Intramolecular Forces
Is the Molecule Polar?
We have already talked about diatomic molecules. 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. Thus giving the molecule
A dipole moment.
But what about molecules made up of more than two molecules?
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 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
Intermolecular Forces
1. London Forces (Dispersion Forces)
2. Dipole-Dipole Interactions
3. Ion-Dipole Interactions (Salt dissolving in solution)
4. Hydrogen Bonding
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a 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 are feeble; but
without them, life as we know it would
be impossible. Water would not
condense from vapor into solid or liquid
forms if its molecules didn't attract each
other. Intermolecular forces are
responsible for many properties of
molecular compounds, including crystal
structures (e. g. the shapes of
snowflakes), melting points, boiling
points,
heats of fusion and vaporization,
surface tension, and densities.
Intermolecular forces pin gigantic
molecules like enzymes, proteins, and
DNA into the shapes required for
biological activity.
http://www.nationmaster.com/encyclopedia/Image:Myoglobin.png
Intermolecular Forces
1. London Forces (Dispersion Forces)
2. Dipole-Dipole Interactions
3. Ion-Dipole Interactions (Salt dissolving in solution)
4. Hydrogen Bonding
Dispersion Forces
Occur between every compound and arise from the net attractive forces
which is produced from induced charge imbalances
Figure 10-8 Olmsted Williams
The magnitude of the Dispersion Forces
is dependent upon how easily it
is to distort the electron cloud.
The larger the molecule the greater
it’s Dispersion Forces are.
Figure 10-9 Olmsted Williams
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2
Dipole Forces occur between molecules containing a dipole moment.
The positive end of the dipole moment on one molecule is attracted to the
Negative end of the dipole moment on a nearby molecule.
Consider 2-methyl propane
(left) and acetone (right)
Both compounds are about
Equal in size and shape therby
Having similar dispersion forces,
But Acetone contains an
Oxygen (red) and causes the
Molecule to have a dipole
Moment allowing it to have
Dipole forces and thus a
Higher boiling point
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
Hydrogen Bond
The hydrogen bond is a special dipole-dipole interaction
between they hydrogen atom in a polar N-H, O-H, or F-H bond
and an electronegative O, N, or F atom.
A
H…B
or
A
H…A
A & B are N, O, or F
11.2
Fig 10-16B
Pg 444
Crystals of
benzoic acid
contain pairs of
molecules held
together head to
head by
hydrogen bonds.
These pairs
then stack in
planes which are
Held together by
dispersion forces.
Courtesy Stephen Frisch
Intermolecular Forces
1. London Forces (Dispersion Forces)
2. Dipole-Dipole Interactions
3. Ion-Dipole Interactions (Salt dissolving in solution)
4. Hydrogen Bonding
These forces affect how molecules will interact with each other and
As a general rule as the strength of the force increases the boiling
Point of the compound increases