3.6 Intermolecular forces, electronegativity and bond properties

Define polar.

A molecule with an uneven distribution of charge.

Define a hydrogen bond.

The strongest type of intermolecular force.

Explain hydrogen bonding.

A hydrogen bond is an attraction between a hydrogen atom covalently bonded to a very electronegative atom and a lone pair of electrons on another electronegative atom.

Give examples of hydrogen bonding in water and ammonia.

• H₂O: Each H–O bond is polar, oxygen has 2 lone pairs, δ⁺ hydrogen of one molecule strongly attracts lone pairs of another oxygen → up to 4 H bonds per molecule.
• NH₃: Each N–H bond is polar, nitrogen has 1 lone pair, δ⁺ hydrogen attracts lone pair of another N → hydrogen bonding. Each NH₃ forms fewer hydrogen bonds (max 2).

State anomalous properties of water due to hydrogen bonding.

Relatively high melting/boiling points → strong H bonds need more energy to break.

High surface tension → H bonds at surface create strong cohesive force.

Ice less dense than water → open lattice structure with spaces, so ice floats.

Define a dipole.

A molecule with areas of partial positive and negative charge.

What factors affect dipole strength?

• Difference in electronegativity (greater → stronger dipole).
• Bond length (longer → weaker dipole).

How can dipoles cancel or not in whole molecules?

• If molecule is linear and dipoles are equal and opposite → cancel → non-polar overall.
• If molecule is bent → dipoles do not cancel → polar overall.
• If dipoles don’t cancel in any shape → polar molecule.

Define Van der Waals’ forces.

The weak forces of attraction between molecules caused by the formation of temporary dipoles.

State the types of Van der Waals’ forces.

Instantaneous dipole-induced dipole (id-id) → London dispersion forces, caused by fluctuations in electron density, present in all molecules, stronger in bigger molecules.

Permanent dipole-permanent dipole (pd-pd) → attraction between positive and negative ends of polar molecules.

Hydrogen bonding → strong permanent dipole-dipole interaction when H is covalently bonded to N, O, or F.

Compare bond strength.

Ionic, covalent, and metallic bonds are stronger than intermolecular forces. Hydrogen bonding is stronger than other Van der Waals’ forces but weaker than covalent bond