Explain why carbon nanotubes exhibit exceptionally high tensile strength and are also able to conduct electricity along their length.

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).

Carbon nanotubes (CNTs) are cylindrical allotropes of carbon in which the carbon atoms are arranged in a hexagonal lattice rolled into a seamless tube. They have tensile strengths far exceeding that of steel and can conduct electricity along their axis.

Model answer (4 marks)

Each carbon atom in a CNT is sp² hybridised, forming three σ bonds with neighbouring carbons in a trigonal‑planar arrangement that creates a continuous, rigid covalent network around the tube.
The σ bonds are strong, directional covalent bonds that require a large amount of energy to break, giving the CNT an exceptionally high tensile strength.
The remaining valence electron on each carbon occupies a p orbital perpendicular to the tube wall, producing a delocalised π system.
These delocalised π electrons can move freely along the tube axis, allowing electrical conduction along the length of the CNT.

Examiner tips

  • Use the word ‘sp² hybridised’ and ‘σ bonds’ to show understanding of bonding. Explain that σ bonds are strong and directional. Mention the delocalised π system and its role in conductivity. Keep the answer concise and directly linked to the marks scheme.

Common mistakes

  • Confusing sp³ with sp² hybridisation. Forgetting to mention the delocalised π electrons. Using vague terms like ‘strong bonds’ without specifying σ bonds.

Mark scheme (4 marks)

  1. Each carbon atom forms three sigma (σ) bonds to neighbouring carbon atoms in a trigonal planar / sp² hybridised arrangement, creating a continuous, rigid covalent network along and around the tube.
  2. The strong, directional covalent bonds require large amounts of energy to break, accounting for the exceptionally high tensile strength.
  3. The fourth valence electron of each carbon atom is not involved in sigma bonding and occupies a p orbital perpendicular to the tube wall, forming a delocalised π system.
  4. These delocalised π electrons are mobile along the length of the tube and can carry charge, allowing electrical conduction along the nanotube axis.

Key terms in this question

carbon nanotube · tensile strength

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