Iron is produced industrially using a blast furnace. A chemist is investigating a different industrial process in which iron(III) oxide is reduced by carbon monoxide in a reversible reaction carried out in a closed system: Fe₂O₃(s) + 3CO(g) ⇌ 2Fe(s) + 3CO₂(g). The forward reaction is exothermic. Explain how increasing the temperature and increasing the pressure would each affect the position of equilibrium in this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A reversible reaction reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the backward reaction. Le Chatelier's principle states that if a system at equilibrium is subjected to a change, the position of equilibrium moves to counteract that change.
Model answer (5 marks)
Increasing the temperature supplies heat, which the system removes by shifting the equilibrium toward the endothermic direction. The forward reaction is exothermic, so the reverse reaction is endothermic; therefore the equilibrium shifts to the left, producing more Fe₂O₃ and CO.
Increasing the pressure favours the side with fewer moles of gas. Both sides of the balanced equation contain 3 mol of gas (3 mol CO on the left, 3 mol CO₂ on the right), so the pressure change does not alter the equilibrium position.
Increasing the pressure favours the side with fewer moles of gas. Both sides of the balanced equation contain 3 mol of gas (3 mol CO on the left, 3 mol CO₂ on the right), so the pressure change does not alter the equilibrium position.
Examiner tips
- State the effect of temperature on the equilibrium direction and justify it with the exothermic nature of the forward reaction.
- Explain the pressure effect by comparing the number of gas moles on each side of the equation.
- Use the exact wording from the mark scheme to demonstrate understanding.
Common mistakes
- Claiming that pressure will shift the equilibrium because of the presence of CO and CO₂, without noting the equal gas moles.
- Saying the temperature shift is to the right (forward) instead of left (reverse).
Mark scheme (5 marks)
- Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction
- This means the equilibrium shifts in the backward/reverse direction (as the forward reaction is exothermic, the backward is endothermic)
- Increasing pressure shifts the equilibrium towards the side with fewer moles of gas
- There are 3 moles of gas on the left (reactant side) and 3 moles of gas on the right (product side), so the number of moles of gas is equal on both sides
- Therefore increasing pressure has no effect on the position of equilibrium
Key terms in this question
closed system · reversible reaction
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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