A chemical engineer is studying the following reversible reaction used in the production of methanol, which is carried out in a closed system at high pressure: CO(g) + 2H₂(g) ⇌ CH₃OH(g). The forward reaction is exothermic. Explain what happens to the position of equilibrium when the temperature of the closed system is increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methanol (CH₃OH) is an important industrial chemical used as a fuel and as a solvent. It is manufactured from carbon monoxide and hydrogen gases in a reversible reaction carried out in a closed system.
Model answer (4 marks)
The equilibrium shifts to the left, i.e. toward the reactants, because the forward reaction is exothermic. Increasing the temperature adds heat, which the system counteracts by favouring the endothermic reverse reaction. Consequently more CO and H₂ are produced and the concentration of CH₃OH decreases until a new equilibrium is reached.
Examiner tips
- Use the phrase "shifts to the left" for 1 mark
- Explain that the forward reaction is exothermic and the reverse is endothermic
- Show that the system favours the endothermic direction to counteract the temperature rise
- Mention the change in concentrations of reactants and product
Common mistakes
- Saying the equilibrium shifts to the right or that the reaction becomes more exothermic
- Failing to identify that the forward reaction is exothermic
- Not linking the temperature change to Le Chatelier’s principle
Mark scheme (4 marks)
- The equilibrium position shifts in the direction of the endothermic reaction / shifts in the reverse direction / shifts to the left
- This is because the system acts to counteract the increase in temperature / opposes the change (Le Chatelier's principle)
- More reactants / CO and H₂ are formed (at the expense of products)
- The concentration of methanol / CH₃OH decreases (and the concentration of CO and H₂ increases) until a new equilibrium is reached
Key terms in this question
closed system · reversible reaction
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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