Intermolecular Forces
and
Liquids and Solids
Chapter 12
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.
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
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
Dispersion
forces usually
increase with
molar mass.
12.2
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.
S
SO2
SO2 is a polar molecule: dipole-dipole forces. There are
also dispersion forces between SO2 molecules.
12.2
Intermolecular Forces
Hydrogen bonding (H-bonding)
A special kind of dipole-dipole interaction
• 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?
Intermolecular Forces
Hydrogen bonding (H-bonding)
A special kind of dipole-dipole interaction
• 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.
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
H…B
or
A
H…A
A & B are N, O, or F
12.2
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
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
Summary of intermolecular forces
Ion-induced dipole and dipole-induced dipole are additional types of
dispersion forces.
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
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
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
Adhesion
Cohesion
water
mercury
12.3
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
High
viscosity
12.3
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
Water is a Unique Substance
Maximum Density
40C
Thermal expansion
Ice is less dense than water
H2O trapping in
cavities
12.3
Solids
Crystalline—high order
(regular repeating pattern)
Amorphous—low order