AQA A-Level Chemistry Paper 3, 2024: Question 28

Nylon-6,6: spotting the correct statement

What this question tests

Recognising nylon-6,6 as a condensation polyamide, knowing its monomers, and linking the amide (–CONH–) link to reactions such as hydrolysis in aqueous NaOH. Also tests avoiding typical polymer misconceptions (addition vs condensation; polymer Mr spread).

Part (a) Multiple-choice (1 mark)

Question: Which statement concerning nylon-6,6 is correct?

✅ Correct answer (what earns the mark)

C — Nylon-6,6 can be hydrolysed by aqueous sodium hydroxide.

Mark scheme: C (1 mark, AO2). You only get the mark for selecting C.

💡 Key knowledge

Nylon-6,6 is a polyamide made by condensation polymerisation (not addition).
It contains amide links: –CONH– .
Amide links hydrolyse under acidic conditions or under alkaline conditions (e.g. aqueous NaOH), breaking the polymer chain.
“6,6” refers to the carbon counts in the two monomers: hexane-1,6-diamine and hexanedioic acid (adipic acid) or its diacyl chloride.

🧠 Exam technique

For polymer MCQs, first identify the link in the polymer: nylon has amide (like peptides).
If it has amide or ester links, immediately consider hydrolysis as a likely true statement.
Be suspicious of absolutes like “all molecules have the same Mr” — polymers usually have a distribution of chain lengths.

❌ Common errors (and why they lose marks)

Choosing B (addition polymer): nylon-6,6 forms by condensation with elimination of small molecules (e.g. H₂O / HCl), not by alkene addition.
Choosing A (butanedioic acid): nylon-6,6 uses hexanedioic acid, not butanedioic acid. “6” means a 6‑carbon monomer unit.
Choosing D (same Mr): polymer samples contain chains of different lengths → different relative molecular masses (Mr).

Why option C is true (linking to chemistry)

💡 What “hydrolysed by aqueous NaOH” means for nylon-6,6

Nylon-6,6 contains amide bonds which can be broken by alkaline hydrolysis. Aqueous NaOH provides OH⁻ which attacks the carbonyl carbon in the amide link, ultimately splitting the chain.

Products (conceptual): smaller molecules with carboxylate salts (–COO⁻ Na⁺) and amines (–NH₂) at chain ends.

🧠 Examiner-style tip: how to “spot” hydrolysis questions

If you see –CONH– (amide) or –COO– (ester), expect hydrolysis to be possible.
AQA often tests: condensation polymers can be hydrolysed (unlike many simple addition polymers such as poly(ethene)).

Quick dismantle of the wrong options (A, B, D)

❌ A: “Butanedioic acid is a reactant…”

Wrong monomer. Nylon-6,6 is made from hexane-1,6-diamine and hexanedioic acid (adipic acid). Butanedioic acid has 4 carbons, so it would not match “6,6”.

❌ B: “Nylon-6,6 is an addition polymer.”

Nylon-6,6 is a condensation polymer. It forms by reaction of two functional groups (diamine + dicarboxylic acid / diacyl chloride) and elimination of a small molecule (commonly H₂O or HCl).

❌ D: “All molecules have the same Mr.”

Polymerisation produces a mixture of chain lengths. Therefore, a polymer sample has a range of relative molecular masses, not a single value.

💡 One-liner memory hook

Nylon = polyamide → hydrolysis possible. “6,6” points to 6‑carbon diamine + 6‑carbon diacid.

Marks & what distinguished full-mark responses

🧠 How marks are awarded

This is a 1-mark MCQ: you must select the single correct option.

Full mark: choose C.
No method marks: even if you “know” nylon chemistry, only the correct letter scores.

❌ Where students typically lose marks

Mixing up addition vs condensation polymerisation.
Not linking the name “6,6” to the correct monomers (6-carbon chains).
Assuming “polymer” implies a single Mr (ignoring chain length distribution).