Chap. 13: Day 2
INTERMOLECULAR
FORCES
Type of Bond determines Forces
Generally strongest
“Interparticle”
Generally weaker
 Ionic
Covalent bonds
Bond
Ion-ion force
“intermolecular forces”


 Metalic
bond
Electronic force
Polar molecules
Dipole forces
Nonpolar molecules
London
Disperson
Induced dipole forces
Melting Point
Energy needed over
come the
intermolecular forces
between molecules in
the solid phase
Temperature at which
molecules in a solid gain
enough potential energy to
change position (become L)
Boiling Point
Energy needed over
come the
intermolecular forces
between molecules in
the liquid phase
Temperature at
which molecules in
a liquid gain enough
potential energy to
change position
(become gas)
POLAR vs NONPOLAR
Molecular Polarity
Molecules will be polar if
a)bonds are polar
AND
b)the molecule is NOT
“symmetric”
8
Polar or Nonpolar?
Compare CO2 and H2O. Which one is polar?
9
Polar or Nonpolar?
• Consider AB3 molecules: BF3, Cl2CO, and NH3.
10
Molecular Polarity, BF3
F
B
F
F
B—F bonds in BF3 are polar.
But molecule is symmetrical
and NOT polar
B atom is
positive and
F atoms are
negative.
11
Molecular Polarity, HBF2
H
B
F
F
B—F and B—H bonds in HBF2
are polar. But molecule is NOT
symmetrical and is polar.
B atom is
positive but
H & F atoms
are negative.
12
Is Methane, CH4, Polar?
Methane is symmetrical
and is NOT polar.
13
Substituted Ethylene
• C—F bonds are MUCH more polar than
C—H bonds.
• Because both C—F bonds are on same
side of molecule, molecule is POLAR.
14
NONPOLAR Molecules
What causes the differences
in intermolecular forces
between molecules?
Why do the intermolecular
forces vary between nonpolar
molecules?
Solid
Liquid
WHY?
Bigger Molecules interact more
DISPERSION
FORCE
Nonpolar Molecules
POLAR Molecules
Why do the
intermolecular
forces vary
between polar
molecules?
Molecular Polarity
Molecules—such as HCl and H2O— can be POLAR (or
dipolar).
They have a DIPOLE MOMENT. The polar HCl
molecule will turn to align with an electric field.
Figure 9.15
20
DIPOLE FORCE
Polar Molecules
More polar stronger force
Why different?
liquid
gas
?
POLAR
Molecules
SOME
Have “special”
Dipole force with
Hydrogen
HYDROGEN
FORCE or
HYDROGEN BOND
HYDOGEN BOND
NEED TO BE POLAR
H is attached to N, F, or O
ALL molecules have LONDON DISPERSION force
Graphing: Intermolecular Forces

400 k
Boiling point
See handout
4 different Families
 Boiling point vs
period

100 k
2
period
6