AQA GCSE Chemistry Chemistry Paper 2 (Higher), June 2025: Question 7

✅ Correct Answer

A: ethanediol (1 mark)

B: hexanedioic acid (1 mark)

💡 Key Knowledge

Count the carbons carefully:

• Molecule A: 2 carbon atoms with two -OH alcohol groups → ethane + diol = ethanediol.
• Molecule B: 1 (in HOOC) + 4 (in CH₂) + 1 (in COOH) = 6 carbons in total with two -COOH carboxylic acid groups → hexane + dioic acid = hexanedioic acid.

🧠 Exam Technique

Always count the carbons inside the brackets or chain carefully. For B, students often miss the carbon atoms contained inside the two -COOH end groups and mistakenly call it "butanedioic acid".

❌ Common Errors

• Writing "ethanol" instead of "ethanediol" (missing that there are two alcohol groups).
• Forgetting the -e- in "ethanediol" or "hexanedioic acid".

✅ Correct Answer

polyester (1 mark)

💡 Key Knowledge

When a diol (monomer with two alcohol groups) reacts with a dicarboxylic acid (monomer with two carboxylic acid groups), they link together via ester links ( -COO- ) to produce a polyester.

✅ Correct Answer

water or H₂O (1 mark)

💡 Key Knowledge

In condensation polymerisation, monomer molecules join together with the simultaneous loss of small molecules, almost always water at GCSE level.

🧠 Exam Technique

Both the chemical name ("water") and the formula ("H₂O") are accepted, but check your subscript formatting to avoid ambiguity.

✅ Correct Answer

polypeptide (1 mark)

(Also allowed: polyamide)

❌ Common Errors

Do NOT write "protein" here! The mark scheme explicitly states: ignore protein.

A single type of amino acid polymerising forms a polypeptide. A protein is formed when different amino acids combine.

✅ Correct Answer

Completing the carboxylic acid group ( -COOH ) fully displayed on the right-hand carbon atom:

Must show: double bond to oxygen ( C=O ) and single bond to an oxygen which has a single bond to hydrogen ( C-O-H ).

🧠 Exam Technique

"Displayed formula" means EVERY bond must be drawn explicitly as a line!

If you write just -OH without showing the bond between the O and the H ( -O-H ), you will lose the mark.

✅ Correct Answer

[  ] Cellulose

[  ] DNA

[✓] Protein (1 mark)

[  ] Starch

💡 Biological Polymers Summary

• Glucose polymers → Starch, Cellulose, Glycogen
• Nucleotide polymer → DNA
• Amino acid polymers → Polypeptides / Proteins

✅ Correct Structure to Draw

• 4 single C-F bonds and 1 single C-C bond (1 mark)
• Two single open bonds extending through the brackets (1 mark)
• Subscript n placed bottom-right outside the brackets (1 mark)

🧠 Exam Technique: 3 Checklist Steps

1. Break the double bond: Change C=C to a single C-C bond.
2. Extend trailing bonds: Draw horizontal bonds from each carbon extending cleanly through the brackets.
3. Add the 'n': Place an n as a subscript at the bottom right.

❌ Common Errors

• Leaving the double bond between the carbons in the polymer product.
• Not drawing the horizontal extension bonds through the brackets.
• Putting a large n in front of the brackets instead of a subscript after.

✅ Correct Answer

• Point 1: Consists of long chain molecules / polymer chains (1 mark)
• Point 2: With covalent bonds (or cross-links) between the chains (1 mark)

💡 Thermosoftening vs Thermosetting

• Thermosoftening: Tangled individual polymer chains held together only by weak intermolecular forces. They melt when heated.
• Thermosetting: Chains are joined by strong covalent cross-links. They do not melt when heated; they char/burn instead.

❌ Common Misconception

Do NOT mention intermolecular forces when describing thermosetting polymers!

The mark scheme explicitly states: ignore intermolecular forces for thermosetting polymers because it is the strong covalent cross-links that determine their structure and properties.