OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 17

✅ Correct Answers

You must give three distinct propagation/formation equations showing radical catalysis:

1. CCl₃F → CCl₂F• + Cl• (Formation of chlorine radical)
2. Cl• + O₃ → ClO• + O₂ (Breakdown of ozone)
3. ClO• + O → Cl• + O₂ (Regeneration of chlorine radical)

💡 Key Knowledge

• CFCs photolyse in the stratosphere via homolytic fission initiated by UV light.
• Chlorine radicals ( Cl• ) act as catalysts because they are regenerated in the overall cycle.
• Unpaired electrons must be shown clearly as a dot ( • ).

🧠 Exam Technique

• Always double-check that every radical species features a clearly visible dot.
• Ensure equations balance in terms of atoms and radicals.

❌ Common Errors

• Omitting the radical dot ( • ), which loses marks instantly.
• Confusing propagation steps with termination steps.

✅ Correct Answers

For the oligomer made from 2-chloro-1,1,2-trifluoroethene ( CFCl=CF₂ ):

Draw the backbone carbon-carbon single bond with extending end bonds passing through brackets or touching the main chain:

-[CFCl-CF₂]- or backbone with F, Cl, F, F attached appropriately.

💡 Key Knowledge

Addition polymerisation converts the carbon-carbon double bond ( C=C ) into a single bond ( C-C ) with open bonds representing continuation.

❌ Common Errors

• Drawing more than one repeat unit (do not write n unless outside brackets, but better to avoid entirely if unsure).
• Leaving out the essential end bonds extending past the repeat units.

✅ Correct Answers

1. Larger molecules have more surface contact / larger surface area between molecules.
2. This leads to stronger / more induced dipole-dipole interactions (London forces), requiring more energy to overcome.

💡 Key Knowledge

As polymer chains get longer, temporary instantaneous dipole-induced dipole interactions accumulate along the length of the backbone.

🧠 Exam Technique

This is a standard comparative question. You must make a comparison linking molecule size/contact surface directly to the energy required to break intermolecular forces.

❌ Common Errors

• Stating that covalent bonds within the polymer chain are broken during boiling (a major conceptual error—intermolecular forces break, not covalent bonds).

✅ Correct Answers

1. 1,4-diaminobutane: H₂N-CH₂-CH₂-CH₂-CH₂-NH₂
2. Benzene-1,4-diacyl dichloride: Benzene ring attached directly to two opposite -COCl groups.
3. Repeat unit of polymer: -[HN-CH₂-CH₂-CH₂-CH₂-NH-CO-(benzene ring)-CO]- with clear amide linkages ( -NH-CO- ) and correct end bonds.

💡 Key Knowledge

Condensation polymerisation between a diamine and a diacyl dichloride forms a polyamide with the elimination of hydrogen chloride ( HCl ).

🧠 Exam Technique

Check the linkage carefully: amine nitrogen ( N ) bonds directly to the carbonyl carbon ( C=O ) of the acyl chloride to create the characteristic peptide/amide bond.

❌ Common Errors

• Drawing ester linkages ( -COO- ) instead of amide linkages ( -CONH- ).
• Retaining the -Cl or extra H atoms inside the polymer repeat unit structure.