Intermolecular forces

MDCAT Chemistry · Chemistry · Liquids. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Covalent bonds hold the atoms together inside a molecule. Intermolecular forces are the much weaker attractions between one molecule and the next. Melting and boiling a molecular substance overcome only these forces, so stronger forces mean a higher boiling point, a higher viscosity and a lower vapour pressure.

There are three kinds. London dispersion forces act between all molecules. Dipole–dipole forces are added when the molecules are polar. A hydrogen bond is an extra strong dipole attraction that needs a hydrogen atom bonded directly to nitrogen, oxygen or fluorine.

Hydrogen bonding explains why water has a high boiling point, why ice floats, and why small alcohols and acids dissolve in water.

Rules to remember

London forces: the only force between non-polar molecules (Br₂, CH₄, noble gases); stronger with more electrons and a longer, less branched shape
Dipole–dipole forces: between polar molecules such as HCl, propanone and ethanal
Hydrogen bond: H joined to N, O or F is attracted to a lone pair on N, O or F of another molecule
For molecules of similar size: hydrogen bond > dipole–dipole > London; all are far weaker than a covalent bond
Hydrogen-bond strength: F–H···F > O–H···O > N–H···N; water still boils higher than HF because each molecule can form more hydrogen bonds
Ice: each H₂O is hydrogen bonded to 4 others in an open tetrahedral structure, so ice is less dense than water; water is densest at 4 °C
Carboxylic acids form hydrogen-bonded dimers; hydrogen bonds hold the two strands of DNA together

Common mistake

Students say that boiling breaks the covalent bonds in the molecule, or that any compound containing hydrogen can hydrogen bond. The hydrogen must be bonded to N, O or F, so CH₃CHO cannot hydrogen bond to itself.

Worked example

Propane, C₃H₈, ethanal, CH₃CHO, and ethanol, C₂H₅OH, have almost the same molar mass (44 to 46 g mol⁻¹). Put them in order of increasing boiling point.

Similar molar masses mean similar London forces, so compare the other forces.
Propane is non-polar: London forces only, so it has the lowest boiling point.
Ethanal has a polar C=O group but no H on O: it adds dipole–dipole forces.
Ethanol has an O–H group, so it forms hydrogen bonds, the strongest of the three.

Answer: propane < ethanal < ethanol

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