# Intermolecular Forces and Properties
AP Chemistry Unit 3 focuses on forces between molecules (not within them) and how these determine physical properties like boiling point, solubility, and phase behaviour.
1. Types of Intermolecular Forces (IMFs)
London Dispersion Forces (LDF)
- Present in ALL molecules
- Temporary dipoles from random electron movement
- Stronger with more electrons (larger molar mass) and greater surface area
- Only IMF in nonpolar molecules
Dipole-Dipole Forces
- Between polar molecules (permanent dipoles)
- δ+ attracts δ−
- Stronger than LDF alone
Hydrogen Bonding
- Special strong dipole-dipole between H (bonded to F, O, or N) and lone pair on F, O, or N
- Explains anomalously high BP of H₂O, NH₃, HF
- ~10× stronger than typical dipole-dipole
Ion-Dipole
- Between ions and polar molecules (e.g. Na⁺ and H₂O)
- Important in dissolving ionic compounds
2. IMFs and Physical Properties
| Property | Stronger IMFs → |
|---|---|
| Boiling point | Higher |
| Melting point | Higher |
| Viscosity | Higher |
| Surface tension | Higher |
| Vapour pressure | Lower |
Ranking Boiling Points
- Identify IMFs present
- More/stronger IMFs → higher BP
- For same IMF type: larger molecule → stronger LDF → higher BP
3. Properties of Solids
| Type | Particles | Forces | Properties |
|---|---|---|---|
| Ionic | Ions | Ionic bonds | High MP, brittle, conduct when dissolved |
| Metallic | Metal ions + e⁻ | Metallic bonds | Conduct, malleable, high MP |
| Molecular | Molecules | IMFs | Low MP, don't conduct |
| Covalent network | Atoms | Covalent bonds | Very high MP, very hard |
4. Solutions and Solubility
"Like dissolves like":
- Polar solutes dissolve in polar solvents
- Nonpolar solutes dissolve in nonpolar solvents
- Ionic compounds dissolve if ion-dipole forces overcome lattice energy
5. Phase Diagrams
- Triple point: all three phases coexist
- Critical point: above this, no distinct liquid/gas boundary
- Lines represent phase boundaries (conditions where two phases coexist)
- Normal BP: temperature at 1 atm on liquid-gas boundary
6. Practice Questions
- Rank in order of increasing BP: CH₄, CH₃OH, CH₃CH₃. Explain.
- Why does HF have a lower BP than H₂O despite F being more electronegative?
- Explain why NaCl dissolves in water but not in hexane.
- Identify the strongest IMF in: (a) HCl, (b) CH₃OH, (c) CO₂.
- Explain why diamond has a much higher MP than ice.
Want to check your answers and get step-by-step solutions?
Summary
- IMFs: LDF < dipole-dipole < hydrogen bonding < ion-dipole
- Stronger IMFs → higher BP, MP, viscosity; lower vapour pressure
- Solid types: ionic, metallic, molecular, covalent network
- "Like dissolves like" — polarity determines solubility
- Phase diagrams show conditions for each phase