AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The repeating units in polyesters (eg Terylene) and polyamides (eg nylon…
Full specification wording
Condensation polymers (A-level only): The repeating units in polyesters (eg Terylene) and polyamides (eg nylon 6,6 and Kevlar) and the linkages between these repeating units.
A repeating unit is the smallest section of chain that repeats, drawn inside brackets with the two bonds crossing the brackets and n written outside.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A repeating unit is the smallest section of chain that repeats, drawn inside brackets with the two bonds crossing the brackets and n written outside. Terylene is a polyester made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol, so its repeating unit contains one aromatic ring, one -CH₂CH₂- unit and two ester links. Nylon 6,6 is a polyamide from hexanedioic acid and hexane-1,6-diamine, each of which contributes six carbon atoms, which is where the name comes from; its repeating unit contains two amide links. Kevlar is also a polyamide, but both monomers are aromatic, benzene-1,4-dicarboxylic acid and benzene-1,4-diamine, which is what makes its chains rigid and flat.
What earns the marks
- The repeating unit drawn with open bonds passing through the brackets and n outside.
- The correct ester linkage in the polyester.
- The correct amide linkage, with the nitrogen bonded to the carbonyl carbon, in the polyamide.
- Naming or drawing both monomers of nylon 6,6 or of Kevlar.
Common mistakes and how to put them right
Drawing the repeating unit with the water still present, so the monomers are simply placed side by side.
Ending the repeating unit with an -OH and an -H, so the bonds do not continue to the next unit.
Reversing the amide link, putting the nitrogen where the oxygen should be.
Drawing Kevlar with aliphatic chains, which makes it nylon rather than Kevlar.
Exam tips
- The bonds must be drawn through the brackets. A repeating unit with tidy closed ends is the commonest way to lose this mark.
- Check your repeat unit by imagining two copies side by side: the ends must join without adding or removing atoms.
Try the lesson
The start of the Condensation polymers (A-level only): The repeating units in polyesters (eg Terylene) and polyamides (eg nylon 6,6 and Kevlar) and the linkages between these repeating units. lesson, made from checked teaching notes for this specification point.
Have a go first
Which linkage is formed between the repeating units in the polyester Terylene?
Listen
A repeating unit is the smallest section of chain that repeats, drawn inside brackets with the two bonds crossing the brackets and n written outside. Terylene is a polyester made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol, so its repeating unit contains one aromatic ring, one -CH₂CH₂- unit and two ester links. Nylon 6,6 is a polyamide from hexanedioic acid and hexane-1,6-diamine, each of which contributes six carbon atoms, which is where the name comes from; its repeating unit contains two amide links. Kevlar is also a polyamide, but both monomers are aromatic, benzene-1,4-dicarboxylic acid and benzene-1,4-diamine, which is what makes its chains rigid and flat. A strong answer earns marks for these points. the repeating unit drawn with open bonds passing through the brackets and n outside. the correct ester linkage in the polyester. the correct amide linkage, with the nitrogen bonded to the carbonyl carbon, in the polyamide. naming or drawing both monomers of nylon 6,6 or of Kevlar. Exam tip: The bonds must be drawn through the brackets. A repeating unit with tidy closed ends is the commonest way to lose this mark.
Key words
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repeating unit
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polyester linkage
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polyamide linkage
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aromatic polyamides
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See every definition
- repeating unit
- The smallest section of chain that repeats, drawn inside brackets with the two bonds crossing the brackets and n written outside.
- polyester linkage
- An ester link between the repeating units of a polyester such as Terylene.
- polyamide linkage
- An amide link between the repeating units of a polyamide, with the nitrogen bonded to the carbonyl carbon.
- aromatic polyamides
- Polyamides such as Kevlar in which both monomers are aromatic, which is what makes its chains rigid and flat.
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