Intermolecular Forces (IMFs)

What Makes Molecules Stick

To One Another?

Intermolecular vs. Intramolecular

Intermolecular forces are the attractive forces between molecules.

Intramolecular forces (bonds) hold atoms together within a molecule and are much stronger

41 kJ to vaporize 1 mole of water (inter)

930 kJ to break all O-H bonds in 1 mole of water (intra)

Dipole-Dipole Interactions

The IMFs that hold polar molecules (permanent dipoles) together

H-bonding is one example…

X - Y ……….…... X - Y ………..…... X - Y

Attractive force

Hydrogen Bonds in Water

How Do Nonpolar Molecules Stick Together?

charge symmetric(no net +, -)

Dispersion (London) Forces

IMF = interaction of induced (temporary) dipoles Individual interaction is weaker than ordinary

dipole-dipole force

+ - + -

nonpolar molecules

attractive force

induced dipoles

dEcoulomb q1q2/d

•q for permanent dipoles > q for induced dipoles

Why Weaker? What’s Different?

Dispersion (London) Forces

Magnitude increases with molecular size (weight)

more e- to polarize large molecules where e-’s are far from

nuclei are easier to polarize than small molecules

Dispersion Forces Are Important In ~All Molecules

Only IMFs for nonpolar molecules Account for up to 85% of total IMF in

polar cmpds Minor only when H-bonding or ion-ion

forces are present

boiling point (l g)

melting point (s l)

Evaporization

Emelting

Esublimation

IMFs Influence Physical Properties and Phase Changes

Phase Changes

sublimatio

n

deposition

GAScondensation

vaporization

freezing or fusion

melting

SOLID LIQUID

E Relationships In Phase Changes

Energy vaporization condensation

gas

liquid

Evaporization= -Econdensation

endothermicexothermic

Energy of Vaporization

Energy to vaporize one mole Increases as IMFs get stronger

Compound Evap, kJ/molHF 25.2HCl 17.5H2O 40.7CH4 8.9He 0.08

IMFs?Hbonding, LondonDipole-dipole, LondonHbonding, LondonLondonLondon