2000 Chemistry Lecture 9: Intermolecular forces
Marc R. Roussel
Bond polarity
When the two atoms at each end of a bond are dierent, they
will not share electrons equally.
This results in bond polarity, i.e. a separation of charge
between the atoms at either end of the bond.
The bond polarity will increase the greater the dierence in
electronegativity between the atoms.
Example: N2 is nonpolar.
Example: CO should be more polar than NO.
Molecular polarity in diatomic molecules
In diatomic molecules, if the bond is polar, then the molecule
is automatically polar.
Molecular polarity can be calculated or measured.
Dipole moment: vector measure of molecular polarity
=
qi ri
atoms
qi = partial charge on atom i
ri = position vector of atom i
For a neutral diatomic molecule, the size of the dipole
moment vector (or just the dipole moment) is
|| = = |q |R
|q | = absolute value of the partial charge carried by each atom
R = bond length
The dipole moment vector calculated from the formula points
toward the positive end of the molecule.
However, chemists draw their dipole moment vectors pointing
the other way, using an arrow with a cross bar at the positive
end of the dipole:
+ .
:N
..
O
..
Measurement of dipole moments
Dipole moments can be measured in two ways:
Temperature dependence of the relative permittivity:
The force between two charges in any medium is
F=
q1 q2
4 r 0 r 2
where r is the relative permittivity of the
medium.
For solids and liquids, the relative permittivity is
easily measured by measuring the capacitance of
a pair of parallel plates separated rst by
vacuum and then by the material under study.
r depends on temperature in a way which
allows us to recover both the dipole moment
and the polarizability of the medium.
Stark eect in rotational spectroscopy:
Molecules in the gas phase have discrete
rotational energy levels.
Molecules with a permanent dipole moment can
undergo pure rotational transitions.
These are studied by microwave spectroscopy.
In the presence of an added electric eld, the
rotational levels shift (Stark eect).
The size of the shift tells us the size of the
molecular dipole moment.
A mess of units...
The SI unit for the dipole moment is the C m.
Dipole moments in C m are small ( 1030 C m).
Example: The dipole moment of HCl is 3.60 1030 C m.
Although its use is discouraged, many people like to use the
debye (D), a non-SI unit, as a unit for dipole moments
because these values are of order unity.
1 D = 3.335 64 1030 C m
Example: The dipole moment of HCl is 1.08 D.
A more modern approach which gives nice values is to use
the dipole length = /e .
is the distance we would have to separate two elementary
charges (+e and e ) to get the dipole moment .
Example: The dipole length of HCl is 22.5 pm.
(Compare: HCl bond length = 127 pm)
Polarity in polyatomic molecules
We can get molecular dipole moments in polyatomic
molecules by vector addition of bond dipoles.
For our purposes, it will suce to carry out this procedure
graphically (and qualitatively).
Example: CO2 bond dipoles:
..
O
..
C
..
O
..
Because of the symmetry, the bond dipoles cancel, so
CO2 is nonpolar ( = 0).
Example: OCS bond dipoles:
..
O
..
C
..
S
..
The molecular dipole moment is nonzero, and the
corresponding dipole moment vector points toward O.
Example: BF3 bond dipoles:
F
B
F
F
To add vectors, place them tip to tail:
The bond dipoles add to zero, so BF3 has a zero
dipole moment.
Example: CH4 .
For simplicity, Ill only draw the vectors without their
cross bars:
Adding the bottom two vectors gives a vector that points
straight down:
Adding the top two vectors gives a vector that points up with
the same magnitude as the one shown above.
Adding all four vectors therefore gives zero.
Conclusion: Methane is nonpolar.
Example: CH2 Cl2 .
Cl
Cl
C
H
H
Red = C-Cl bond dipoles. Green = C-H bond dipoles.
Blue = net dipole moment (not to scale).
Intermolecular forces
The properties of chemical systems are strongly aected by
the forces which act between molecules in these systems.
There are dierent kinds of intermolecular forces.
Some are stronger than others.
We will survey the dierent types of intermolecular forces,
from strongest to weakest.
Ion-ion forces
The Coulomb force is the strongest one which operates on the
length scales of interest to chemists.
Because of the strength of this force, ionic compounds
typically have high boiling points.
When we dissolve an ionic compound in a solvent, forces
between the ions result in a relatively organized state where,
on average, anions are closer to cations than to other anions.
Ion-dipole forces
There are also fairly strong forces between ions and dipoles.
When we dissolve an ionic compound in a polar solvent, this
results in substantial organization of the solvent around the
ions.
Mg 2+
Dipole-dipole forces
Dipoles exert forces on each other.
Dipoles tend to align themselves so that the positive end of
one is close to the negative end of another.
F
F
F
F
N
N
F
F
Dipole-induced dipole forces
If we place a molecule next to a polar molecule, the electric
eld of the dipole exerts a force on the electrons of the other
molecule, resulting in an induced dipole.
The polarizability is a measure of how large an induced dipole
is produced for a given electric eld.
The more loosely held the electrons are, the larger the
polarizability.
= atomic polarizability increases as we move down a group.
All other things being equal, the polarizability will be larger
for long molecules because a small charge separation over a
large distance can result in a large dipole moment ( = |q |d ).
The induced dipoles orientation will always generate an
attractive force to the polar molecule.
