2.2.9—IMF strength and properties
- Syllabus
- First assessment 2025
- Objective
- 2.2.9
- Level
- SL
IMF Strength and Properties
For the relative comparison in this topic, London dispersion forces are weaker than dipole-dipole forces, which are weaker than hydrogen bonding. Molecular size also affects dispersion strength.
Stronger intermolecular attractions generally reduce volatility. Explain conductivity and solubility by considering whether charged particles are available and whether solute–solvent attractions are favourable.
Compare like evidence: pentane has stronger dispersion forces and a higher boiling point than butane because its electron cloud is larger. The simple London < dipole–dipole < hydrogen-bond ordering is a guide for comparable molecules, not a rule that ignores molecular size and the number of interaction sites.
Explaining IMF-Dependent Properties
Assessment in practice
Representative question
Question 1
[Maximum number: 2]
Explain, in terms of the intermolecular forces present, the trend in the boiling points of the first four alkenes.
| Alkene | Boiling point / K |
|---|---|
| ethene | 169 |
| propene | 225 |
| but-1-ene | 267 |
| pent-1-ene | 303 |
London (dispersion) forces «only»
stronger forces of attraction with increasing chain length / larger electron cloud /molar mass
Marking guidance:
Accept dispersion forces/ instantaneous / transient / induced dipole attractions for M1. Do not accept van der Waals' forces for M1.
Accept "more electrons" for "larger electron cloud" in M2.
Accept increased surface area.
The Covalent Model Summary
Retrieve the covalent pathway: shared pairs and bond order lead to geometry, polarity and molecular polarity; structure determines network properties, IMF behaviour and chromatography; HL representations extend to resonance, formal charge, sigma/pi bonds and hybridization.
Check the representation first, then count domains, apply geometry, identify polarity or forces, and connect the structure to the requested property or HL bonding description.