Chemistry
Second Edition
Julia Burdge
Lecture Powerpoints
Jason A. Kautz
University of Nebraska-Lincoln
12
Intermolecular Forces and the
Physical Properties of Liquids
and Solids
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
12
Intermolecular Forces and the Physical properties
of Liquids and Solids
12.1 Intermolecular Forces
Dipole-Dipole Interactions
Hydrogen Bonding
Dispersion Forces
Ion-Dipole Interactions
12.2 Properties of Liquids
Surface Tension
Viscosity
Vapor Pressure
12.3 Crystal Structure
Unit Cells
Packing Spheres
Closest Packing
12.4 Types of Crystals
Ionic Crystals
Covalent Crystals
Molecular Crystals
Metallic Crystals
12.5 Amorphous Solids
12.6 Phase Changes
Liquid-Vapor Phase Transition
Solid-Liquid Phase Transition
Solid-Vapor Phase Transition
12.7 Phase Diagrams
12.1
Intermolecular Forces
Intermolecular forces are attractive forces that hold particles together in
the condensed phases.
The magnitude (and type) of intermolecular forces is what determines
whether the particles that make up a substance are a gas, liquid, or solid.
Gas
Liquid
Solid
Intermolecular Forces
Attractive forces that act between atoms or molecules in a pure
substance are collectively called van der Waals forces.
Dipole-dipole interactions are attractive forces that act between polar
molecules.
The magnitude of the attractive forces depends on the magnitude of the
dipole.
Intermolecular Forces
Intermolecular Forces
Hydrogen bonding is a special type of dipole-dipole interaction.
Hydrogen bonding only occurs in molecules that contain H bonded to a
small, highly electronegative atom such as N, O, or F.
F
H
F
Intermolecular Forces
Intermolecular Forces
Dispersion forces or London dispersion forces result from the
Coulombic attractions between instantaneous dipoles of non-polar
molecules.
Intermolecular Forces
Intermolecular Forces
What kinds of intermolecular forces exist in the following molecules:
CH3CH2CH2CH2CH3
This compound is nonpolar; only dispersion forces
CH3CH2OH
This compound is polar and contains an O‒H bond.
Dispersion, dipole-dipole, and hydrogen bonding
Intermolecular Forces
Ion-dipole interactions are Coulombic attractions between ions (either
positive or negative) and polar molecules.
12.2
Properties of Liquids
Surface tension is the amount of
energy required to stretch or
increase the surface of a liquid by a
unit area.
The stronger the intermolecular
forces, the higher the surface
tension.
Properties of Liquids
Capillary action is the movement of a liquid up a narrow tube.
Two types of forces bring about capillary action:
cohesion is the attraction between like molecules
adhesion is the attraction between unlike molecules
Adhesive forces are
greater than cohesive
forces
Cohesive forces are
greater than adhesive
forces
Properties of Liquids
Viscosity is a measure of a fluid’s
resistance to flow.
The higher the viscosity the more slowly
a liquid flows.
Liquids that have higher intermolecular
forces have higher viscosities.
Properties of Liquids
Vapor pressure is also dependent on intermolecular forces.
If a molecule at the surface of a liquid has enough kinetic energy, it can
escape to the gas phase in a process called vaporization.
The number of molecules
with enough kinetic energy
to escape.
T1 < T2
Properties of Liquids
The vapor pressure increases until the rate of evaporation equals the
rate of condensation.
H2O(l) ⇌ H2O(g)
Evaporation:
H2O(l) → H2O(g)
Condensation:
H2O(l) ← H2O(g)
When the forward process and
reverse process are occurring at
the same rate, the system is in
dynamic equilibrium.
Properties of Liquids
The vapor pressure increases until the rate of evaporation equals the
rate of condensation.
H2O(l) ⇌ H2O(g)
Properties of Liquids
The vapor pressure increases with temperature.
Properties of Liquids
The Clausius-Clapeyron equation relates the natural log of vapor
pressure and the reciprocal of absolute temperature.
ln P  
Hvap
RT
ln P = natural log of vapor pressure
ΔHvap = the molar heat of vaporization
R = the gas constant (8.314 J/K•mol)
T = the kelvin temperature
C is an experimentally determined constant
C
Properties of Liquids
The Clausius-Clapeyron equation:
ln P  
Hvap
RT
C
Plotting ln P versus 1/T is a line with
a slope of −ΔH/R.
7
ΔH is assumed to be independent of
temperature.
ln P
6
5
4
3
2
0.0016
0.0018
0.002
1/T
0.0022
Properties of Liquids
The Clausius-Clapeyron equation can be rearranged into a two point
form:
P1 Hvap  1 1 
ln 
  
