Chemistry / Unit 4: Chemical Bonding
4.5 Intermolecular Forces
Notes
Intermolecular forces (IMFs) are attractions between molecules. They are much weaker than bonds but control boiling point, melting point and solubility.
- London dispersion forces: in all molecules; caused by temporary dipoles. Stronger in larger molecules with more electrons.
- Dipole-dipole forces: between polar molecules.
- Hydrogen bonding: a strong dipole-dipole force when H is bonded to N, O or F. Found in water, ammonia and alcohols.
- Stronger IMFs mean higher boiling points. Water (hydrogen bonding) boils at 100 °C; methane (only dispersion) boils at -162 °C.
Practice
Which substance has hydrogen bonding between its molecules?
In $\text{NH}_3$, H is bonded directly to N, so hydrogen bonds form.
Which halogen has the highest boiling point?
All are nonpolar, so only dispersion forces act; $\text{I}_2$ has the most electrons and the strongest dispersion forces.
When water boils, what is broken?
Boiling separates molecules from each other; the molecules themselves stay intact.
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