Ethanol and ethanoic acid react together in a closed system to form ethyl ethanoate and water. The equation for this reaction is: C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O. At a certain temperature, the equilibrium concentrations are: [C₂H₅OH] = 0.50 mol/dm³, [CH₃COOH] = 0.50 mol/dm³, [CH₃COOC₂H₅] = 1.50 mol/dm³, [H₂O] = 1.50 mol/dm³. Write the expression for the equilibrium constant, Kc, for this reaction, calculate its value, and state what a Kc value greater than 1 tells you about the position of equilibrium.

WJEC A-Level Chemistry (Wales) — 3.8 Equilibrium constants · State · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Ethanol and ethanoic acid react in a closed system. At equilibrium, ethyl ethanoate and water are both present at a concentration of 1.50 mol/dm³, while ethanol and ethanoic acid are both present at a concentration of 0.50 mol/dm³.

Model answer (5 marks)

Kc = [CH₃COOC₂H₅][H₂O]/[C₂H₅OH][CH₃COOH]
Kc = (1.50 × 1.50)/(0.50 × 0.50) = 9
A Kc value greater than 1 indicates that, at equilibrium, the concentration of products is greater than that of reactants – the equilibrium lies to the right.

Examiner tips

  • Write the full expression with correct stoichiometric coefficients. Show the substitution of the given concentrations. Calculate the product of the numerators and denominators. State the implication of Kc>1 in terms of product dominance.

Common mistakes

  • Using the wrong order of reactants and products in the expression. Forgetting to square the 1.50 values or mis‑multiplying. Adding units to Kc or writing an incorrect conclusion about the equilibrium position.

Mark scheme (5 marks)

  1. Correct Kc expression: Kc = [CH₃COOC₂H₅][H₂O] / [C₂H₅OH][CH₃COOH]
  2. Correct substitution of values into the Kc expression: (1.50 × 1.50) / (0.50 × 0.50)
  3. Correct numerical value of Kc = 9 (no units required for this expression)
  4. A Kc value greater than 1 means the products are favoured at equilibrium
  5. A Kc value greater than 1 means the concentration of products is greater than the concentration of reactants at equilibrium

Key terms in this question

equilibrium constant · Kc · closed system · position of equilibrium · concentration

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