Intermolecular Forces Van der Waals Forces (Johannes van der Waals) Dipole-Dipole Attractions (Polar) Dispersion Forces (Nonpolar and Polar, induced dipole) - Dipole-induced (London Dispersion) - Ion-induced

1. Intermolecular Forces
O+ -
H
H

2. Van der Waals Forces (Johannes van der Waals)
Dipole-Dipole Attractions (Polar)
Dispersion Forces (Nonpolar and Polar, induced dipole)
- Dipole-induced (London Dispersion)
- Ion-induced

3. Dipole-Dipole Attraction
• Attractive forces that exist
between molecules that have
permanent dipoles.
• These exist in any polar
substance.
• Weaker than Ion-Dipole force
• Increased polarity, stronger
dipole-dipole attraction
H Cl
+ -
H Cl
+ -
dipole
dipole
Attractive force

4. The dipole–dipole interactions in (a) crystalline CH3CN and (b) liquid CH3CN.
Example

5. Molecular weights, dipole moments, and boiling points of several simple
organic substances.
For molecules of approximately
equal mass and size, the
strength of intermolecular
attractions
increases with increasing
polarity.

6. Determine which of the
following would exhibit
Dipole-Dipole forces:
H2S, CO2, C2H4
H2S is Polar. Dipole-Dipole.
CO2 is Nonpolar.
C2H4 is Nonpolar.
..
H-S-H
..
Polar
.. ..
O=C=O
.. ..
Nonpolar
H-C=C-H
| |
H H
C - H bonds are
nonpolar, and C = C
bond is nonpolar.
Nonpolar

7. Dispersion Force:London Dispersion/London
Force (Fritz London)
• A temporary dipole is induced in
a non polar molecule due to
electron movements.
• These INDUCE similar but
opposite forces in neighboring
molecules which cause weak
momentary attractions.
• These are the WEAKEST
attractive forces that exist
between molecules.
+ -
Electrons
repelled
Induced dipole
Weak short lived
attraction
+ -
Temporary dipole
Electrons
moving

8. Dispersion forces. “Snapshots” of the charge distribution
for a pair of helium atoms at three instants.

9. • How would the
strength of London
forces be affected
by the size of the
molecule??
number of electrons in an atom or molecule, Polarizability
atomic or molecular size, molecular weight, Polarizability
Polarizability, Strength of dispersion forces
Example:
Boiling points of the
halogens and noble
gases. This plot shows
how the boiling points
increase as the
molecular weight
increases due to
stronger
dispersion forces.
List the substances
CCl4, CBr4, and CH4
in order of increasing
boiling point.
CH4 < CCl4 < CBr4

10. • Molecular shape also influences the magnitudes of dispersion forces.
Example: C5H12