AQA A-Level Chemistry Paper 3, 2024: Question 33

AQA A-Level Chemistry • Multiple choice

Zwitterions in amino acids (identify the correct structure)

What this question tests

Core idea (AO1)

Recognising the zwitterionic form of an amino acid: must contain –NH₃⁺ and –COO⁻ in the same molecule.
Checking for overall neutrality: internal + and − charges cancel (net charge = 0).
Spotting when an option is not a simple amino acid zwitterion (e.g. extra charged groups giving net charge ≠ 0).

Marking focus: 1 mark for selecting the single correct structure (no working required).

Part (a) 1 mark • AO1

Which structure shows a zwitterion of an amino acid?

A zwitterion has both a positive and negative charge on different atoms within the same molecule.

✅ Correct answer (from mark scheme)

What you should see in option D:

H₃N⁺—CH—COO⁻ with side chain CH₂—SH

This is the zwitterion of cysteine: the amino group is protonated ( –NH₃⁺ ) and the carboxyl group is deprotonated ( –COO⁻ ), giving net charge 0.

Mark award: 1 mark for selecting D.

💡 Key knowledge: what counts as a zwitterion?

Must include –NH₃⁺ and –COO⁻ (for an amino acid zwitterion).
Typical amino acid skeleton: H₂N—CH(R)—COOH
Zwitterion form in water: H₃N⁺—CH(R)—COO⁻
Net charge check: +1 and −1 sums to 0 (unless there are extra charged groups).

🧠 Exam technique (how to secure the mark fast)

Scan each option for NH₃⁺ (not NH₂ ).
Scan for COO⁻ (not COOH ).
Check there isn’t an extra charge that makes the molecule overall not neutral.
Pick the one with exactly the internal + and − charges expected for the amino acid zwitterion.

Examiner-style tip: this is a recognition question—don’t overcomplicate by thinking about pH unless needed.

❌ Common errors (why students lose this 1 mark)

Choosing an option with NH₂ instead of NH₃⁺ (that’s not the zwitterionic amino group).
Ignoring extra charged groups and assuming “any + and − somewhere” must be correct.
Mixing up COO⁻ with neutral COOH or drawing/choosing both as charged incorrectly.

Quick option-by-option check (examiner commentary style)

Option A

Contains H₃N⁺ and COO⁻ (so looks promising).
But also has an additional –NH₃⁺ on the side chain.
Net charge becomes +1 overall (two positives, one negative), so not the neutral zwitterion form expected here.

Why marks are lost: students spot “+ and −” but don’t check overall charge.

Option B

Has H₃N⁺ but has two COO⁻ groups.
Net charge becomes −1 overall (one positive, two negatives).

Common misconception: “more charges means more zwitterion” — not true; zwitterion refers to internal charges but many exam questions expect the neutral overall form.

Option C

Has COO⁻ but the amino group is H₂N (uncharged).
Not a zwitterion because there is no internal positive charge.

High-scoring responses: explicitly look for NH₃⁺ , not just “an amine group”.

Option D

Contains H₃N⁺ and COO⁻ .
No extra charged groups; side chain CH₂SH is neutral here.
Net charge = 0 → correct zwitterion.

Distinguishes top answers: a quick net charge check confirms it.

Extra support

Mini-checklist for zwitterions (1-mark MCQ version)

🧠 10-second checklist

Is there NH₃⁺ ?
Is there COO⁻ ?
Any extra charges making net charge ≠ 0?
If all good → choose it.

💡 Reminder: amino acids in water

Most amino acids exist mainly as zwitterions in aqueous solution: proton transfer occurs from –COOH to –NH₂ to give –COO⁻ and –NH₃⁺ .

❌ Trap to avoid

Don’t assume “any molecule with both + and − somewhere” is the required answer. In MCQs like this, AQA typically targets the standard amino acid zwitterion with one NH₃⁺ and one COO⁻ and overall neutrality.