Explain why high-density polyethylene (HDPE) has a higher melting point and greater tensile strength than low-density polyethylene (LDPE), despite both polymers having the same empirical repeat unit, –CH₂–.

IB DP Chemistry Higher Level (2023 syllabus) — S2.4 From models to materials · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

LDPE has significant chain branching, whereas HDPE chains are largely unbranched/linear.
Linear HDPE chains can pack more closely and regularly together, increasing crystallinity.
Closer packing leads to greater London/van der Waals dispersion forces between chains in HDPE.
Stronger intermolecular forces in HDPE require more energy to overcome, explaining its higher melting point and greater tensile strength compared with LDPE.

Examiner tips

  • Use the word ‘because’ to link cause and effect clearly.
  • Mention ‘branching’ and ‘crystallinity’ – these are key terms the mark scheme looks for.
  • Show the logical sequence: branching → less packing → lower crystallinity → weaker forces → lower melting point and tensile strength.
  • Keep the answer concise – 4 marks, so one sentence per point is enough.

Common mistakes

  • Confusing the terms ‘density’ with ‘molecular weight’ – both polymers have the same repeat unit. Forgetting to mention the role of crystallinity. Using vague phrases like ‘more energy’ without linking to intermolecular forces.

Mark scheme (4 marks)

  1. LDPE has significant chain branching whereas HDPE chains are largely unbranched/linear
  2. Linear HDPE chains can pack more closely/regularly together, increasing crystallinity
  3. Closer packing leads to greater London/van der Waals dispersion forces between chains in HDPE
  4. Stronger intermolecular forces in HDPE require more energy to overcome, explaining its higher melting point and greater tensile strength compared with LDPE

Key terms in this question

HDPE · LDPE · tensile strength · melting point

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