Explain why the pH of a solution of ammonium chloride (NH₄Cl) is less than 7 at 25 °C, making reference to the relevant equilibrium and the relative strengths of the species involved.

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)

1. NH₄Cl dissociates completely in water to give NH₄⁺ and Cl⁻.
2. NH₄⁺ is a weak Brønsted–Lowry acid and reacts with water:
NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺.
3. The equilibrium lies to the left, but some H₃O⁺ is produced, giving an acidic solution.
4. NH₄⁺ is a stronger acid than H₂O (or NH₃ a weaker base than OH⁻), so the conjugate base NH₃ cannot neutralise all H₃O⁺, leaving pH < 7.

Examiner tips

  • Use the word ‘dissociates’ for the salt, ‘weak acid’ for NH₄⁺, give the proton‑transfer equation, and state the direction of the equilibrium and the relative strengths to justify acidity.

Common mistakes

  • Saying NH₄Cl is a weak acid instead of its ion NH₄⁺; confusing NH₃ as a strong base; omitting the equilibrium equation or the comparison with water’s acidity.

Mark scheme (4 marks)

  1. NH₄Cl is a fully dissociated salt, so the relevant species in solution is the ammonium ion, NH₄⁺.
  2. NH₄⁺ acts as a weak Brønsted–Lowry acid, donating a proton to water: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺.
  3. The equilibrium lies to the left but does produce some H₃O⁺, making the solution acidic (pH < 7).
  4. NH₄⁺ is a stronger acid than water (or equivalently, NH₃ is a weaker base than water / OH⁻), so the conjugate base NH₃ cannot fully neutralise the H₃O⁺ produced, leaving the solution acidic.

Key terms in this question

equilibrium

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