Intermolecular Forces and
Liquids and Solids
Chapter 11
1
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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
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
boiling point
forces are much weaker
melting point
than intramolecular forces.
DHvap
DHfus
DHsub
3
Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
4
Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
5
Interaction Between Water and Cations
in solution
6
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
7
Induced Dipoles Interacting With Each Other
8
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
9
Example 11.2
Which of the following can form hydrogen bonds with water?
CH3OCH3
CH4
F2
HCOOH
Na+
Example 11.2
Example 11.1
What type(s) of intermolecular forces exist between the
following pairs?
(a) HBr and H2S
(b) Cl2 and CBr4
(c) I2 and
(d) NH3 and C6H6
Total attraction
• Hydrogen bond
– Dipole-dipole
– Dispersion
• Dipole-dipole
– Dispersion
• Dispersion
13
van der Waals forces
• Dipole-dipole
• Dipole-induced dipole
• Dispersion
14
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.
15
Differences in Melting and
Boiling point
• Melting and boiling
points increase as the
number of electrons in
a molecule increases
• More electrons =
larger molar mass
16
Why is the hydrogen bond considered a “special”
dipole-dipole interaction?
Decreasing molar mass
Decreasing boiling point
17
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
18
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
19
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
High
viscosity
20
3-D Structure of Water
Water is a Unique Substance
Maximum Density
40C
Density of Water
Ice is less dense than water
21
A crystalline solid possesses rigid and long-range order. In a
crystalline solid, atoms, molecules or ions occupy specific
(predictable) positions.
An amorphous solid does not possess a well-defined
arrangement and long-range molecular order.
A glass is an
optically
transparent fusion
product of
inorganic
materials that has
cooled to a rigid
state without
crystallizing
Crystalline
quartz (SiO2)
Non-crystalline
quartz glass
22
An Arrangement for Obtaining the X-ray Diffraction Pattern
of a Crystal.
23
Reflection of X-rays from Two Layers of Atoms
Extra distance = BC + CD = 2d sinq = nl
(Bragg Equation)
24
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
CsCl
ZnS
CaF2
25
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
carbon
atoms
diamond
graphite
26
Types of Crystals
Molecular Crystals
• Lattice points occupied by molecules
•
Held together by intermolecular forces
•
Soft, low melting point
•
Poor conductor of heat and electricity
water
benzene
27
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
Cross Section of a Metallic Crystal
nucleus &
inner shell emobile “sea”
of e-
28
Types of Crystals
29
Phase Changes
Least
Order
Greatest
Order
30
Effect of Temperature on Kinetic Energy
T2 > T1
31
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
32
Measurement of Vapor Pressure
Before
Evaporation
At
Equilibrium
33
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
T = temperature (K)
R = gas constant (8.314 J/K•mol)
Vapor Pressure Versus Temperature
34
Alternate Forms of the Clausius-Clapeyron Equation
At two temperatures
or
35
Example 11.7
Diethyl ether is a volatile, highly flammable organic liquid that is
used mainly as a solvent.
The vapor pressure of diethyl ether is 401 mmHg at 18°C.
Calculate its vapor pressure at 32°C.
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.
37
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.
38
The Critical Phenomenon of SF6
T < Tc
T > Tc
T ~ Tc
T < Tc
39
Solid-Liquid Equilibrium
H2O (s)
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.
40
Molar heat of fusion (DHfus) is the energy required to melt
1 mole of a solid substance at its freezing point.
41
Heating Curve
42
Solid-Gas Equilibrium
H2O (s)
H2O (g)
Molar heat of sublimation
(DHsub) is the energy required
to sublime 1 mole of a solid.
DHsub = DHfus + DHvap
( Hess’s Law)
43
Example 11.8
Calculate the amount of energy (in kilojoules) needed to heat
346 g of liquid water from 0°C to 182°C.
Assume that the specific heat of water is 4.184 J/g · °C
over the entire liquid range and that the specific heat of steam
is 1.99 J/g · °C.
A phase diagram summarizes the conditions at which a
substance exists as a solid, liquid, or gas.
Phase Diagram of Water
45
Phase Diagram of Carbon
Dioxide
At 1 atm
CO2 (s)
CO2 (g)
46
Effect of Increase in Pressure on the Melting Point
of Ice and the Boiling Point of Water
47