In the industrial synthesis of ammonia, N2(g) + 3H2(g) ⇌ 2NH3(g), the reaction is exothermic. Which of the following conditions is used to maximize the yield of ammonia at equilibrium?
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Correct answer
A. Low temperature and high pressure
Principle or equation
According to Le Chatelier's principle, for an exothermic reaction, lowering temperature shifts equilibrium toward products (increasing yield). Also, the reaction produces fewer moles of gas (2 mol NH3 from 4 mol reactants), so increasing pressure shifts equilibrium toward products.
Why this answer is correct
To maximize yield, both low temperature and high pressure favor the formation of ammonia. Low temperature favors the exothermic forward reaction, and high pressure favors the side with fewer gas molecules.
Example
In the Haber process, a compromise temperature (around 450°C) and high pressure (150-300 atm) are used to achieve a reasonable yield with an adequate rate, but the equilibrium yield is higher at lower temperatures.
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