Alkenes & Polymers

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FreeScienceLessons · AQA ChemistryAlkenes and Polymers

Cognito · AQA ChemistryAddition polymerisation

Key facts

Alkenes

Alkenes = unsaturated hydrocarbons with a C=C double bond. General formula: CₙH₂ₙ.
Ethene C₂H₄, propene C₃H₆, butene C₄H₈.
Test for alkene — bromine water. Alkene → bromine water decolourises (goes from orange to colourless). Alkane → no reaction.
Addition reaction — another molecule adds across the double bond, which opens up.
Ethene + H₂O (steam) → ethanol (hydration). Requires catalyst and high temp/pressure.

Polymerisation

Addition polymerisation — many alkene monomers join together. Double bond opens → long chain forms.
Ethene → poly(ethene): n CH₂=CH₂ → –(CH₂–CH₂)ₙ–
The repeat unit in the polymer has single bonds only.
Condensation polymerisation — monomers with two functional groups join releasing small molecules (water or HCl).
Nylon (polyamide): amine + carboxylic acid → amide bond + water.
Polyester: diol + dicarboxylic acid → ester bond + water.

Exam questions — 4 questions · 10 marks

1 markAlkene testAQA 8462 P2 style

How can you distinguish between an alkane and an alkene using a simple chemical test?

A Add universal indicator — alkenes turn it red

B Add bromine water — alkenes decolourise it from orange to colourless; alkanes have no effect

C Burn both — alkenes produce a smoky flame; alkanes burn cleanly

D Add sodium hydroxide — alkenes react but alkanes do not

1 markAddition polymerisationAQA 8462 P2 style

Which type of monomer is required for addition polymerisation?

A A molecule with two carboxylic acid groups

B A molecule with two amine groups

C An unsaturated molecule with a carbon–carbon double bond

D Any molecule that can dissolve in a monomer solution

4 marksCompare addition and condensation polymerisationAQA 8462 P2 style

Compare addition polymerisation and condensation polymerisation. Give the type of monomer needed for each, what bonds form, and whether any small molecule is released. (4 marks)

Hint: Two clear contrasts: monomer type, bond formed, whether small molecule released. Cover both types with examples.

+40 XP

3 marksBromine water testAQA 8462 P2 style

A student has two unlabelled liquids — one is hexane and one is hexene. Describe a test to identify which is which and explain the chemistry. (3 marks)

Hint: Test: add bromine water. Result: alkene → decolourises. Alkane → stays orange. Explain: addition reaction across double bond.

+30 XP

Alkenes

Polymerisation

Module complete! 🎉