Explain why a buffer solution containing ethanoic acid and sodium ethanoate resists a significant change in pH when a small amount of hydrochloric acid is added to it.

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).

A buffer solution is prepared by mixing ethanoic acid (CH₃COOH) and sodium ethanoate (CH₃COONa) in water.

Model answer (4 marks)

A buffer contains a significant amount of the conjugate base CH₃COO⁻.
When HCl is added, the H⁺ ions are taken up by CH₃COO⁻, forming CH₃COOH.
Thus the free H⁺ concentration does not rise appreciably and the pH stays almost the same.
The buffer can keep resisting pH changes until the CH₃COO⁻ reservoir is depleted, i.e. until the buffer capacity is exhausted.

Examiner tips

Use the term ‘conjugate base reservoir’ to show understanding of buffer composition. Explain the reaction CH₃COO⁻ + H⁺ → CH₃COOH to show how H⁺ is removed. Mention that pH remains unchanged because [H⁺] is kept low. State that the buffer works only while the base is available – buffer capacity.

Common mistakes

Forgetting to mention the conjugate base reservoir. Saying the buffer ‘creates’ H⁺ instead of neutralising it. Not recognising that the buffer capacity limits the amount of acid it can neutralise.

Mark scheme (4 marks)

The buffer contains a reservoir of the conjugate base, ethanoate ions (CH₃COO⁻), in significant concentration.
The added H⁺ ions (from HCl) are consumed / neutralised by the ethanoate ions acting as a Brønsted–Lowry base.
Because the H⁺ ions are removed / converted to undissociated ethanoic acid, the H⁺ concentration and therefore pH remains almost unchanged.
The system can continue to resist pH change as long as the ethanoate ion reservoir is not exhausted (buffer capacity is maintained).

Key terms in this question

buffer solution