Explain why a mixture of ammonia solution and ammonium chloride solution acts as a buffer, maintaining a nearly constant pH when a small amount of strong acid is added.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A buffer contains a weak base (NH₃) and its conjugate acid (NH₄⁺) in significant concentrations.
When a small amount of strong acid is added, the NH₃ reacts with H⁺ to form NH₄⁺:
NH₃ + H⁺ → NH₄⁺.
This removes the added protons, so the concentration of H⁺ (and therefore pH) changes only slightly.
The equilibrium NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ shifts to keep the ratio [NH₄⁺]/[NH₃] almost constant.
Because the buffer components are present in large excess, the ratio changes minimally, maintaining a nearly constant pH.
When a small amount of strong acid is added, the NH₃ reacts with H⁺ to form NH₄⁺:
NH₃ + H⁺ → NH₄⁺.
This removes the added protons, so the concentration of H⁺ (and therefore pH) changes only slightly.
The equilibrium NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ shifts to keep the ratio [NH₄⁺]/[NH₃] almost constant.
Because the buffer components are present in large excess, the ratio changes minimally, maintaining a nearly constant pH.
Examiner tips
- State that the buffer contains NH₃ and NH₄⁺. Explain the reaction of NH₃ with added H⁺. Mention the equilibrium shift that keeps the ratio constant. Show why the ratio changes only slightly due to large concentrations.
Common mistakes
- Confusing NH₃ with NH₄⁺ as the base. Forgetting to mention the equilibrium shift. Claiming the pH stays exactly constant rather than nearly constant.
Mark scheme (4 marks)
- A buffer contains a weak acid and its conjugate base (or a weak base and its conjugate acid) in significant concentrations.
- When a small amount of strong acid (H⁺/H₃O⁺) is added, the base component (NH₃) reacts with and removes the added protons.
- The equilibrium NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (or NH₄⁺ ⇌ NH₃ + H⁺) shifts to maintain the ratio of weak acid to conjugate base, so [H⁺] (and hence pH) remains nearly unchanged.
- The system works because the concentrations of both NH₃ and NH₄⁺ are large relative to the amount of added acid, so the ratio [NH₄⁺]/[NH₃] (and therefore pH) changes only minimally.
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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