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 pressure of about 200 atm is used?

High pressure increases the rate of reaction by increasing the activation energy.
High pressure shifts the equilibrium toward more ammonia because the forward reaction produces fewer gas moles.
High pressure prevents the catalyst from being poisoned.
High pressure is needed to keep nitrogen and hydrogen in the liquid state.

Show the answer and explanation

Correct answer

B. High pressure shifts the equilibrium toward more ammonia because the forward reaction produces fewer gas moles.

Principle or equation

According to Le Chatelier's principle, increasing pressure shifts equilibrium toward the side with fewer gas moles; forward reaction has 4 moles gas to 2 moles gas.

Why this answer is correct

The forward reaction reduces the number of gas moles from 4 to 2, so high pressure favors ammonia production. The catalyst increases rate but does not affect equilibrium. Activation energy is not changed by pressure. The gases are not liquid at these conditions.

Example

For N2 + 3H2 ⇌ 2NH3, increasing pressure from 1 atm to 200 atm increases the yield of NH3.

This published item includes a stored explanation and passed the platform’s publication workflow. It is independent preparation material, not a claim of an official or recalled examination question.