Intermolecular Forces
A phase is a homogeneous part of the system in
contact with other parts of the system but
separated from them by a well-defined boundary.
2 Phases
Solid phase - ice
Liquid phase - water
11.1
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
amount molecules 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
The boiling point of alkanes increase with the length of the carbon
chain. Long-chain alkanes have larger dispersion forces because
of the increased polarizability of their larger electron cloud.
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.
Long thin molecules can also lie closer together - these attractions are
at their most effective if the molecules are really close.
For example, the hydrocarbon molecules butane and 2-methylpropane
both have a molecular formula C4H10, but the atoms are arranged
differently. In butane the carbon atoms are arranged in a single chain,
but 2-methylpropane is a shorter chain with a branch.
Butane has a higher boiling point because the dispersion forces are
greater. The molecules are longer (and so set up bigger temporary
dipoles) and can lie closer together than the shorter, fatter 2methylpropane molecules.
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?
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
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 mole 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
Liquids and Surface Tension
Surface tension is the amount of energy required to stretch
or increase the surface of a liquid by a unit area.
Strong
intermolecular
forces
High
surface
tension
11.3
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
11.3
T2 > T1
Condensation
Evaporation
Least
Order
Greatest
Order
11.8
The equilibrium vapor pressure is the vapor pressure
measured when a dynamic equilibrium exists between
condensation and evaporation
H2O (l)
H2O (g)
Dynamic Equilibrium
Rate of
Rate of
= evaporation
condensation
A substance with a high
Vapor pressure is considered
To be volitile therefore, the lower
The boiling point the higher the
Vapor pressure and the weaker
The intermolecular forces
11.8
The boiling point is the temperature at which the
(equilibrium) vapor pressure of a liquid is equal to the
external pressure.
The normal boiling point is the temperature at which a liquid
boils when the external pressure is 1 atm.
11.8
The critical temperature (Tc) is the temperature above which
the gas cannot be made to liquefy, no matter how great the
applied pressure.
The critical pressure
(Pc) is the minimum
pressure that must be
applied to bring about
liquefaction at the
critical temperature.
11.8
The melting point of a solid
or the freezing point of a
liquid is the temperature at
which the solid and liquid
phases coexist in equilibrium
Freezing
H2O (l)
Melting
H2O (s)
11.8
11.8
Molar heat of sublimation
(DHsub) is the energy required
to sublime 1 mole of a solid.
Deposition
H2O (g)
Sublimation
H2O (s)
DHsub = DHfus + DHvap
( Hess’s Law)
11.8
A phase diagram summarizes the conditions at which a
substance exists as a solid, liquid, or gas.
Phase Diagram of Water
11.9
11.9