PowerPoint Lecture Presentation
by
J. David Robertson
University of Missouri
ed. Brad Collins
Intermolecular Forces
and
Liquids and Solids
Chapter 11
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
The principal difference between condensed states (liquids
and solids) and gases is the distance between the molecules.
Because the molecules are so close together, intermolecular
forces are effective in liquids and solids.
Molecules in liquids have moderate intermolecular attractions.
Molecules in solids have strong intermolecular attractions, which
allows solids to be arranged in regular configurations in 3 dimensions.
11.1 (1)
2
It is common for different states of a substance to be present
in a system at the same time.
Example: Water and Ice
The different states, water and ice, are referred to as “phases”
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 (1)
3
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)
Generally,
intermolecular
forces are much
weaker than
intramolecular
forces.
“Measure” of intermolecular force
boiling point
melting point
ΔHvap
ΔHfus
ΔHsub
11.2 (1)
4
Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2 (1)
5
Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
11.2 (1)
6
11.2 (1)
7
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 (1)
8
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 (1)
9
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
11.2 (1)
10
Hydrogen Bond
Formic Acid
Water
11.2 (1)
11
Why is the hydrogen bond considered a
“special” dipole-dipole interaction?
Decreasing molar mass
Decreasing boiling point
11.2 (1)
12
13
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 (1)
14
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
When Adhesion is
stronger than Cohesion,
liquid will rise in the
capillary until Adhesive
forces equal the weight
of liquid in the tube.
Adhesion
Cohesion
When Cohesion is
stronger than Adhesion,
liquid will not rise in the
capillary.
11.3 (1)
15
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
High
viscosity
Viscosity is also affected by temperature. Higher temperature, lower viscosity.
11.3 (1)
16
Water is a Unique Substance
High specific heat
Raising temperature requires
breaking H-bonds - more energy
Maximum Density
4 0C
Water molecules form 2 H-bonds. Each molecule
coordinates with 2 oxygen atoms leading to an
open 3-D structure.
Ice is less dense than water
{
Density of Water
Liquid
water
molecules
trapped in
spaces in
ice 3-D
structure
11.3 (1)
17
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 unit cell is the basic repeating structural unit of a crystalline
solid.
At lattice points:
lattice
point
Unit Cell
Unit cells in 3 dimensions
•
Atoms
•
Molecules
•
Ions
11.4 (2)
18
11.4 (2)
19
11.4 (2)
20
11.4 (2)
21
11.4 (2)
22
Shared by 8
unit cells
Shared by 2
unit cells
11.4 (2)
23
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)
11.4 (2)
24
Fig. 11.20 Closest Packing
Closest packing
Third layer
duplicates
first layer
Third layer
doesn’t
duplicate
first layer
25
Fig. 11.21 Closest Packing
Face Centered
Cubic Unit Cell
26
Worked Examples 11.5 - Sodium Chloride cell type
27
11.4 (2)
28
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
V = a3 = (409 pm)3 = 6.83 x 10-23 cm3
4 atoms/unit cell in a face-centered cubic cell
1 mole Ag
107.9 g
-22 g
x
m = 4 Ag atoms x
=
7.17
x
10
mole Ag 6.022 x 1023 atoms
7.17 x 10-22 g
m
3
=
=
10.5
g/cm
d=
V
6.83 x 10-23 cm3
11.4 (2)
29
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
11.6 (2)
30
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
11.6 (2)
31
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
11.6 (2)
32
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-
11.6 (2)
33
Types of Crystals
11.6 (2)
34
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
11.7 (2)
35
Chemistry In Action: High-Temperature Superconductors
YBa2Cu3Ox
Conducts at 97K
Missing oxygen
36
Gas - Liquid
Condensation
Evaporation
A change from one physical state (phase) to
another, results in a change in molecular
order
Least
Order
Energy
required
to melt or
vaporize
T2 > T1
Greatest
Order
11.8 (3)
37
Before
Evaporation
At
Equilibrium
Remember: Vapor pressure is the pressure of a gas over its liquid phase.
11.8 (3)
38
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
condensation
Rate of
= evaporation
Rate of evaporation is constant at any
given temperature
11.8 (3)
39
Molar heat of vaporization (ΔHvap) is the energy required to
vaporize 1 mole of a liquid.
Clausius-Clapeyron Equation
ΔHvap
ln P = +C
RT
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
11.8 (3)
40
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 (3)
41
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.
• At the boiling point bubbles form within the liquid.
• Pressure on the bubble = external pressure
• External pressure ≈ atmospheric pressure
• Pressure in the bubble = vapor pressure
• If vapor pressure < atmospheric pressure, bubble collapses
• If vapor pressure = external pressure, bubble rises
Because boiling point is related to vapor pressure, boiling
point is also related to Hvap
The higher the Hvap, the higher the boiling point
11.8 (3)
42
The critical temperature (Tc) is the temperature above which
the gas cannot be made to liquefy, no matter how great the
applied pressure.
Essentially no difference between liquid and gas above Tc, have a fluid.
The critical pressure
(Pc) is the minimum
pressure that must be
applied to bring about
liquefaction at the
critical temperature.
Tc is affected by intermolecular forces, strong forces raise Tc
11.8 (3)
43
Solid - Liquid
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 (3)
44
Molar heat of fusion (ΔHfus) is the energy required to melt
1 mole of a solid substance.
11.8 (3)
45
11.8 (3)
46
Solid - Gas
Molar heat of sublimation
(ΔHsub) is the energy required
to sublime 1 mole of a solid.
Deposition
H2O (g)
Sublimation
H2O (s)
ΔHsub = ΔHfus + ΔHvap
( Hess’s Law)
11.8 (3)
47
A phase diagram summarizes the conditions at which a
substance exists as a solid, liquid, or gas.
Phase Diagram of Water
11.9 (3)
48
11.9 (3)
49
Increased pressure on ice, decreases its melting point
Increased pressure on water, increases its boiling point
11.9 (3)
50
Chemistry In Action: Liquid Crystals
11.9 (3)
51