AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to draw the structures of amino acids as…
Full specification wording
Amino acids (A-level only): 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.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- The zwitterion drawn with both charges shown and balancing.
- The cation in excess acid, with COOH and NH₃⁺ and a single positive charge overall.
- The anion in excess alkali, with COO⁻ and NH₂.
- The R group and the rest of the skeleton drawn unchanged in all three forms.
Common mistakes and how to put them right
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.
Exam tips
- 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.
Try the lesson
The start of the Amino acids (A-level only): 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. lesson, made from checked teaching notes for this specification point.
Have a go first
Which structure represents the zwitterion formed by glycine, H₂NCH₂COOH?
Listen
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. A strong answer earns marks for these points. the zwitterion drawn with both charges shown and balancing. the cation in excess acid, with COOH and NH₃⁺ and a single positive charge overall. the anion in excess alkali, with COO⁻ and NH₂. the R group and the rest of the skeleton drawn unchanged in all three forms. Exam tip: Count the hydrogens on the nitrogen every time: two in NH₂, three in NH₃⁺.
See it
Key words
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zwitterion
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isoelectric point
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cation in acid
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anion in alkali
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See every definition
- zwitterion
- The form present around the isoelectric point, with NH₃⁺ and COO⁻ and both charges shown and balancing.
- isoelectric point
- The pH at which the positive and negative centres balance.
- cation in acid
- In excess acid the carboxylate ion is protonated back to COOH while the nitrogen keeps its extra proton, giving a single positive charge overall.
- anion in alkali
- In excess alkali the OH⁻ removes a proton from NH₃⁺, leaving NH₂ and COO⁻.
Label the diagram
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1Box 1
2Box 2
3Box 3
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Test your understanding of Amino acids (A-level only): 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.
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