Amino acids (A-level only) — AQA A-Level Chemistry
Test yourself on Amino acids (A-level only) with AQA A-Level practice questions.
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Amino acids (A-level only) explained
An α-amino acid carries a basic -NH₂ group and an acidic -COOH group on the same carbon, so each can react with the other.
Read the full explanation
The proton moves from the carboxyl group to the amine group within the same molecule, giving a zwitterion: ⁺H₃N-CHR-COO⁻, with a positive centre and a negative centre but no overall charge. That internal ionic character is why amino acids are crystalline solids with high melting points and dissolve well in water, unlike the small covalent molecules they otherwise resemble. Add acid and the COO⁻ is protonated, so the molecule becomes an overall cation. Add alkali and the NH₃⁺ loses its proton, so the molecule becomes an overall anion. The amino acid is therefore amphoteric.
Students should be able to draw the structures of amino acids as zwitterions and the ions formed from amino acids: in acid solution in alkaline solution.
Draw the skeleton once and then decide only which groups carry a hydrogen. The zwitterion has NH₃⁺ and COO⁻ and is the form present around the isoelectric point, the pH at which the positive and negative centres balance. In excess acid there is plenty of H⁺, so the weaker base, the carboxylate ion, is protonated back to COOH while the nitrogen keeps its extra proton, so the species is a cation carrying a single positive charge overall. In excess alkali the OH⁻ removes a proton from NH₃⁺, leaving NH₂ and COO⁻, so the species is an anion. The R group and the carbon skeleton are untouched in all three forms, so only the ends change.
Your focus
- Draw the zwitterion of a named amino acid and show that the charges cancel.
- Explain why amino acids have unexpectedly high melting points and are soluble in water.
- Predict what an amino acid does when hydrochloric acid or sodium hydroxide solution is added.
Show all 6 objectives
- Draw glycine as a zwitterion, as its cation and as its anion, labelling the overall charge on each and showing three hydrogens on the nitrogen in the zwitterion and the cation, two in the anion.
- Deduce the overall charge on an amino acid at a stated pH relative to its isoelectric point.
- Explain which group gains or loses a proton when acid or alkali is added, and why.
Amino acids (A-level only) exam tips
Marking Points
- one mark for the zwitterion drawn with NH₃⁺ and COO⁻ and no overall charge
- one mark for the cation in acid, with COOH and NH₃⁺
- one mark for the anion in alkali, with COO⁻ and NH₂
- one mark for explaining a high melting point by the attraction between the oppositely charged ions
- one mark for the zwitterion drawn with both charges shown and balancing
- one mark for the cation in excess acid, with COOH and NH₃⁺ and a single positive charge overall
- one mark for the anion in excess alkali, with COO⁻ and NH₂
- one mark for the R group and the rest of the skeleton drawn unchanged in all three forms
Examiner Tips
- 💡Decide what the added reagent does before you draw: acid supplies H⁺, so something gains a proton; alkali removes H⁺, so something loses one.
- 💡Check the charges cancel in a zwitterion, and that they do not in the ions formed in acid or alkali.
- 💡Count the hydrogens on the nitrogen every time: two in NH₂, three in NH₃⁺.
- 💡Skeletal formulae are accepted, but the hydrogens on the nitrogen must still be shown - NH₂ or NH₃⁺. The skeletal exemption covers only aldehydes, secondary amines and N-substituted amides, not a primary amine. A negative sign on the nitrogen is always wrong.
Common Mistakes
- drawing a zwitterion that carries an overall positive or negative charge
- protonating the amine group but leaving COO⁻ unchanged when acid is added
- saying an amino acid is neutral so it will not react with hydrochloric acid
- assuming the zwitterion exists only in solution, when it is also the form in the solid
- moving a negative charge onto the nitrogen atom, which AQA does not accept
- changing the R group when the pH changes
- drawing a positive nitrogen and a neutral COOH and calling it a zwitterion
- drawing NH₃⁺ with only two hydrogen atoms on the nitrogen