Crude oil contains hydrocarbons with different chain lengths. As the length of the hydrocarbon chain increases, the properties of the hydrocarbons change. Describe how the following properties change as the chain length of a hydrocarbon increases, and explain why each property changes in the way it does: (i) boiling point and (ii) viscosity.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.2 Crude oil · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

(i) Boiling point rises with chain length because longer chains have more London dispersion forces; the greater surface area gives more points of contact, so more energy is required to overcome the forces and vapourise.
(ii) Viscosity also rises with chain length. Longer chains are more entangled and have stronger dispersion forces, so they resist flow and the liquid becomes thicker.

Examiner tips

  • Use the word ‘rises’ or ‘increases’ for both properties; link to London dispersion forces and surface area.
  • Explain that higher boiling point means more energy needed to break intermolecular forces; higher viscosity means greater resistance to flow due to entanglement and stronger forces.

Common mistakes

  • Saying boiling point decreases or viscosity decreases; confusing the direction of change.
  • Failing to mention London dispersion forces or surface area as the cause; giving a vague or incorrect explanation.

Mark scheme (4 marks)

  1. Boiling point increases as chain length increases
  2. Longer chains have stronger intermolecular forces / more points of contact between molecules, so more energy is needed to separate them
  3. Viscosity increases as chain length increases / longer chain hydrocarbons flow less easily
  4. Longer chains become more entangled with each other / stronger intermolecular forces between longer chains resist flow

Key terms in this question

hydrocarbons · boiling point · viscosity · chain length

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