Ethanol can be produced industrially by the reversible reaction of ethene with steam. The reaction reaches dynamic equilibrium in a closed system. Explain how changing the conditions affects the position of equilibrium and the yield of ethanol, and describe what is meant by dynamic equilibrium in this context.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethene reacts with steam in a reversible reaction: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g). The forward reaction is exothermic. The reaction is carried out in a closed system under high pressure.
Model answer (4 marks)
At dynamic equilibrium the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of C₂H₄, H₂O and C₂H₅OH remain constant.
Increasing the pressure (the reaction is in a closed system) favours the side with fewer gas molecules. The forward reaction has 2 moles of gas and the reverse 3, so the equilibrium shifts to the right, giving more ethanol.
Because the forward reaction is exothermic, lowering the temperature removes heat and favours the exothermic direction. Thus decreasing temperature shifts the equilibrium to the right, increasing the ethanol yield.
Dynamic equilibrium therefore means that the reaction continues in both directions at the same rate, keeping the concentrations of all species constant.
Increasing the pressure (the reaction is in a closed system) favours the side with fewer gas molecules. The forward reaction has 2 moles of gas and the reverse 3, so the equilibrium shifts to the right, giving more ethanol.
Because the forward reaction is exothermic, lowering the temperature removes heat and favours the exothermic direction. Thus decreasing temperature shifts the equilibrium to the right, increasing the ethanol yield.
Dynamic equilibrium therefore means that the reaction continues in both directions at the same rate, keeping the concentrations of all species constant.
Examiner tips
- Show the rate equality and constant concentrations first. Mention the mole change for pressure. Explain exothermic effect on temperature. Use correct terminology (dynamic equilibrium).
Common mistakes
- Confusing pressure effect with temperature. Saying the equilibrium shifts to the left when pressure increases. Forgetting that the reaction is exothermic when discussing temperature.
Mark scheme (4 marks)
- At dynamic equilibrium, the rate of the forward reaction equals the rate of the backward reaction
- At dynamic equilibrium, the concentrations of reactants and products remain constant
- Increasing pressure shifts the equilibrium to the right / towards the products, increasing the yield of ethanol
- Decreasing temperature shifts the equilibrium to the right / towards the products (because the forward reaction is exothermic), increasing the yield of ethanol
Key terms in this question
dynamic equilibrium · reversible reaction · position of equilibrium
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Dynamic equilibria, calculations involving volumes of gases (Chem only) questions
- A student is investigating the contact process, used to manufacture sulfuric aci…
- The Haber process is used to manufacture ammonia. The reaction is a reversible r…
- Nitrogen monoxide, NO, is produced as a by-product during combustion reactions i…
- Explain what is meant by dynamic equilibrium and describe what happens to the co…