Reviewed CSCA Chemistry question · Standard

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 would decrease the yield of ammonia?

The catalyst becomes less active at higher temperatures.
The equilibrium shifts to the left because the forward reaction is exothermic.
The rate of the reverse reaction increases more than the forward reaction.
The gases expand and the pressure decreases.

Show the answer and explanation

Correct answer

B. The equilibrium shifts to the left because the forward reaction is exothermic.

Principle or equation

According to Le Chatelier's principle, for an exothermic reaction, increasing temperature shifts the equilibrium toward the reactants (left), decreasing the yield of products.

Why this answer is correct

The forward reaction is exothermic, so heat is a product. Increasing temperature adds heat, shifting equilibrium to the left, reducing NH3 yield. The catalyst's activity and rate effects are not the primary reason for the yield decrease.

Example

For N2 + 3H2 ⇌ 2NH3 + heat, raising temperature favors the reverse reaction, so less NH3 is produced.

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