Structure 2.2.8 — Intermolecular Forces (IMFs)

Intermolecular forces are weak attractive forces between covalent molecules. These forces are not bonds (like covalent or ionic) but are responsible for many physical properties of substances such as boiling point, melting point, solubility, and volatility.

Main Types of Intermolecular Forces:

1. London (Dispersion) Forces

• • • Present in all covalent molecules, especially non-polar molecules.
    • Arise due to temporary dipoles caused by momentary electron distribution.
    • Strength increases with increased number of electrons and molecular size.
    • Example: Found in \( \text{I}_2 \), \( \text{CH}_4 \), noble gases like \( \text{Ne} \), \( \text{Ar} \).

2. Dipole–Dipole Interactions

• • Occur between permanently polar molecules (molecules with a permanent dipole).
  • Stronger than dispersion forces but weaker than hydrogen bonding.
  • Result from electrostatic attraction between the δ⁺ end of one molecule and the δ⁻ end of another.
  • Example: \( \text{HCl} \), \( \text{CH}_3\text{Cl} \), \( \text{SO}_2 \).

3. Dipole–Induced Dipole Forces

• • Occur when a polar molecule induces a dipole in a non-polar molecule.
  • Weaker than dipole–dipole forces and usually occur in mixtures.
  • Example: Oxygen dissolving in water: \( \text{H}_2\text{O} \) (polar) induces a dipole in \( \text{O}_2 \).

4. Hydrogen Bonding

• • Strongest type of intermolecular force (but still weaker than covalent bonds).
  • Occurs when hydrogen is bonded to highly electronegative atoms: N, O, or F.
  • Results from strong dipole–dipole attraction between the hydrogen atom (δ⁺) and lone pair of N/O/F (δ⁻).
  • Example: \( \text{H}_2\text{O} \), \( \text{NH}_3 \), \( \text{HF} \), DNA base pairing.

Key Point: The type and strength of intermolecular forces depend on the polarity, electron distribution, and molecular size.

How to Deduce the Type of IMF:

1. Is the molecule polar or non-polar? (Use shape and electronegativity differences)
2. Does it contain hydrogen directly bonded to N, O, or F? → Hydrogen bonding possible.
3. If polar → dipole–dipole; if non-polar → only London dispersion.
4. If polar + non-polar mixture → dipole–induced dipole possible.

Trends in Physical Properties:

• Boiling point: increases with stronger IMF (hydrogen bonding > dipole–dipole > dispersion).
• Volatility: substances with stronger IMFs are less volatile (i.e., evaporate less easily).
• Solubility: “like dissolves like” – polar substances dissolve in polar solvents (due to similar IMFs).