Intermolecular Forces
Intramolecular Forces:
▫ The attractive forces between atoms and ions within a
molecule
▫ Types: Ionic bond, covalent bonds (e.g. polar, nonpolar,
double, triple)
▫ Relatively strong
Intermolecular Forces:
▫ The attractive forces between molecules
▫ Weak (in comparison to intramolecular forces)
▫ Ex: much less energy to melt H2O (inter) than for it to
decompose into H2 and O2 (intra)
▫ 3 types:
 Dipole-Dipole
 London Dispersion
 Hydrogen Bonding
Intermolecular Forces & Physical
Properties
The strength of the intermolecular forces determines:
• State
• Melting point and boiling point
• Surface tension
• Hardness/texture
• Solubility
1. Dipole-dipole
• Forces of attraction between oppositely charged
ends of polar molecules.
• Relatively strong intermolecular force
Dipole-dipole force
London Dispersion
• Attractive forces between all
molecules, including
nonpolar molecules
• Result of temporary
displacements of the
electron cloud around atoms
in a molecule (extremely
short-lived dipoles)
• Therefore, weaker than
dipole-dipole
Hydrogen Bonding
• Very strong dipole-dipole
force between the positive
hydrogen atom of one
H-bond
molecule and highly
electronegative atom of
another molecule (N, O, F)
Discussion/Recap:
• Rank the intermolecular forces from weakest to
strongest.
• If one compound was gas at room temperature, and
another was solid, what could you assume about
their intermolecular forces?
• How might intermolecular forces affect melting
point?
Practice
• List the intermolecular forces in the following
compounds:
▫ Cl2
▫ H2O
▫ HCl
Surface Tension
• Molecules in a liquid are
attracted by molecules on
all sides
• But, molecules at the
surface are only attracted
down or sideways.
Practice!
• Penny Activity
• p. 85 # 6
• P. 88 # 10, 11