Intermolecular Forces
4.3.1 Describe the types of intermolecular force
(hydrogen bond, dipole-dipole attraction and
van der Waals' forces) and explain how they
arise from the structural features of molecules.
4.3.2 Describe and explain how intermolecular
forces affect the boiling points of substances.
Intramolecular forces

Bonding forces that exist within each
molecule and influence the chemical
properties of the substance
Intermolecular Forces
Non-bonding force that exist between the
molecules and influence the physical
properties of the substance.
 Ex: boiling points are higher for molecules
that have stronger intermolecular forces


As we discussed with the melting point trends
for the halogens in the periodicity unit
Dipole-Dipole Forces



Dipole = polar
molecule
Dipoles will change
their direction so that
their oppositely
charged ends are near
to one another.
The electrostatic
attraction between the
ends is dipole-dipole
force
Ion-Dipole Forces



The force of attraction
between an ion and a
polar molecule.
The charge of the ion
will attract the
oppositely partial
charge of the dipole.
Na+ is a cation, so it
attracts the slightly
negative end of the
water (the oxygen)
London (dispersion) Forces


weakest
intermolecular force
between non polar
molecules
It is a temporary
attractive force that
results when the
electrons in two
adjacent atoms
occupy positions that
make the atoms form
temporary dipoles
London (dispersion) forces

Why does the temporary dipole occur?



It results from the random movements of the
electrons, especially the valence electrons,
that produces and inductive effect on the other
molecules close by.
The temporary dipole will cause the electrons
in another molecule to be repelled (or move
away). This will cause an induced dipole (since
the electrons moved away, that end of the
molecule is more +, while the other end
becomes more -)
The net result is that there is on average a
stronger attractive force between molecules
than repulsive.
Dipole-Dipole forces
The electrostatic attraction between the
molecules with permanent dipoles
 Cause an increase in B.P for those
molecules of similar size that are not
permanent dipoles.
 Ex: HCl B.P is 188 K and F2 B.P is 85 K


Both have London (dispersion) or van der
Waal’s forces between them, but only HCl also
has dipole-dipole forces.
Hydrogen Bonding


It is the attractive
force between the
hydrogen attached to
an electronegative
atom of one molecule
and an
electronegative atom
of a different
molecule.
It is the strongest of
the intermolecular
forces
Hydrogen bonding
The molecule involved with hydrogen
bonding should have at least one lone
pair. More lone pairs= stronger hydrogen
bonds.
 Hydrogen bonding results between the
interaction of the lone pair of one atom
(small and highly electronegative, like N,
O or F) with the hydrogen atom of another
molecule
 Part way between a dipole-dipole bond
and a dative covalent bond

Formula
Intermolecular
force
Polar or Nonpolar
H2O2
BP 431 K
(hydrogen
bonding)
polar
C3H8
BP 231 K (van
Non-polar
der Waals)
BP 294 K (no
polar
hydrogen bond)
CH3COH
(ethanal)
CH3CH2OH
(ethanol)
BP 352 K
(hydrogen
bond)
polar
Boiling points
Tetrahedrals with all the same surrounding
atoms are NON-POLAR and tend to have
low B.P because there is very little
intermolecular forces involved.
 The more lone pairs on the oxygen atom,
the more hydrogen bonding, the higher
the B.P (ex: H2O)

Strength of intermolecular force
based on atomic size





Melting point is a measure of the difference in
strength of the forces between particles between
the solid and liquid state.
M.P for HF is -118 oC
M.P for HCl is -114.2 oC
M.P for HBr is -86.8 oC
M.P for HI is -50.80 oC


The larger the atomic size, the more energy it requires
to melt the molecule.
The larger the size, the more electrons there are moving
randomly around, so the more likely and probably
stronger van der Waals forces occuring between each
molecule.
Readings

Pages 450-456



Look over sample problems and be able to
interpret graphs.
Pay attention to the table 12.2 for a good
review of all intra and intermolecular forces
Practice problems

Page 495 #12.37, 12.38, 12.39, 12.41, 12.52,
12.55
Websites and resources:

http://www.ausetute.com.au/intermof.html


http://www.chem.purdue.edu/gchelp/liquids/indd
ip.html


Induced dipoles (ion induced or dipole induced)
http://www.mikeblaber.org/oldwine/chm1045/not
es/Forces/Intermol/Forces02.htm


Intermolecular forces and trends in physical properties
review
Review intermolecular forces
http://en.wikipedia.org/wiki/Hydrogen_bond

Hydrogen bonding