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Intermolecular Forces and
Liquids and Solids
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Chapter 12
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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A phase is a homogeneous part of the system in contact with
other parts of the system but separated from them by a welldefined boundary.
2 Phases
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Solid phase - ice
Liquid phase - water
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Intermolecular Forces
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Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
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Intermolecular vs Intramolecular
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41 kJ to vaporize 1 mole of water (inter)
•
930 kJ to break all O-H bonds in 1 mole of water (intra)
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“Measure” of intermolecular force
Generally, intermolecular
forces are much weaker
than intramolecular forces.
boiling point
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melting point
DHvap
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DHfus
DHsub
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Intermolecular Forces
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Dipole-Dipole Forces
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Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
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Intermolecular Forces
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Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
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Ion-Dipole Interaction
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Interaction Between Water and Cations
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in solution
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Intermolecular Forces
Dispersion Forces
Attractive forces that arise as a result of temporary
dipoles induced in atoms or molecules
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ion-induced dipole interaction
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dipole-induced dipole interaction
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Induced Dipoles Interacting With Each Other
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Intermolecular Forces
Dispersion Forces Continued
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Polarizability is the ease with which the electron distribution
in the atom or molecule can be distorted.
Polarizability increases with:
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greater number of electrons
•
more diffuse electron cloud
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Dispersion forces usually
increase with molar mass.
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What type(s) of intermolecular forces exist between each of the
following molecules?
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HBr
HBr is a polar molecule: dipole-dipole forces. There are
also dispersion forces between HBr molecules.
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CH4
CH4 is nonpolar: dispersion forces.
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S
SO2
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SO2 is a polar molecule: dipole-dipole forces. There are
also dispersion forces between SO2 molecules.
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Intermolecular Forces
Hydrogen Bond
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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
H…B
or
A
H…A
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A & B are N, O, or F
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Hydrogen Bond
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HCOOH and water
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Why is the hydrogen bond considered a “special”
dipole-dipole interaction?
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Decreasing molar mass
Decreasing boiling point
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Properties of Liquids
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Surface tension is the amount of energy required to stretch
or increase the surface of a liquid by a unit area.
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Strong
intermolecular
forces
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High
surface
tension
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Properties of Liquids
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Cohesion is the intermolecular attraction between like molecules
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Adhesion is an attraction between unlike molecules
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Adhesion
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Cohesion
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Properties of Liquids
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Viscosity is a measure of a fluid’s resistance to flow.
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Strong
intermolecular
forces
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High
viscosity
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3-D Structure of Water
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Water is a Unique Substance
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Maximum Density
4 °C
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Density of Water
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Ice is less dense than water
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A crystalline solid possesses rigid and long-range order. In a
crystalline solid, atoms, molecules or ions occupy specific
(predictable) positions.
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An amorphous solid does not possess a well-defined
arrangement and long-range molecular order.
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A unit cell is the basic repeating structural unit of a crystalline
solid.
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lattice
point
Unit Cell
At lattice points:
Unit cells in 3 dimensions
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Atoms
•
Molecules
•
Ions
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Seven Basic Unit Cells
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Three Types of Cubic Unit Cells
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Arrangement of Identical Spheres in a Simple Cubic Cell
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Arrangement of Identical Spheres in a Body-Centered
Cubic Cell
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A Corner Atom, a Edge-Centered Atom and a
Face-Centered Atom
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Shared by 8
unit cells
Shared by 2
unit cells
Shared by 4
unit cells
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Number of Atoms Per Unit Cell
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1 atom/unit cell
2 atoms/unit cell
4 atoms/unit cell
(8 x 1/8 = 1)
(8 x 1/8 + 1 = 2)
(8 x 1/8 + 6 x 1/2 = 4)
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Relation Between Edge Length and Atomic Radius
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When silver crystallizes, it forms face-centered cubic cells. The
unit cell edge length is 409 pm. Calculate the density of silver.
d=
m
V
d=
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V = a3 = (409 pm)3 = 6.83 x 10-23 cm3
4 atoms/unit cell in a face-centered cubic cell
m = 4 Ag atoms x
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1 mole Ag
107.9 g
x
= 7.17 x 10-22 g
mole Ag 6.022 x 1023 atoms
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7.17 x 10-22 g
m
=
= 10.5 g/cm3
V
6.83 x 10-23 cm3
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Types of Crystals
Ionic Crystals
• Lattice points occupied by cations and anions
• Held together by electrostatic attraction
• Hard, brittle, high melting point
• Poor conductor of heat and electricity
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CsCl
ZnS
CaF2
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Types of Crystals
Covalent Crystals
• Lattice points occupied by atoms
• Held together by covalent bonds
• Hard, high melting point
• Poor conductor of heat and electricity
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carbon
atoms
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graphite
diamond
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Types of Crystals
Molecular Crystals
• Lattice points occupied by molecules
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Held together by intermolecular forces
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Soft, low melting point
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Poor conductor of heat and electricity
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water
benzene
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Types of Crystals
Metallic Crystals
• Lattice points occupied by metal atoms
• Held together by metallic bonds
• Soft to hard, low to high melting point
• Good conductors of heat and electricity
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Cross Section of a Metallic Crystal
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nucleus &
inner shell e-
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mobile “sea”
of e-
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Phase Changes
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Least
Order
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Greatest
Order
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The equilibrium vapor pressure is the vapor pressure
measured when a dynamic equilibrium exists between
condensation and evaporation
H2O (l)
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H2O (g)
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Dynamic Equilibrium
Rate of
condensation
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Rate of
= evaporation
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Measurement of Vapor Pressure
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Before
Evaporation
At
Equilibrium
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Molar heat of vaporization (DHvap) is the energy required to
vaporize 1 mole of a liquid at its boiling point.
Clausius-Clapeyron Equation
DHvap
ln P = +C
RT
P = (equilibrium) vapor pressure
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T = temperature (K)
R = gas constant (8.314 J/K•mol)
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Vapor Pressure Versus Temperature
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Alternate Forms of the Clausius-Clapeyron Equation
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At two temperatures
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or
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The boiling point is the temperature at which the
(equilibrium) vapor pressure of a liquid is equal to the
external pressure.
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The normal boiling point is the temperature at which a liquid
boils when the external pressure is 1 atm.
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The critical temperature (Tc) is the temperature above which
the gas cannot be made to liquefy, no matter how great the
applied pressure.
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The critical pressure
(Pc) is the minimum
pressure that must be
applied to bring about
liquefaction at the
critical temperature.
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The Critical Phenomenon of SF6
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T < Tc
T > Tc
T ~ Tc
T < Tc
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Solid-Liquid Equilibrium
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H2O (s)
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H2O (l)
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The melting point of a solid
or the freezing point of a
liquid is the temperature at
which the solid and liquid
phases coexist in equilibrium
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Molar heat of fusion (DHfus) is the energy required to melt
1 mole of a solid substance at its freezing point.
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Solid-Gas Equilibrium
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H2O (s)
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H2O (g)
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Molar heat of sublimation
(DHsub) is the energy required
to sublime 1 mole of a solid.
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DHsub = DHfus + DHvap
( Hess’s Law)
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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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Phase Diagram of Water
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Phase Diagram of Carbon Dioxide
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At 1 atm
CO2 (s)
CO2 (g)
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Effect of Increase in Pressure on the Melting Point
of Ice and the Boiling Point of Water
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CH-12
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Questions and Problems
Page 429 - 433
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12.10, 12.12, 12.14, 12.16, 12.18, 12.20, 12.42, 12.76.
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