Explain why the pH at the equivalence point of a titration of 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid with 0.100 mol dm⁻³ sodium hydroxide solution is greater than 7 at 25 °C.

IB DP Chemistry Higher Level (2023 syllabus) — R3.1 Proton transfer reactions · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

At the equivalence point all the ethanoic acid has reacted with NaOH, leaving only sodium ethanoate in solution.

The ethanoate ion is the conjugate base of a weak acid and therefore acts as a Brønsted–Lowry base.

It reacts with water:
CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq)

The formation of OH⁻ ions makes the solution basic, so [OH⁻] > [H⁺] and the pH is greater than 7.

Examiner tips

  • Mention the species present at equivalence (CH₃COONa).
  • Explain that CH₃COO⁻ is a weak base and give the hydrolysis equation.
  • State that OH⁻ production raises pH above 7.

Mark scheme (4 marks)

  1. At the equivalence point, all the ethanoic acid has been neutralised and the solution contains only sodium ethanoate (CH₃COONa).
  2. The ethanoate ion is the conjugate base of a weak acid and acts as a Brønsted–Lowry base, accepting a proton from water.
  3. The relevant equilibrium is: CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq)
  4. The production of OH⁻ ions makes the solution basic, so [OH⁻] > [H⁺] and pH > 7.

Key terms in this question

equivalence point

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