Example 1
Why does H2O have a higher boiling point than H2S despite its lower molar mass?
Water molecules form extensive hydrogen bonding; H2S does not meet the usual FON hydrogen-bonding requirement.
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Year 11 · SACE Stage 1
Intermolecular forces and physical properties
Classify attractions and compare simple boiling-point patterns.
Method and worked examples
Dispersion attractions occur between all atoms and molecules through temporary and induced dipoles. They generally become stronger as electron-cloud size and polarisability increase, and molecular shape affects contact. Polar molecules also have permanent dipole–dipole attractions. Hydrogen bonding is a particularly strong interaction involving H bonded to N,O or F and a suitable lone pair on a neighbouring molecule.
For comparable molecular sizes, stronger intermolecular attractions generally mean more energy is required to separate molecules and a higher boiling point. Do not compare only the label of the strongest force when molecular sizes differ greatly. Boiling water mainly overcomes attractions between molecules; it does not usually break the O–H covalent bonds inside each water molecule. All three kinds of attraction may coexist in one substance.
Example 1
Example 2
Why does iodine boil at a higher temperature than bromine?
I2 has a larger, more polarisable electron cloud than Br2, producing stronger dispersion attractions.
Watch out: Hydrogen bonding is an attraction involving H bonded to N,O or F and a neighbouring suitable lone pair.