3.2.4—Physical property trends
- Syllabus
- First assessment 2025
- Objective
- 3.2.4
- Level
- HL
Organic Physical-Property Trends
Melting and boiling points depend on chain length, branching, polarity, and intermolecular-force strength. Larger molecules often have stronger dispersion forces, while branching changes contact and packing.
Explain a comparison by naming the relevant structural difference and the resulting change in intermolecular attraction or packing.
Straight-chain pentane has a larger contact surface and higher boiling point than more highly branched isomers of the same formula; lengthening a series usually strengthens dispersion forces. For different functional groups, include polarity and hydrogen bonding before attributing the trend to size alone.
Explaining Organic Trends
Assessment in practice
Representative question
Question 1
[Maximum number: 2]
Explain why the boiling point increases from methane to propane.
London/dispersion forces «only»
strength «of intermolecular forces» increases as size of electron cloud/number of electrons increases
Marking guidance:
Accept strength of intermolecular forces
increases as mass/size of molecule
increases for M2.
Organic Analysis Summary
Retrieve the route: translate formulae, identify functional groups and series, name and classify isomers, then use mass, IR, and NMR evidence together to determine structure.
Check connectivity, functional-group evidence, formula/mass constraint, shifts and integration, splitting neighbours, and agreement across every technique.