Intermolecular Forces:

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Chapter 12
Intermolecular Forces
Liquids, Solids, and Phase Changes
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Intermolecular Forces Liquids, Solids, and
Phase Changes
12.1 An Overview of Physical States and Phase
Changes
12.2 Quantitative Aspects of Phase Changes
12.3 Types of Intermolecular Forces
12.4 Properties of the Liquid State
12.5 The Uniqueness of Water
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Table 12.1
A Macroscopic Comparison of Gases, Liquids, and
Solids
State
Shape and Volume
Compressibility
Ability to Flow
Gas
Conforms to shape and volume of container
high
high
Liquid
Conforms to shape of container volume limited by
surface
very low
moderate
Solid
Maintains its own shape and volume
almost none
almost none
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Figure 12.1
Heats of vaporization and fusion for several
common substances.
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Phase changes and their enthalpy changes
Figure 12.2
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Figure 12.3
A cooling curve for the conversion of gaseous
water to ice
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Heat supplied
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Quantitative Aspects of Phase Changes
Within a phase, a change in heat is accompanied
by a change in temperature which is associated
with a change in average Ek as the most probable
speed of the molecules changes.
q (amount)(molar heat capacity)(DT)
During a phase change, a change in heat occurs at
a constant temperature, which is associated with
a change in Ep, as the average distance between
molecules changes.
q (amount)(enthalpy of phase change)
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The specific heat (s) of a substance is the
amount of heat (q) required to raise the
temperature of one gram of the substance by one
degree Celsius.
The heat capacity (C) of a substance is the
amount of heat (q) required to raise the
temperature of a given quantity (m) of the
substance by one degree Celsius.
C ms
Heat (q) absorbed or released
q msDt
q CDt
Dt tfinal - tinitial
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Liquid-gas equilibrium
Figure 12.4
T is constant
H2O (l) H2O (g)
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The effect of temperature on the distribution of
molecular speed in a liquid
Figure 12.5
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Molar heat of vaporization (DHvap) is the energy
required to vaporize 1 mole of a liquid.
P (equilibrium) vapor pressure
T temperature (K)
R gas constant (8.314 J/Kmol)
Vapor pressure as a function of temperature and
intermolecular forces
A linear plot of vapor pressure- temperature
relationship
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The Clausius-Clapeyron Equation
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SAMPLE PROBLEM 12.1
Using the Clausius-Clapeyron Equation
SOLUTION
34.90C 308.0K

T2 350K 770C
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A phase diagram summarizes the conditions at
which a substance exists as a solid, liquid, or
gas.
Phase Diagram of Water
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A phase diagram summarizes the conditions at
which a substance exists as a solid, liquid, or
gas.
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Intermolecular Forces
Intermolecular forces are attractive forces
between molecules.
Intramolecular forces hold atoms together in a
molecule.

• Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O-H bonds in 1 mole of water
  (intra)

Measure of intermolecular force boiling
point melting point DHvap DHfus DHsub
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Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
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Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar
molecule
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Intermolecular Forces
Dispersion Forces
Attractive forces that arise as a result of
temporary dipoles induced in atoms or molecules
ion-induced dipole interaction
dipole-induced dipole interaction
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Intermolecular Forces
Dispersion Forces Continued
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

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Intermolecular Forces
Hydrogen Bond
The hydrogen bond is a special dipole-dipole
interaction between they 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
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Why is the hydrogen bond considered a special
dipole-dipole interaction?
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Dipole moment and boiling point
Figure 12.13
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SAMPLE PROBLEM 12.2
Drawing Hydrogen Bonds Between Molecules of a
Substance
SOLUTION
(a) C2H6 has no H bonding sites.
(c)
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Figure 12.16
separated Cl2 molecules
instantaneous dipoles
Dispersion forces among nonpolar molecules
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Figure 12.17
Molar mass and boiling point
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Molecular shape and boiling point
Figure 12.18
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SAMPLE PROBLEM 12.3
Predicting the Type and Relative Strength of
Intermolecular Forces
PROBLEM
For each pair of substances, identify the
dominant intermolecular forces in each substance,
and select the substance with the higher boiling
point.
(a) MgCl2 or PCl3
(b) CH3NH2 or CH3F
(c) CH3OH or CH3CH2OH

• Bonding forces are stronger than
  nonbonding(intermolecular) forces.
• Hydrogen bonding is a strong type of
  dipole-dipole force.
• Dispersion forces are decisive when the
  difference is molar mass or molecular shape.

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SAMPLE PROBLEM 12.3
Predicting the Type and Relative Strength of
Intermolecular Forces
continued
SOLUTION
(a) Mg2 and Cl- are held together by ionic
bonds while PCl3 is covalently bonded and the
molecules are held together by dipole-dipole
interactions. Ionic bonds are stronger than
dipole interactions and so MgCl2 has the higher
boiling point.
(b) CH3NH2 and CH3F are both covalent compounds
and have bonds which are polar. The dipole in
CH3NH2 can H bond while that in CH3F cannot.
Therefore CH3NH2 has the stronger interactions
and the higher boiling point.
(c) Both CH3OH and CH3CH2OH can H bond but
CH3CH2OH has more CH for more dispersion force
interaction. Therefore CH3CH2OH has the higher
boiling point.
(d) Hexane and 2,2-dimethylbutane are both
nonpolar with only dispersion forces to hold the
molecules together. Hexane has the larger
surface area, thereby the greater dispersion
forces and the higher boiling point.
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Properties of Liquids
Viscosity is a measure of a fluids resistance to
flow.
Strong intermolecular forces
High viscosity
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Table 12.4 Viscosity of Water at Several
Temperatures
Viscosity (Ns/m2)
Temperature(0C)
20
1.00x10-3
40
0.65x10-3
0.47x10-3
60
80
0.35x10-3
The units of viscosity are newton-seconds per
square meter.
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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
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Table 12.3
Surface Tension and Forces Between Particles
Surface Tension (J/m2) at 200C
Substance
Formula
Major Force(s)
diethyl ether
dipole-dipole dispersion
CH3CH2OCH2CH3
1.7x10-2
ethanol
H bonding
2.3x10-2
CH3CH2OH
butanol
H bonding dispersion
2.5x10-2
CH3CH2CH2CH2OH
water
H bonding
7.3x10-2
H2O
mercury
metallic bonding
48x10-2
Hg
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Properties of Liquids
Cohesion is the intermolecular attraction between
like molecules
Adhesion is an attraction between unlike molecules
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Shape of water or mercury meniscus in glass
Figure 12.20
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Water is a Unique Substance
Density of Water
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The hexagonal structure of ice
Figure 12.22
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The H-bonding abilitiy of the water molecule
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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.
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The macroscopic properties of water and their
atomic and molecular roots.