4.3 Intermolecular forces

What are intermolecular forces?

Forces of attraction between molecules (not within them).

Define London (dispersion) forces.

Weak intermolecular forces caused by temporary dipoles from electron movement.

Why do London forces increase with molar mass?

More electrons → greater temporary dipoles → stronger attraction.

Define dipole–dipole forces.

Attractions between permanent dipoles of polar molecules.

Define hydrogen bonding.

Strong dipole–dipole attraction between molecules where H is bonded to N, O, or F.

Why is hydrogen bonding unusually strong?

Large electronegativity difference + small size of H → strong attraction.

Define polarity of a molecule.

Uneven distribution of charge due to shape and bond polarity.

What two factors determine molecule polarity?

Bond polarity and molecular geometry.

Why do polar molecules have higher boiling points than nonpolar molecules of similar mass?

Stronger dipole–dipole forces require more energy to break.

Why does water have a high boiling point?

Extensive hydrogen bonding between molecules.

Define van der Waals forces.

A collective term for London forces and dipole–dipole interactions.

Which intermolecular force is present in all molecules?

London (dispersion) forces.

What determines the strength of dipole–dipole forces?

The magnitude of the molecular dipole moment.

Why do linear molecules often have stronger London forces?

Greater surface area for contact → stronger induced dipoles.