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AS · Practice questions · Intermolecular forces

Pull molecules apart.

Six original Cambridge-style questions on intermolecular forces: how id-id forces arise, the effect of electrons and shape, polar and non-polar molecules and boiling point comparisons.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Name every force, then compare.

01
Explain
[3 marks]

Explain how instantaneous dipole-induced dipole forces arise between two argon atoms.

  • Electrons move constantly, so at any instant the electron distribution in one atom may be uneven, giving an instantaneous dipole. ✓
  • This dipole distorts the electron cloud of a neighbouring atom, inducing a dipole in it. ✓
  • The δ+ end of one attracts the δ− end of the other. ✓
02
Explain
[2 marks]

The boiling points of the noble gases increase from helium to xenon. Explain this trend.

  • The number of electrons per atom increases down the group. ✓
  • Larger electron clouds are more easily polarised, so the id-id forces between atoms are stronger and more energy is needed to separate them. ✓
03
Explain
[2 marks]

Tetrachloromethane, CCl4, contains polar C–Cl bonds but is a non-polar molecule. Explain why.

  • The molecule is tetrahedral and symmetrical. ✓
  • The four equal C–Cl bond dipoles point to the corners of the tetrahedron and cancel, so there is no overall dipole. ✓
04
Explain
[3 marks]

Butane (Mr 58, b.p. −1 °C) and propanone (Mr 58, b.p. 56 °C) have similar numbers of electrons. Explain the difference in boiling point.

  • Both have id-id forces of similar strength, because they have similar numbers of electrons. ✓
  • Propanone contains a polar C=O bond and is a polar molecule, so it also has permanent dipole-permanent dipole forces. ✓
  • More energy is needed to overcome the intermolecular forces in propanone. ✓
05
Analysis
[2 marks]

Place these in order of increasing boiling point and give a reason: 2,2-dimethylpropane, pentane, hexane.

  • 2,2-dimethylpropane < pentane < hexane. ✓
  • Hexane has the most electrons; of the two C5H12 isomers, the unbranched pentane has the larger contact area, so stronger id-id forces. ✓
06
Explain
[2 marks]

A student writes: “When iodine sublimes, the I–I bonds break.” Correct this statement.

  • Sublimation overcomes the weak id-id forces between I2 molecules. ✓
  • The covalent I–I bonds within the molecules are not broken; iodine vapour is still I2. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.