Explain why a solution of ammonium chloride (NH₄Cl) is acidic at 25 °C, with reference to the Brønsted–Lowry theory of acids and bases.

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).

Model answer (4 marks)

NH₄Cl fully dissociates in water to give NH₄⁺ and Cl⁻ ions.
NH₄⁺ is a Brønsted–Lowry acid because it donates a proton to water:
NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺.
The reaction produces H₃O⁺ (or H⁺) in solution.
Cl⁻ is the conjugate base of the strong acid HCl and does not accept protons from water, so it does not affect the pH; the net effect is an increase in [H₃O⁺] making pH < 7.

Examiner tips

  • Show the dissociation step first, then the acid–base reaction with water, and finish with the pH consequence.
  • Use the correct Brønsted–Lowry terminology: acid, base, conjugate base, conjugate acid.

Common mistakes

  • Writing NH₄⁺ as a base or claiming Cl⁻ is a base that accepts protons.
  • Omitting the water molecule in the proton‑transfer equation.
  • Using the wrong ion (NH₃) as the acid instead of NH₄⁺.

Mark scheme (4 marks)

  1. NH₄Cl fully dissociates in water to give NH₄⁺ and Cl⁻ ions
  2. NH₄⁺ is a Brønsted–Lowry acid because it donates a proton to water
  3. The reaction produces H₃O⁺ (or H⁺) in solution, shown by the equation NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺
  4. Cl⁻ is the conjugate base of the strong acid HCl and does not accept protons from water, so it does not affect the pH; the net effect is an increase in [H₃O⁺] making pH < 7

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