a sufficient rate of reaction, even though it lowers the equilibrium yield.

Why this answer is correct

The Haber process uses a compromise temperature of about 450°C. At lower temperatures, the equilibrium yield of ammonia is higher, but the rate is too slow for industrial viability. The iron catalyst is effective at this temperature, and the higher temperature increases the rate at which equilibrium is attained, making the process economically feasible.

Example

At 25°C, the equilibrium constant for ammonia synthesis is very large, but the reaction rate is negligible. At 450°C, the rate is fast enough to produce ammonia at a practical rate, even though the equilibrium yield is lower.