3.1.3.7 - Forces between molecules

Permanent dipole-dipole forces

In substance made of molecules with permanent dipoles, will be weak electrostatic forces of attraction between

Van der Waals forces

• Electrons in clouds always moving quickly
  At any moment, electrons are likely to be more one side than the other - at this point, atom has temporary dipole

• Temp dipole can cause another temp dipole in opposite direction on neighbouring atom - dipoles attracted to each other

• 2nd dipole can cause dipole on 3rd atom

• Because electrons constantly moving, dipoles being created/destroyed all the time

• Overall effect = atoms attracted to each other

Stronger Van der Waals forces = (higher/lower) boiling point

HIGHER

• Larger molecules → larger electron cloud → stronger VdW

• Molecule shape affects VdW strength

  • Long, straight molecules lie closer together than branched → closer molecules = stronger VdW

• When boiling, need to overcome intermolecular forces
  Stronger intermolecular forces → more energy → higher BP

Hydrogen bonding only happens when hydrogen is covalently bonded to ___

Fluorine, nitrogen or oxygen

How does hydrogen bonding form?

• F, N, O very electronegative → attract bonding electrons away from H atom

• Bond is polarised
  → H has such high charge density (because it’s small) that H atoms form weak bonds with LP on F, N, O

Molecules with hydrogen bonding usually contain ___ groups

-OH or -NH

Water + ammonia both have H bonding

Substances with hydrogen bonds have ___ melting and boiling points

HIGHER

Because of extra energy needed to break hydrogen bonds