Poly(ethene) is a simple molecular substance used to make plastic bags. Explain why poly(ethene) has a low melting point and does not conduct electricity.

OCR GCSE Chemistry A: Gateway Science (J248) — C2.3 Properties of materials · Explain · 4 marks · View as Markdown

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

Poly(ethene) is a polymer made up of long chains of carbon and hydrogen atoms. It is widely used in packaging because it is lightweight and flexible.

Model answer (4 marks)

Poly(ethene) has weak London dispersion forces between its chains, so only a small amount of energy is required to overcome these forces and the material melts at a low temperature.

Poly(ethene) contains only C and H atoms and has no free or delocalised electrons. Without mobile charge carriers it cannot carry an electric current, so it does not conduct electricity.

Examiner tips

Use the word ‘weak’ to describe the forces; mention London dispersion. Explain that low energy → low melting point. State that only C and H atoms give no free electrons. Link lack of charge carriers to no conductivity.

Common mistakes

Confusing polymer with simple molecule; describing covalent bonds as the reason for low melting point. Claiming that poly(ethene) conducts electricity because it is a solid. Omitting the role of free electrons in electrical conduction.

Mark scheme (4 marks)

Poly(ethene) has weak intermolecular forces between its molecules
Only a small amount of energy is needed to overcome these intermolecular forces, so the melting point is low
Poly(ethene) has no free/delocalised electrons
Therefore there are no charged particles free to move and carry charge, so it cannot conduct electricity

Key terms in this question

simple molecular substance