Topic 7 · page 1 of 14

Forces between molecules

Boiling separates whole molecules from each other and leaves the atoms inside them joined. So boiling temperature measures the forces between molecules, and there are three kinds of them, in order of strength.

10 min read7.1

Boiling separates molecules 7.1

Water boils at 100 °C. Hydrogen sulfide, H₂S, boils at −60 °C, so it is a gas at room temperature.

H₂O H₂S
MrM_\text{r} 18 34
State at 25 °C liquid gas
Boiling temperature 100 °C −60 °C

The H₂S molecule is almost twice as heavy, yet it boils 160 °C lower. A heavier molecule usually boils higher, so mass cannot be the reason. The reason lies in what boiling actually does.

Two boxes of water molecules. In the liquid, 11 molecules sit close together, joined by dashed lines that stand for the forces between molecules. In steam, 4 molecules sit far apart with no dashed lines. Every molecule in both boxes is still H₂O.
Boiling pulls whole molecules apart. Steam is still H₂O.

When a liquid boils, its molecules gain enough energy to move far apart from each other. Each molecule stays whole. The covalent bonds inside the molecule are not broken. What is overcome is the attraction between one molecule and the next.

Melting works the same way: a solid melts when its particles gain enough energy to move past each other, which overcomes some of the forces between them. So a high boiling temperature means strong forces between molecules, because more energy is needed to separate them.

Three forces, in order of strength 7.1

The specification names three intermolecular forces. You meet each one on its own page.

Force Found in Strength
London forces (instantaneous dipole–induced dipole) every molecule weakest; grows with the number of electrons
Permanent dipole–permanent dipole interactions molecules with a net dipole stronger than London forces between molecules with similar numbers of electrons
Hydrogen bonds molecules with H bonded to N, O or F the strongest of the three

Every molecule has London forces. A polar molecule has permanent dipole–dipole interactions as well. A molecule with an O–H, N–H or H–F bond has hydrogen bonds as well. The forces add up. They do not replace one another.

All three are far weaker than a covalent bond. That is why a liquid boils long before any molecule falls apart.

Check yourself

  1. What happens to the molecules of ethanol, C₂H₅OH, when it boils? Is any covalent bond broken?
  2. Define an intermolecular force.
  3. List the three intermolecular forces in order of increasing strength.
  4. A student writes: "Water boils at 100 °C because the O–H bonds are strong and need a lot of energy to break." Give the mistake and write a correct sentence.
  5. Hydrogen sulfide has a larger MrM_\text{r} than water but a much lower boiling temperature. Say what this tells you about the forces between the molecules.
  6. Ethanol has hydrogen bonds but boils at 78 °C. Octane has only London forces but boils at 126 °C. Why does this not contradict the ladder of strengths?

Answers cover these until you have tried the questions

  1. The molecules gain energy and move far apart from each other, so the liquid becomes a vapour. Each molecule stays as C₂H₅OH. No covalent bond is broken; only the forces between molecules are overcome.
  2. A force of attraction between one molecule and another (it acts between molecules, not inside them).
  3. London forces < permanent dipole–permanent dipole interactions < hydrogen bonds.
  4. The mistake is saying that the O–H bonds break. Boiling leaves the molecules whole. Correct: "Water boils at a high temperature because hydrogen bonds between the molecules (as well as London forces) are strong, and a lot of energy is needed to overcome them."
  5. Mass does not decide boiling temperature here. The forces between water molecules must be much stronger than those between H₂S molecules (this is due to hydrogen bonds in water; page 7).
  6. The ladder only compares molecules with similar numbers of electrons. Octane has 66 electrons (C₈H₁₈: 8 × 6 + 18 = 66) and ethanol only 26 (C₂H₆O: 12 + 6 + 8 = 26), so London forces between octane molecules are much stronger than those between ethanol molecules, enough to outweigh the extra hydrogen bonds in ethanol.