Intermolecular Forces and Physical Properties

Intermolecular Forces

interactions between molecules

• van der Waal’s forces (every molecule has these)

• dipole-dipole interactions

• hydrogen bonding

van der Waal’s

induced dipole formed as e- clouds pass eachother

• areas of partial negative and partial positive form as molecules are distorted

strength of van der Waal’s depends on polarizability (surface area), LARGER molecules=more surface area

dipole-dipole interaction

electrostatic interaction between two molecules with PERMANENT dipoles

• stronger than vdw (temporary dipoles)

hydrogen bonding

NOT REAL BONDS

VERY strong dipole interaction

• electrostatic attraction between H-atom bonded to O, N, F and lone pair on O, N, F in another molecule

IMFs and Physical Properties

IMFs control physical properties

• greater/stronger IMFs = higher BP, MP

• IMFs control solubility

Phase Transition: Boiling

BP= temp where liquid is converted to gas

stronger IMF = more energy required to break up interactions between molecules = higher BP

Phase Transition: Melting

MP = temp where solid → liquid

stronger IMF = more energy required to break up interactions between molecules = higher MP

Packing, only for MP

If all else is equal (IMF), consider which structure is more symmetrical

more symmetry = pack efficiently = more stable structure = more energy required to break up

Solubility

Like dissolves like

• interactions between solute and solvent must make up for loss of interaction between solute molecules

Matching IMFs = dissolves