Intermolecular Forces and
Liquids and Solids
Chapter 11
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11-1 The kinetic molecular theory of liquids and solids
A phase is a homogeneous part of the system in contact
with other parts of the system but separated from them by
a well-defined boundary.
2 Phases
Solid phase - ice
Liquid phase - water
11.1
11-2 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
Generally,
intermolecular
forces are much
weaker than
intramolecular
forces.
boiling point
melting point
DHvap
DHfus
DHsub
11.2
Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2
Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
11.2
11.2
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
11.2
Induced Dipoles Interacting With Each Other
11.2
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
Dispersion
forces usually
increase with
molar mass.
11.2
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.
11.2
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
11.2
Hydrogen Bond
11.2
Why is the hydrogen bond considered a
“special” dipole-dipole interaction?
Decreasing molar mass
Decreasing boiling point
11.2
WORKED EXAMPLE 11.1
WORKED EXAMPLE 11.2
11-3 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
11.3
PG 459
FIGURE 11.9
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
11.3
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
High
viscosity
11.3
The structure and properties of water
Water is a Unique Substance
Maximum Density
40C
Density of Water
Ice is less dense than water
11.3
FIGURE 11.11
11-8
Least
Order
T2 > T1
Condensation
Phase change is the transformation
from one phase to another occur when
energy (usually in the form of heat) is
added or removed. Phase changes are
physical changes characterized by
changes in molecular order, molecules
in the solid phase have the greatest
order, and those in the gas phase have
the greatest randomness.
Evaporation
Liquid – vapor equilibrium
Greatest
Order
11.8
The equilibrium vapor pressure is the vapor pressure
measured when a dynamic equilibrium exists between
condensation and evaporation
H2O (l)
H2O (g)
Dynamic Equilibrium
Rate of
Rate of
= evaporation
condensation
11.8
Before
Evaporation
At
Equilibrium
11.8
Molar heat of vaporization (DHvap)
is the energy required to
vaporize 1 mole of a liquid at its boiling point. The molar heat of
vaporization is directly related to the strength of intermolecular forces that
exist in the liquid.
Clausius-Clapeyron Equation
ln P = -
DHvap
RT
+C
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
Vapor Pressure Versus Temperature
11.8
WORKED EXAMPLE 11.7
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.
11.8
The critical temperature (Tc) is the temperature above which
the gas cannot be made to liquefy, no matter how great the
applied pressure.
The critical pressure
(Pc) is the minimum
pressure that must be
applied to bring about
liquefaction at the
critical temperature.
11.8
The Critical Phenomenon of SF6
T < Tc
T > Tc
T ~ Tc
T < Tc
11.8
Liquid – solid equilibrium
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
Freezing
H2O (l)
Melting
H2O (s)
11.8
Molar heat of fusion (DHfus) is the energy required to melt 1
mole of a solid substance at its freezing point.
11.8
Heating Curve
11.8
Solid – vapor equilibrium
Molar heat of sublimation
(DHsub) is the energy
required to sublime 1 mole
of a solid.
Deposition
H2O (g)
Sublimation
H2O (s)
DHsub = DHfus + DHvap
( Hess’s Law)
11.8
11-9 phase diagram
A phase diagram summarizes the conditions at which a
substance exists as a solid, liquid, or gas.
Phase Diagram of Water
11.9
Phase Diagram of Carbon Dioxide
At 1 atm
CO2 (s)
CO2 (g)
11.9
Effect of Increase in Pressure on the Melting Point
of Ice and the Boiling Point of Water
Boiling point of water decreases with decreasing pressure so its hard to
boiling an egg on a mountain top because the amount of heat delivered to
the egg is not enough for cooking.
Why is the food in the pressure cookers will be cook faster?
The melting point of ice decreases with increasing external pressure. This
phenomenon helps to make ice skating possible.
11.9
Chemistry In Action: Liquid Crystals