1. Intermolecular Forces (IMFs)
What Makes Molecules Stick
To One Another?

2. 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)

3. Dipole-Dipole Interactions
d d-
d d-
d d-
X - Y ……….…...
X - Y ………..…...
X - Y
Attractive force
The IMFs that hold polar molecules (permanent dipoles) together
H-bonding is one example…

4. Hydrogen Bonds in Water

5. How Do Nonpolar Molecules Stick Together?
charge symmetric
(no net d+, d-)

6. Dispersion (London) Forces
nonpolar molecules
attractive force
induced dipoles
IMF = interaction of induced (temporary) dipoles
Individual interaction is weaker than ordinary dipole-dipole force

7. Why Weaker? What’s Different?
Ecoulomb a q1q2/d
q for permanent dipoles > q for induced dipoles

8. 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

9. 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

10. IMFs Influence Physical Properties and Phase Changes
boiling point (l ® g)
melting point (s ® l)
DEvaporization
DEmelting
DEsublimation

11. Phase Changes GAS LIQUID SOLID sublimation condensation deposition
vaporization
freezing or fusion
LIQUID
SOLID
melting

12. E Relationships In Phase Changes
gas
Energy
vaporization
condensation
endothermic
exothermic
liquid
DEvaporization= -DEcondensation

13. Energy of Vaporization
Energy to vaporize one mole
Increases as IMFs get stronger
Compound DEvap, kJ/mol HF
HCl
H2O CH He
Hbonding, London
Dipole-dipole, London
London
IMFs?