Structure 2.2.9 — Intermolecular Forces and Physical Properties of Covalent Substances

Covalent compounds consist of molecules held together by intermolecular forces (IMFs), not ionic or metallic bonds. These forces are weaker than intramolecular covalent bonds, but determine many physical properties.

Types of Intermolecular Forces (in increasing strength)

1. London (Dispersion) Forces – temporary dipoles between all molecules; only type in non-polar substances.
2. Dipole–Dipole Interactions – occur between polar molecules with permanent dipoles.
3. Hydrogen Bonding – strong dipole–dipole interaction when H is bonded to N, O, or F.

General Rule: For substances of similar molar mass, the strength follows:

\( \text{London Forces} < \text{Dipole–Dipole} < \text{Hydrogen Bonding} \)

Physical Properties of Covalent Substances (explained via structure and IMFs)

Volatility:

Covalent substances with weak intermolecular forces (e.g. London forces) are volatile (evaporate easily), e.g. hexane. Stronger IMFs (e.g. hydrogen bonds in water) decrease volatility.

Melting/Boiling Point:

Affected by IMF strength. Stronger IMF = higher boiling point.

For example:

• • \( \text{CH}_4 \) (non-polar, London forces) → very low boiling point
  • \( \text{H}_2\text{O} \) (hydrogen bonding) → much higher boiling point

Electrical Conductivity:

Most simple covalent molecules do not conduct electricity as they have no free ions or electrons.

Solubility:

Depends on polarity:

• • Polar covalent substances (e.g. ethanol) dissolve in polar solvents (e.g. water)
  • Non-polar molecules (e.g. iodine) dissolve in non-polar solvents (e.g. hexane)
  • “Like dissolves like” principle applies