Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
(Example: water molecule to water molecule)
Intramolecular forces hold atoms together within in a molecule.
(Example: H to O bond within a water molecule).
Intermolecular vs Intramolecular
•
41 kJ to vaporize 1 mole of water (inter)
•
930 kJ to break all O-H bonds in 1 mole of water (intra)
“Measure” of intermolecular force
Generally,
intermolecular
forces are much
weaker than
intramolecular
forces.
boiling point
melting point
DHvap
DHfus
DHsub
11.2
Intermolecular Forces
1. London Forces (a.k.a. Dispersion Forces) Weakest
2. Dipole-Dipole Interactions
3. Ion-Dipole Interactions
-(Salt dissolving in solution; Na+ and Cl- )
4. Hydrogen Bonding
(STRONGEST)
London Dispersion Forces: Weakest
Occur between every compound and arise from the net attractive forces
amount molecules which is produced from induced charge imbalances
Figure 10-8 Olmsted Williams
The larger the molecule the greater
it’s Dispersion Forces are.
Animation:
http://chemmovies.unl.edu/ChemAnim
e/LONDOND/LONDOND.html
Figure 10-9 Olmsted Williams
The boiling point of long molecules increase with the length of the
chain.
Olmsted Williams
Fig 10-10
Pg 437
How molecular shape affects the strength of the dispersion forces
The shapes of the molecules also matter. Long thin molecules can
develop bigger temporary dipoles due to electron movement than short
fat ones containing the same numbers of electrons.
Butane has a higher boiling point because the dispersion forces are
greater.
http://www.chemguide.co.uk/atoms/bonding/vdw.html
Polarizability
the ease with which the electron distribution in the atom or
molecule can be distorted.
Polarizability increases with:
•
greater number of electrons
•
more diffuse electron cloud
Dispersion
forces usually
increase with
molar mass.
11.2
Is the Molecule Polar?
The more Electronegative atom will pull the electron density of the bond
Closer to itself giving it a partial negative charge leaving the other
Atom with a partially positive charge. This is a dipole moment.
Molecules with 3 Atoms
Even though the C-O bond is polar, the bonds
cancel each other out because the molecule is
linear the dipole moments are equal and in
opposite directions.Therefore CO2 is non-polar.
CO2
HCN
SO2
The dipole moment between H-C points in
the direction of C. The dipole moment points
between C-N points in the direction of the N.
Therefore the dipole vectors are additive and
HCN is polar
SO2 is a polar molecule because the S-O
dipole Moments don’t cancel each other out
due to the angle
Molecules with 4 Atoms
CCl4 is non-polar
CHCl3 is polar
How to Determine if a Molecule
Is Polar
1. Draw Lewis Structure
2. If all of the regions of electron density are
bound to the same thing (CCl4; CO2 ) than the
molecule is non-polar
3. If the regions of electron density are not bound to
the same thing (atom) than the molecule
is polar (HCN; SO2)
Which of the following molecules are polar (have a
dipole moment)?H2O, CO2, SO2, and CH4
O
S
dipole moment
polar molecule
dipole moment
polar molecule
H
O
C
O
no dipole moment
nonpolar molecule
H
C
H
H
no dipole moment
nonpolar molecule
10.2
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
Animation: http://chemmovies.unl.edu/ChemAnime/DIPOLED/DIPOLED.html
11.2
Dipole Forces occur between molecules containing a dipole moment.
The positive end of the dipole moment on one mole is attracted to the
Negative end of the dipole moment on a nearby molecule.
2-methyl propane
(left) and acetone (right)
Both compounds are about
Equal in size and shape
Figure 10-11
Olmsted Williams
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
The larger the charge the stronger the force
11.2
Olmsted Williams
Fig 10-34
A molecular picture showing the ion-dipole
Interaction that helps a solid ionic crystal dissolve
in water. The arrows indicate ion-dipole interactions.
What type(s) of intermolecular forces exist
between each of the following molecules?
HBr
HBr is a polar molecule: dipole-dipole forces. There are
also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
S
SO2
SO2 is a polar molecule: dipole-dipole forces. There are
also dispersion forces between SO2 molecules.
11.2
The Hydrogen Bond
STRONGEST
INTERMOLECULAR
FORCE
http://chemmovies.unl.edu/ChemAnime/HYBOND/HYBONDD.html
Animation
water has
water
water
thehas
has
water
thehas
highest
theheat
highest
highest
the
heat
highest
of
water
has
the
lowest
of
fusion
melting
vaporization
point
boiling point
molar
mass
The melting point, boiling point, heat of fusion and heat of
vaporization of water are extremely high and do not fit the
trend of properties relative to molar mass within Group
VIA.
Water exhibits these unusual properties
because of hydrogen bonding between
water molecules.
• A hydrogen bond is an intermolecular
bond.
• A hydrogen bond is formed between
polar molecules that contain hydrogen
covalently bonded to a small, highly
electronegative atom: F, O, N.
F—H
O—H
N—H
• it will be attracted to another F, O, or N, on
another molecule.
• A dipole-dipole bond bond will be
formed between the two molecules
which is called a hydrogen bond.
hydrogen
bond
covalent
bond
covalent
bond
Water in the liquid and solid states exists as groups in
which the water molecules are linked together by
hydrogen bonds.
13.8