Intermolecular forces

What are intermolecular forces (IMF)?

Forces of attraction between molecules that determine physical properties such as boiling/melting points, viscosity, evaporation rate, and surface tension.

Interatomic vs intermolecular forces?

Interatomic (covalent bonds) hold atoms together inside a molecule. Intermolecular forces act between molecules.

When do ion-dipole forces occur?

When an ionic substance dissolves in a polar solvent. Example: NaCl in water.

What are London (dispersion) forces?

Weak temporary attractions between non-polar molecules caused by momentary uneven electron distribution creating temporary dipoles.

What three factors increase London force strength?

1. More electrons (greater polarisability).

2. Larger electron cloud/higher molar mass.

3. Greater surface area (linear > branched).

State the two conditions for hydrogen bonding.

1. H must be covalently bonded to N, O, or F.

2. A lone pair must exist on N, O, or F of the neighbouring molecule.

Why does HF have an unexpectedly high boiling point?

Strong hydrogen bonding raises the bp from an expected ~−100°C to the actual +19°C.

Why does ice float on water?

H-bonds in ice are directional, forming a more open crystal structure that is less dense than liquid water.

Stronger IMF → what physical effects?

Higher melting/boiling points, more viscous, lower evaporation rate, higher surface tension.

What is viscosity?

The resistance of a liquid to flow. Stronger IMF = more viscous.

Boiling point

The temperature at which the vapour pressure of a substance equals atmospheric pressure.

Melting point

The temperature at which the solid and liquid phases of a substance are at equilibrium.

Vapour pressure

The pressure exerted by a vapour at equilibrium with its liquid in a closed system.

London forces

Non-polar + non polar

Dipole dipole

polar + polar

Dipole-induced dipole

polar + non-polar

Ion-induced dipole

Ion + non-polar

Ion dipole

ion + polar

Linear

2 groups
0 lone pairs

Angle: 180

Trigonal planar

3 groups
0 lone pairs

Angle: 120

Tetrahedral

4 groups
0 lone pairs

Angle: 109,5

Angular/ bent

4 groups

2 lone pairs

Angle: 104,5

Trigonal pyramidal

4 groups

1 lone pair

Angle: 107

Trigonal bipyramidal

5 groups

0 lone pairs

Angle: 90 and 120

Octahedral

6 groups

0 lone pairs

Angle: 90