5 Properties of liquids and solids.2015.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?

Van der Waal Forces
• Van der Waal forces are the attractive
forces between molecules that are
responsible for:
– Real gas behaviour (Van der Waal “a”)
– Recall: 𝑃 +
𝑎 𝑛
2
𝑉
2
𝑉 − 𝑛𝑏 = 𝑛𝑅𝑇
• There are a number of such forces resulting
in different strengths of interactions
between molecules
CHEM 1000 3.0 Structure of liquids and solids 4

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
Normal
Instantaneous
dipole
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.

Effects of London Forces
CHEM 1000 3.0 Structure of liquids and solids 9

CHEM 1000 3.0 Structure of liquids and solids 10
Effect of London Forces
• Boiling Point (He) = 4K
• Boiling Point (Rn) = 211K Rn is larger
____________________________________
n-pentane is more elongated
CH
3
-CH
2
-CH
2
-CH
2
-CH
3
Boiling Point 36.1
o
C
Boiling Point 9.5
o
C
C C H
3
CH
3
CH
3
CH
3

Effects of London Forces
Compound Polarizability, *
10 -25 cm 3
Molar Mass, amu Boiling Point, K
H
2
7.90 2.0158 20.35
O
2
16.0 31.9988 90.19
N
2
17.6 28.0134 77.35
CH
4
26.0 16.04 109.15
C
2
H
6
44.7 30.07 184.55
Cl
2
46.1 70.906 238.25
C
3
H
8
62.9 44.11 231.05
CCl
4
105 153.81 349.95
CHEM 1000 3.0 Structure of liquids and solids 11
Adapted from Petrucci et al., 2011

CHEM 1000 3.0 Structure of liquids and solids 12
Van der Waal Forces
Dipole-Dipole interactions
• Bonds where the elements have different
electronegativity have a permanent charge
separation.