H
O
+
H
+
N
N
Fluctuation-induced dipole
Due to the random essentially motion of electrons around a
molecule, from time to time, molecules will acquire a dipole
moment when there happen to be more electrons on one side
of the molecule than the other.
This short-term uctuation will in turn induce dipoles in
adjacent molecules.
Fluctuation-induced dipole forces are also known as London
dispersion forces.
The strength of dispersion forces depends on the polarizability.
Dispersion forces can occur even in molecules which have a
permanent dipole moment.
Induced dipoles can be produced in directions orthogonal to .
Example: In H2 O, there is a permanent dipole moment parallel
to the symmetry axis of the molecule.
H
O
H
Dipole moment components can be induced
perpendicular to this axis (perpendicular to molecular
plane, or parallel to the H-H axis).
Boiling points
All other things being equal, boiling points will be higher if
the intermolecular forces are larger because it takes more
energy to remove a molecule from the liquid.
Example: Group 14 boiling points
200
180
Tb /K
160
140
120
100
(Why?)
CH4
SiH4
GeH4
400
Group 15
Group 16
Group 17
H2O
350
Tb /K
300
250
Normal trend
HF
NH3
H2S
200
PH3
HCl
H2Se
AsH3 HBr
H2Te
SbH3
HI
150
2
3
4
Period
Why do NH3 , H2 O and HF break the trend?
5
Hydrogen bonding
In molecules containing O-H and N-H bonds and in HF, we
observe dramatically increased intermolecular forces.
We call this additional type of intermolecular force hydrogen
bonding.
Molecules involved in hydrogen bonding are much closer
together than molecules experiencing (e.g.) dipole-dipole
interactions, but not as close as covalent bonding distances.
(Known from X-ray and neutron scattering experiments)
Hydrogen bonding (continued)
Hydrogen bonding is due to a combination of factors:
Large electronegativity of N, O and F
No core orbitals in H
Availability of lone pairs on N, O and F
The rst two factors result in a very small electron density on
the side of H away from a covalent bond to N, O or F.
This allows H to interact with a lone pair. This interaction is
partly a dipole-dipole interaction, but also has some covalent
character.
Hierarchy of intermolecular forces
1. Ion-ion
2. Ion-dipole
3. Hydrogen bonding
4. Dipole-dipole
5. Dipole-induced dipole
6. Induced dipole-induced dipole (London dispersion)
Higher members of the hierarchy are stronger.
Higher members are usually accompanied by some or all of
the lower members.
Example: Water is subject to all the forces from hydrogen
bonding to London dispersion.
Some manifestations of intermolecular forces in liquids
Other than boiling points, there are other macroscopic
manifestations of the varying strengths of intermolecular
forces in liquids.
surface tension
viscosity
Surface tension
Bubbles and drops tend to be round. Why?
A sphere is the shape of minimum area-to-volume ratio.
Increasing the surface area takes work.
Well see why in a minute.
surface tension =
work
increase in surface area
On average, the intermolecular forces on a molecule in the
bulk (away from the surface) sum to zero.
A molecule at the surface experiences a net force directed
inward.
Adding molecules to the surface (increasing the surface area)
requires that we push them to the surface against this
unbalanced force.
This is the origin of surface tension.
Surfactants
Suppose that we add a solute to a solvent.
Suppose also that the forces between the solute and solvent
are weaker than the forces between the solute molecules.
As a rule in a situation like this, the solute-solute forces would
also be weaker than solvent-solvent forces.
For this kind of solute-solvent system, the solute molecules
would tend to congregate on the surface because the energy
required to put a solute on the surface is less than the energy
required to put a solvent molecule on the surface.
Accordingly, such a solute would tend to lower the surface
tension.
Solutes with these properties are called surfactants (surface
active agents).
Surfactants
Surfactants vary greatly in chemical and physical properties.
Detergents are a particularly important class of surfactants.
They have a polar (or charged) head and a nonpolar tail.
Example: the dodecylsulfate ion, CH3 (CH2 )11 OSO
3
Surface wetting
When we put a drop of liquid on a solid surface, we can
observe one of two qualitative situations:
Wetting: the drop spreads out on the surface.
Non-wetting: the drop assumes a spheroidal form.
Wetting occurs when molecules in the liquid are more strongly
attracted to the solid than to each other.
In this case, part of the liquid surface, which
requires energy to create, is replaced by an
energetically favorable contact with the solid.
Wetting is responsible for the familiar meniscus
seen when water is placed in a glass container.
Non-wetting is seen when the interactions between molecules of
the liquid are stronger than between the liquid and
solid.
In this case, we get an inverted meniscus, for
instance when we put mercury in a glass
container.
Capillary rise
Wetting is also responsible for capillary rise.
A capillary is a narrow tube.
The attractive force between the tube and liquid causes liquid
to rise within the capillary.
The solid-liquid attraction has to support the weight of the
column of water, so narrower capillaries lead to higher
capillary rise.
In non-wetting situations, we get capillary depression.
Viscosity
Viscosity is a liquids resistance to uid ow.
It is analogous to friction as one part of the liquid moves
relative to another.
Viscosity is directly related to intermolecular forces: stronger
intermolecular forces, higher viscosity.
Some of the highest viscosities are seen in ionic liquids.
Example: The viscosity of (bmim)(BF4 )
H 3C
+
N
CH 3
N
F
B
F
is more than 100 times that of water.
F
F