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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
Solid phase - ice
Liquid phase - water
Intermolecular Forces and
Liquids and Solids
Chapter 11
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Intermolecular Forces
Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Dipole-Dipole Forces
Intramolecular forces hold atoms together in a molecule.
Attractive forces between polar molecules
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)
Orientation of Polar Molecules in a Solid
―Measure‖ of intermolecular force
boiling point
Generally, intermolecular
forces are much weaker
than intramolecular forces.
melting point
Hvap
Hfus
Hsub
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Intermolecular Forces
Polarity and
Dipole-to-Dipole Attraction
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
CH3CH2CH3
MolarMass
(g/mol)
44
Boiling
Point, °C
-42
Dipole
Size, D
0
CH3-O-CH3
46
-24
1.3
CH3 - CH=O
44
20.2
2.7
CH3-C N
41
81.6
3.9
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10–5
5
Ion-Dipole Interaction
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Interaction Between Water and Cations
Intermolecular Forces
Dispersion Forces
Attractive forces that arise as a result of temporary
dipoles induced in atoms or molecules
ion-induced dipole interaction
in solution
dipole-induced dipole interaction
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Dispersion Forces (weak Intermolecular forces of attraction)
• also known as London Forces or
Induced Dipoles
• caused by electrons on one
molecule distorting the electron
cloud on another
• all molecules have dispersion forces
- +
- -
-
- +
- -
-
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10–9
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Dispersion Force and Molar Mass
Noble
Gas
He
Molar
Mass, (g/mol)
4.00
Boiling
Point, (K)
4.2
Ne
20.18
27
Ar
39.95
87
Kr
83.80
120
Xe
131.29
165
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10–10
10
Molecular Shape and Polarizability
Long skinny
molecule …
… higher
boiling point.
… giving weaker
dispersion forces and
a lower boiling point.
… can have greater separation of
charge along its length. Stronger
forces of attraction, meaning …
Induced Dipoles Interacting With Each Other
In the compact isomer, less
possible separation of charge …
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10–11
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Intermolecular Forces
What type(s) of intermolecular forces exist between each of the
following molecules?
Dispersion Forces Continued
Polarizability is the ease with which the electron distribution
in the atom or molecule can be distorted.
HBr
Polarizability increases with:
•
greater number of electrons
•
more diffuse electron cloud
HBr is a polar molecule: dipole-dipole forces. There are
also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
Dispersion forces usually
increase with molar mass.
S
SO2
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SO2 is a polar molecule: dipole-dipole forces. There are
also dispersion forces between SO 2 molecules.
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Hydrogen Bond
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
H…B
or
A
H…A
A & B are N, O, or F
HCOOH and water
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Why is the hydrogen bond considered a ―special‖
dipole-dipole interaction?
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Hydrogen Bonding in Ice
Decreasing molar mass
Decreasing boiling point
Hydrogen bonding arranges
the water molecules into an
open hexagonal pattern.
17
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“Hexagonal” is reflected in the
crystal structure. “Open”
means reduced density of the
solid (vs. liquid).
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Properties of Liquids
Properties of Liquids
Surface tension is the amount of energy required to stretch
or increase the surface of a liquid by a unit area.
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Strong
intermolecular
forces
High
surface
tension
Cohesion
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Properties of Liquids
Meniscus Formation
Viscosity is a measure of a fluid’s resistance to flow.
What conclusion
can we draw about
the cohesive forces
in mercury?
Strong
intermolecular
forces
High
viscosity
Water wets the
glass (adhesive
forces) and its
attraction for glass
forms a concaveup surface.
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3-D Structure of Water
10–21
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Water is a Unique Substance
22
Effect of Temperature on Kinetic Energy
Maximum Density
40 C
Density of Water
T2 > T1
Ice is less dense than water
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The equilibrium vapor pressure is the vapor pressure
measured when a dynamic equilibrium exists between
condensation and evaporation
H2O (l)
Measurement of Vapor Pressure
H2O (g)
Dynamic Equilibrium
Rate of
condensation
Rate of
= evaporation
At
Equilibrium
Before
Evaporation
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Molar heat of vaporization ( Hvap) is the energy required to
vaporize 1 mole of a liquid at its boiling point.
Vapor Pressure
Clausius-Clapeyron Equation
Hvap
ln P = +C
RT
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
Vapor Pressure Versus Temperature
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10–27
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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.
The normal boiling point is the temperature at which a liquid
boils when the external pressure is 1 atm.
Vapor pressure increases with
temperature; why?
Boiling point: the temperature at which the vapor
pressure of the liquid equals the external pressure.
Normal boiling point: boiling point at 1 atm.
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10–29
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Table 10.10 Boiling Point of Water at
Various Locations
Phase Changes Involving Solids
• Melting (fusion): transition of solid  liquid.
• Melting point: temperature at which melting occurs.
– Same as freezing point!
• Enthalpy of fusion, Hfusion, is the quantity of heat
required to melt a set amount (one gram, one mole)
of solid.
• Sublimation: transition of solid  vapor.
– Example: Ice cubes slowly ―disappear‖ in the freezer.
• Enthalpy of sublimation, Hsubln, is the sum of the
enthalpies of fusion and vaporization.
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10–31
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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 Phenomenon of SF6
The critical pressure
(Pc) is the minimum
pressure that must be
applied to bring about
liquefaction at the
critical temperature.
T < Tc
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T ~ Tc
T < Tc
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Molar heat of fusion ( Hfus) is the energy required to melt
1 mole of a solid substance at its freezing point.
Solid-Liquid Equilibrium
H2O (s)
T > Tc
H2O (l)
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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Heating Curve
Solid-Gas Equilibrium
H2O (s)
H2O (g)
Molar heat of sublimation
( Hsub) is the energy required
to sublime 1 mole of a solid.
Hsub = Hfus + Hvap
( Hess’s Law)
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Triple point: all three
phases—solid, liquid,
vapor—are in
equilibrium
Phase Diagrams
A phase diagram is a
graphical representation of
the conditions of
temperature and pressure
under which a substance
exists as a solid, liquid, a
gas, or some combination of
these in equilibrium.
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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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Critical temperature
(Tc): the highest
temperature at which
a liquid can exist.
The critical
pressure, Pc, is the
vapor pressure at the
critical temperature.
The condition
corresponding to a
temperature of Tc and
a pressure of Pc is
called the critical
40
point.10–40
Phase Diagram of Carbon Dioxide
Phase Diagram of Water
At 1 atm
CO2 (s)
CO2 (g)
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Phase Diagram for H2O
Effect of Increase in Pressure on the Melting Point
of Ice and the Boiling Point of Water
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10–44
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Chemistry In Action: Ice Skating
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