AQA A-Level Chemistry Paper 3, June 2018: Question 22

1 mark · Easy difficulty · Multiple Choice

Identify the statement explaining why plastics made from a given polyester chain only soften at high temperatures.

Practise this question

Question

Question 22 shows the chemical structure of a repeating unit of a polyester, polyethylene terephthalate (PET), consisting of a benzene-1,4-dicarboxylate group linked to an ethylene unit via ester linkages. The question asks which statement correctly explains why plastics made from this polyester only soften at high temperatures, offering four options: A (Hydrogen bonds and van der Waals' forces exist between polyester chains), B (Permanent dipole-dipole forces and van der Waals' forces exist between polyester chains), C (The carbon-carbon bonds in the chain are strong), and D (The carbon-oxygen bonds in the chain are strong).
Question text

22 The structure of part of a polyester chain is shown.

Which statement correctly explains why plastics made from this polyester only soften at

high temperatures?

[1 mark]

A Hydrogen bonds and van der Waals’ forces exist between

polyester chains.

B Permanent dipole-dipole forces and van der Waals’ forces exist

between polyester chains.

C The carbon-carbon bonds in the chain are strong.

D The carbon-oxygen bonds in the chain are strong.

Mark scheme

Show the mark scheme Mark scheme for question 22 showing the correct answer as B.

22 B

How to answer it

Polyester Structure & Thermal Properties (PET)

What this question tests

This question evaluates your understanding of polymer physical properties and bonding:

  • Intermolecular forces vs covalent bonds: Recognising that melting/softening a thermoplastic polymer involves overcoming intermolecular forces between chains, not breaking covalent bonds along the polymer backbone.
  • Types of intermolecular forces in polyesters: Identifying that the polar ester linkages (Cδ+=Oδ-) give rise to permanent dipole–dipole attractions alongside van der Waals' (induced dipole–dipole) forces, while noting the absence of hydrogen bonding.
Question 22 Breakdown

Multiple Choice Analysis [1 Mark]

Identifying the force responsible for high softening temperatures in polyesters

✅ Correct Answer: B

Permanent dipole–dipole forces and van der Waals' forces exist between polyester chains.

Award 1 mark for selecting option B.

💡 Key Knowledge

  • The Polymer: The molecule shown is poly(ethylene terephthalate), commonly known as PET or Terylene.
  • Softening/Melting: When a thermoplastic polymer softens, chains slide past one another. Only intermolecular forces between adjacent chains are overcome; the strong covalent backbone remains intact.
  • Polar Groups: Carbonyl bonds (C=O) and C–O bonds are polar due to electronegativity differences between C and O. This produces permanent dipole–dipole attractions between chains.
  • Induced Dipoles: All molecular chains experience van der Waals' (dispersion) forces, which are significant here due to the large, extended electron clouds of the benzene rings and overall long chain length.
  • No Hydrogen Bonds: While oxygen atoms with lone pairs are present, there is no H atom directly bonded to an O, N, or F. Therefore, polyesters cannot form hydrogen bonds between their chains.

🧠 Exam Technique: Two-Step Elimination

  1. Step 1: Check the process (Intermolecular vs Intramolecular)
    Does softening break covalent bonds? No. Softening is a physical change involving weak forces between molecules.
    → Instantly eliminate options C and D.
  2. Step 2: Identify the correct intermolecular forces
    Are there H-bonds? Polyamides (like Kevlar and Nylon) have N–H bonds and can form hydrogen bonds. Polyesters only have C=O and C–O, with H bonded solely to C.
    → No H-bonding eliminates option A.
    → Leaves B as the only correct choice.

❌ Common Errors & Misconceptions

  • Option A Confusing Polyesters with Polyamides: Students often see oxygen atoms and immediately assume "hydrogen bonding". Remember: H-bonding requires a hydrogen atom attached to an electronegative atom (–O–H or –N–H). Polyamides have N–H···O=C hydrogen bonds; polyesters do not.
  • Options C & D Confusing Covalent Bonds with Intermolecular Forces: Covalent bonds (C–C and C–O) are indeed very strong, but they are not broken when plastics soften or melt. Breaking covalent bonds represents thermal decomposition (pyrolysis), not melting/softening.

📐 Summary Comparison: Polyesters vs Polyamides

Polymer Type Linkage Intermolecular Forces Present H-Bonding?
Polyester (e.g. Terylene) –COO– Permanent dipole–dipole + van der Waals' No
Polyamide (e.g. Nylon 6,6, Kevlar) –CONH– Hydrogen bonds + van der Waals' Yes (N–H···O=C)

Topics

Organic Chemistry · Physical Chemistry · 3.1.3 Bonding · 3.3.12 Polymers

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.