Explain why the pH of a solution of carbonic acid (H₂CO₃) is higher than the pH of a hydrochloric acid solution of the same concentration.

IB DP Chemistry Standard 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).

Carbonic acid (H₂CO₃) and hydrochloric acid (HCl) are both present in water at a concentration of 0.10 mol dm⁻³.

Model answer (4 marks)

HCl is a strong acid and dissociates completely in water, giving a maximum concentration of H⁺ ions. In contrast, H₂CO₃ is a weak acid and only partially dissociates; the equilibrium lies to the left. Therefore, at the same 0.10 mol dm⁻³ concentration the H⁺ (or H₃O⁺) concentration in the H₂CO₃ solution is lower than in the HCl solution. A lower [H⁺] gives a higher pH, so the H₂CO₃ solution has a higher pH than the HCl solution.

Examiner tips

  • Use the command word ‘Explain’ – give a clear cause–effect chain. Include the key terms ‘strong acid’, ‘weak acid’, ‘complete dissociation’, ‘partial dissociation’, ‘equilibrium lies to the left’, ‘lower [H⁺]’, ‘higher pH’.
  • Show the logical progression: HCl → complete dissociation → high [H⁺] → low pH; H₂CO₃ → partial dissociation → lower [H⁺] → higher pH.
  • Keep the answer concise – 4 marks, so one sentence per point is sufficient.

Common mistakes

  • Confusing ‘weak acid’ with ‘weak base’ or not stating that H₂CO₃ only partially dissociates.
  • Failing to link the lower [H⁺] to the higher pH, or omitting the comparison of dissociation extents.

Mark scheme (4 marks)

  1. HCl is a strong acid that dissociates completely/fully in water, producing the maximum possible concentration of H⁺ ions.
  2. H₂CO₃ is a weak acid that only partially dissociates/ionises in water, so the equilibrium lies to the left.
  3. At the same concentration, the H⁺ (or H₃O⁺) ion concentration in the H₂CO₃ solution is lower than in the HCl solution.
  4. A lower [H⁺] corresponds to a higher pH, so the H₂CO₃ solution has a higher pH than the HCl solution.

Key terms in this question

pH

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