Explain how the functional groups present in an amino acid molecule account for its ability to act as a buffer in aqueous solution.

IB DP Chemistry Standard Level (2023 syllabus) — S3.2 Functional groups: classification of organic compounds · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Amino acids are the monomers of proteins. Glycine, the simplest amino acid, has the molecular formula C₂H₅NO₂ and contains two distinct functional groups.

Model answer (4 marks)

Amino acids contain an amino group (–NH₂) and a carboxyl group (–COOH). The carboxyl group can donate a proton, acting as an acid: –COOH → –COO⁻ + H⁺, neutralising added base. The amino group can accept a proton, acting as a base: –NH₂ + H⁺ → –NH₃⁺, neutralising added acid. Because the molecule can respond to both increases and decreases in H⁺ concentration, it resists changes in pH, acting as a buffer.

Examiner tips

  • Use the exact functional group names (amino, carboxyl).
  • Show both acid and base reactions.
  • Explain the buffering action in one sentence.

Common mistakes

  • Confusing the direction of proton transfer (e.g. writing –COO⁻ → –COOH).
  • Omitting the amino group’s role as a base.

Mark scheme (4 marks)

  1. Amino acids contain an amino group (–NH₂) and a carboxyl group (–COOH).
  2. The carboxyl group can donate a proton (act as an acid), neutralising added base: –COOH → –COO⁻ + H⁺.
  3. The amino group can accept a proton (act as a base), neutralising added acid: –NH₂ + H⁺ → –NH₃⁺.
  4. Because the molecule can respond to both increases and decreases in H⁺ concentration, it resists changes in pH, acting as a buffer.

Key terms in this question

buffer

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