Intermolecular Forces and

1
Intermolecular Forces and Liquids and Solids
Chapter 12
2

• 12.7 The compounds Br2 and ICl have the same
  number of electrons, yet Br2 melts at -7.2oC,
  whereas ICl melts at 27.2oC. Explain.
• 12.9 The binary hydrogen compounds of the Group
  4A elements are CH4 (-162oC), SiH4 (-112oC), GeH4
  (-88oC), and SnH4 (-52oC). The temperature in
  parentheses are the corresponding boiling points.
  Explain the increase in boiling points from CH4
  to SnH4.
• 12.10 List the types of intermolecular forces
  that exist in each of these species (a) benzene
  (C6H6), (b) CH3Cl, (c) PF3, (d) NaCl, (e) CS2.

3
Intermolecular Forces
Dispersion Forces (London forces)
Attractive forces that arise as a result of
temporary dipoles induced in atoms or molecules
Instantaneous-induced dipole
The strength of dispersion forces tends to
increase with increased molecular weight. Larger
atoms have larger electron clouds, which are
easier to polarize.
The tendency of an electron cloud to distort in
this way is called polarizability.
12.2
4
Intermolecular Forces
Dispersion Forces
Polarizability is 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

12.2
5
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.
SO2
SO2 is a polar molecule dipole-dipole forces.
There are also dispersion forces between SO2
molecules.
12.2
6
Intermolecular Forces

• The strength of dispersion forces tends to
  increase with increaseing molecular weight.
• Increased strength of dispersion forces usually
  leads to an increased boiling point (see blue
  curve)
• Why do the binary hydrogen compounds of Group 6A
  not follow this trend?

7
Intermolecular Forces

• Why do the binary hydrogen compounds of Group 6A
  not follow this trend?

These molecules are polar so a dipole-dipole
interaction is also operative. H2O, in
particular is capable of a special
dipole-dipole interaction that is unusually
strong.
8
Intermolecular Forces
Hydrogen Bond
The hydrogen bond is a special dipole-dipole
interaction between the hydrogen atom in a polar
N-H, O-H, or F-H bond and an electronegative O,
N, or F atom.
A B are N, O, or F
12.2
9
Intermolecular Forces
Hydrogen Bond
The dipole-dipole interactions experienced when H
is bonded to N, O, or F are unusually
strong. H-bonding arises, in part, from the high
electronegativity of nitrogen, oxygen, and
fluorine.
12.2
10
Intermolecular Forces
Hydrogen Bond
Fluorine is more electronegative than oxygen so
it is reasonable to expect a stronger H-bond in
liquid HF than in H2O. However, the boiling
point of HF is lower than that of H2O, indicating
a weaker H-bonding network in HF. Explain.
Each H-F can participate in two H-bonds while
each H2O can make four H-bonds.
12.2
11
Summary of intermolecular forces
Ion-induced dipole and dipole-induced dipole are
additional types of dispersion forces.
12
Intermolecular forces affect properties of liquids
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
12.3
13
Intermolecular forces affect properties of liquids
Surface tension is the amount of energy required
to stretch or increase the surface of a liquid by
a unit area.
Surface tension allows water glider walk on
water.
Surface tension results from the net inward
force experienced by the molecules on the surface
of a liquid which causes the surface to tighten
like an elastic film.
12.3
14
Properties of Liquids
Surface tension is responsible for capillary
action.
Cohesion is the intermolecular attraction between
like molecules
Adhesion is an attraction between unlike molecules
Cohesion
12.3
water
mercury
15
Properties of Liquids
Viscosity is a measure of a fluids resistance to
flow.
Strong intermolecular forces
High viscosity
12.3
16
Structure and properties of water

• All life processes involve water
• Excellent solvent for many ionic compounds as
  well as other substances capable of forming
  hydrogen bonds with water (i.e., DNA)
• Moderator of climate owing to a high specific
  heat (absorbs heat in summer and gives off heat
  in winter with only small changes in the
  temperature of water)
• Unique substance in that solid ice is less dense
  than liquid water (usually solids are more dense
  than liquids)

12.3
17
Water is a Unique Substance
12.3
18
Solids

• Crystallinehigh order
• (regular repeating pattern)

Amorphouslow order