5_Properties_of_liquids_and_solids.pdf - Properties of...

CHEM 1000 3.0 Structure of liquids and solids 1
Properties of Liquids and Solids
•Petrucci, Herring Madura and Bissonnette :
Chapter 12

CHEM 1000 3.0 Chemical Bonding 2
Problem set
Chapter 12
questions 1, 22, 29, 36, 41, 51, 65, 87, 105,
108, 125, 127, 139

CHEM 1000 3.0 Structure of liquids and solids 3
Van der Waal Forces
• In the section on gases we said there were
forces between the molecules. Now we
know more about bonding and electronic
structure we can revisit the question.
• What kinds of forces exist between
molecules?

CHEM 1000 3.0 Structure of liquids and solids 4
Van der Waal Forces
• Van der Waal forces are those attractive
forces between molecules that are responsible
for:
– Real gas behaviour (Van der Waal “a”)
– Condensation
• There are a number of such forces resulting in
different strengths of interactions between
molecules.

CHEM 1000 3.0 Structure of liquids and solids 5
Van der Waal Forces
London or Dispersion Force
• The quantum concept of electronic structure
talks about the probability of an electron
being in a certain region at a certain time.
• Even for an atom or a perfectly symmetrical
molecule there is a finite chance that the
electronic charges are not uniformly
distributed.
• This produces an instantaneous dipole .

CHEM 1000 3.0 Structure of liquids and solids 6
Van der Waal Forces
London or Dispersion Force
• The instantaneous dipole can then influence
neighbouring molecules and induce a dipole
in them. (an induced dipole )
• This gives two dipoles that can attract. This
is usually called a dispersion force or a
London force .
Molecule
on average
Instantaneous
dipole

CHEM 1000 3.0 Structure of liquids and solids 7
Van der Waal Forces
London or Dispersion Force
a. Normal
b. Instantaneous dipole
c. Instantaneous dipole (left) induced dipole (right)

CHEM 1000 3.0 Structure of liquids and solids 8
Van der Waal Forces
London or Dispersion Force
• This force is going to be strongest in a molecule
with a large number of electrons or for an
elongated molecule.
• The ability or tendency of a molecule to have
charge separation occur is called polarizability .
• The effect of London forces can often be seen in
the boiling points of similar compounds.

CHEM 1000 3.0 Structure of liquids and solids 9
Effect of London Forces
• Bpt(He) = 4K
• Bpt(Rn) = 211K Rn is larger
____________________________________
n-pentane is more elongated
CH
3
-CH
2
-CH
2
-CH
2
-CH
3
Bpt 36.1
o
C
Bpt 9.5
o
C
C C H
3
CH
3
CH
3
CH
3

CHEM 1000 3.0 Structure of liquids and solids 10
Van der Waal Forces
Dipole-Dipole interactions
• Bonds where the elements have different
electronegativity have a permanent charge
separation.
E.g. HCl
• H →Cl
• If this shows up in the molecule we say the
molecule has a permanent dipole.

CHEM 1000 3.0 Structure of liquids and solids 11
Van der Waal Forces
Dipole-Dipole interactions
• For molecules with permanent dipoles,
Dipole-Dipole interactions can occur.

CHEM 1000 3.0 Structure of liquids and solids 12
Van der Waal Forces
Dipole-Dipole interactions

CHEM 1000 3.0 Structure of liquids and solids 13
Van der Waal Forces
Dipole-Dipole interactions
• Dipole-dipole interactions
are in addition to the
London forces.