2.5a: demonstrate an understanding of the nature of intermolecular forces resulting from interactions between permanent dipoles, instantaneous dipoles and induced dipoles (London forces) and from the formation of hydrogen bonds

VAN DER WAALS OR INDUCED DIPOLE-DIPOLE FORCES

Exist between all molecules, whether polar or non-polar

Weak intermolecular forces caused by attractions between very small dipoles in molecules

Form when electrons move around and create oscillating dipoles

• the oscillating dipole in one electron induces a dipole in the molecule, which induces dipoles in neighbouring molecules, and so on

Strength:

• more electrons (larger atomic radius/molecular size)
• larger oscillating and induced dipoles are created, causing a greater attractive force between molecules, so van der Waals forces are stronger
• points of contact
• straight-chain molecules are able to pack closer together, so have more points of contact, meaning that they have stronger van der Waals forces

Evidence:

• the boiling points of noble gases
• as you go down group 0, electron numbers increase, so there is more shielding, which makes it easier to distort electron clouds
• more oscillating and induced dipoles can form, so van der Waals forces are stronger and boiling point increases

PERMANENT DIPOLE-DIPOLE INTERACTIONS

Small δ+ and δ- charges on polar molecules attract oppositely charged dipoles on other polar molecules

A weak intermolecular force, but stronger than van der Waals forces

HYDROGEN BONDING

A special type of permanent dipole-dipole interaction

Strong intermolecular attraction between:

• an electron deficient hydrogen atom
• and a lone pair of electrons on a highly electronegative atom of F, O or N

The lone pair of electrons is key to hydrogen bonding

• the H+ atom is very small, so the attraction of its nucleus is strong
• the lone pair of electrons is attracted to the hydrogen

Properties of water arising from hydrogen bonding:

• solid H2O is less dense than liquid H2O because the hydrogen bonds hold the water molecules apart in an open lattice structure
• ice has a high melting point and water has a high boiling point, because of the strong hydrogen bonds between H2O molecules

Evidence:

• the boiling points of hydrides (group 6 and 7) increase from H2S to H2Te and from HCl to HI
• H2O and HF have unusually high boiling points compared to them, due to the extra forces from hydrogen bonds

NH3 (ammonia) shows hydrogen bonding in the liquid state