Iron is used as a catalyst in the industrial production of ammonia. A student claims that using a higher temperature always improves the efficiency of an industrial process because it speeds up the reaction. Evaluate this claim in the context of a reversible reaction carried out in a closed system, referring to both the rate of reaction and the position of equilibrium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Many industrial processes use reversible reactions carried out in closed systems. The conditions chosen must balance the rate of reaction with the yield of product obtained.
Model answer (5 marks)
Increasing temperature increases the rate of reaction because particles have more kinetic energy and collide more frequently, so a greater proportion of collisions have the activation energy.
The student is partially correct – a higher temperature does accelerate the reaction, so products are formed more quickly.
However, for a reversible reaction in a closed system, temperature also affects the position of equilibrium. If the forward reaction is exothermic (as in the Haber process for NH₃), raising the temperature shifts the equilibrium towards the reactants, reducing the yield of ammonia.
Thus, a higher temperature does not always improve overall efficiency. In practice a compromise temperature is chosen to give an acceptable rate of reaction and an acceptable product yield.
The student is partially correct – a higher temperature does accelerate the reaction, so products are formed more quickly.
However, for a reversible reaction in a closed system, temperature also affects the position of equilibrium. If the forward reaction is exothermic (as in the Haber process for NH₃), raising the temperature shifts the equilibrium towards the reactants, reducing the yield of ammonia.
Thus, a higher temperature does not always improve overall efficiency. In practice a compromise temperature is chosen to give an acceptable rate of reaction and an acceptable product yield.
Examiner tips
- Mention both rate and equilibrium effects; use the term ‘exothermic’ for the forward reaction; explain why a compromise temperature is chosen
- Show the logical flow: rate ↑ → claim; equilibrium shift ↓ yield → counter‑point; conclusion
Common mistakes
- Assuming higher temperature always increases yield; forgetting that the Haber process is exothermic; not linking temperature to equilibrium shift
Mark scheme (5 marks)
- Increasing temperature increases the rate of reaction because particles have more kinetic energy and collide more frequently / a greater proportion of particles have at least the activation energy
- The student is partially correct — a higher temperature does increase the rate of reaction, so products form more quickly
- However, in a reversible reaction at dynamic equilibrium, increasing temperature shifts the equilibrium position in the direction of the endothermic reaction
- If the forward reaction is exothermic, increasing temperature shifts equilibrium towards the reactants, so the yield of product decreases
- Therefore the student's claim is not fully correct — a compromise temperature is chosen to give an acceptable rate of reaction and an acceptable yield of product
Key terms in this question
rate of reaction · reversible reaction · closed system
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