A student prepares a buffer solution by dissolving excess ammonium chloride in aqueous ammonia. Explain how this buffer solution resists a change in pH when a small amount of hydrochloric acid is added to it, and when a small amount of sodium hydroxide is added to it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A buffer solution maintains a nearly constant pH when small amounts of acid or alkali are added to it. The ammonium/ammonia buffer contains the weak base ammonia (NH₃) and its conjugate acid, the ammonium ion (NH₄⁺), in equilibrium: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
Model answer (5 marks)
The buffer contains a reservoir of NH₄⁺ ions (from the ammonium chloride) and NH₃ molecules (from the aqueous ammonia). When a small amount of HCl is added, the H⁺ ions react with NH₃:
NH₃ + H⁺ → NH₄⁺
Because the NH₃ reservoir is large, the concentration of H⁺ changes only slightly, so the pH remains almost unchanged. When a small amount of NaOH is added, the OH⁻ ions react with NH₄⁺:
NH₄⁺ + OH⁻ → NH₃ + H₂O
Because the NH₄⁺ reservoir is large, the concentration of OH⁻ changes only slightly, so the pH again remains almost unchanged.
NH₃ + H⁺ → NH₄⁺
Because the NH₃ reservoir is large, the concentration of H⁺ changes only slightly, so the pH remains almost unchanged. When a small amount of NaOH is added, the OH⁻ ions react with NH₄⁺:
NH₄⁺ + OH⁻ → NH₃ + H₂O
Because the NH₄⁺ reservoir is large, the concentration of OH⁻ changes only slightly, so the pH again remains almost unchanged.
Examiner tips
- Use the word ‘reservoir’ to show you understand the role of each component.
- Show the reactions for both acid and base addition.
- Explain that the large excess of each species keeps the change in [H⁺] or [OH⁻] negligible.
- Use correct chemical equations and state the effect on pH.
Mark scheme (5 marks)
- The buffer contains a reservoir of NH₄⁺ ions (from the ammonium chloride) and NH₃ molecules (from the aqueous ammonia)
- When hydrochloric acid is added, the H⁺ ions are removed by reacting with NH₃: NH₃ + H⁺ → NH₄⁺
- Because the NH₃ reservoir is large, the change in [H⁺] and therefore pH is negligible when acid is added
- When sodium hydroxide is added, the OH⁻ ions react with NH₄⁺ ions: NH₄⁺ + OH⁻ → NH₃ + H₂O
- Because the NH₄⁺ reservoir is large, the change in [OH⁻] and therefore pH is negligible when alkali is added
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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