P2
R  T2 T1 
Properties of Liquids
The vapor pressure of ethanol is 1.00 x 102 mmHg at 34.9°C. What is its
vapor pressure at 55.8°C? (ΔHvap for ethanol is 39.3 kJ/mol)
Solution:
Step 1: Given the vapor pressure at one temperature, P1 use the
equation below to calculate the vapor pressure at a second
temperature.
ln
P1 Hvap

P2
R
1
1



T
T
 2
1 
1.00×10 2mmHg
39300 J/mol 
1
1

ln




P2
8.314 J/mol  K  328.95 K 308.05 K 
P2 = 265 mmHg
12.3
Crystal Structure
A crystalline solid possess rigid and long-range order; its atoms,
molecules, or ions occupy specific positions.
A unit cell is the basic repeating structural unit of a crystalline solid.
Crystal Structure
There are seven types of unit cells.
Crystal Structure
The coordination number is defined as the number of atoms
surrounding an atom in a crystal lattice.
The value of the coordination number indicates how tightly the atoms are
packed together.
The basic repeating unit in the array of atoms is called a simple cubic
cell.
Crystal Structure
There are three types of cubic cells.
Crystal Structure
In a body-centered cubic cell (bcc) the
spheres in each layer rest in the depressions
between spheres in the previous layer.
The coordination number is 8.
Crystal Structure
In a face-centered cubic cell (fcc) the
coordination number is 12.
Crystal Structure
Most of a cell’s atoms are shared by neighboring cells.
A corner atom is shared
by eight unit cells.
An edge atom is shared
by four unit cells.
A face-centered atom is
shared by two unit cells.
Crystal Structure
A simple cubic cell has the equivalent of only one complete atom contained
within the cell.
1
8 atoms at corners 
 1 equivlent atom
8
Crystal Structure
A body-centered cubic cell has two equivalent atoms:
1
8 atoms at corners 
 1 equivalent atom
8
 1 atom in the center
 2 equivalent atoms total
A face-centered cubic cell contains four complete atoms:
1
8 atoms at corners 
 1 equivalent atom
8
1
 6 atoms on faces 
 3 equivalent atoms
2
 4 equivalent atoms total
Crystal Structure
Hexagonal close-packed (hcp) structure:
Site directly over an
atom in layer A
Close packing starts
with a layer of atoms
(A)
Atoms in the second
layer (B) fit into the
depressions of the
first layer
Hexagonal closepacked structure.
Crystal Structure
Cubic close-packed (ccp) structure:
Site directly over an atom in layer A
(hcp)
Site NOT directly over
an atom in layer A
(ccp)
Cubic close-packed
structure
Crystal Structure
Closest packing:
Hexagonal close-packing
(hcp)
Cubic close-packing (ccp)
corresponds to a facecentered cubit cell.
Crystal Structure
Edge length (a) and radius (r) are related:
Simple cubic
Body-centered cubic
Face-centered cubic
Crystal Structure
When silver crystallizes, it forms face-centered cubic cells. The unit cell
edge length is 4.087 Å. Calculate the density of silver.
Solution:
Step 1: Density is calculated as follows:
mass of the unit cell
d
volume of the unit cell
Step 2: Determine the mass of the unit cell.
4 atoms
1 mol
107.9 g Ag
22
m



7.167

10
g/unit cell
23
unit cell 6.022 10 atoms 1 mol Ag
Crystal Structure
When silver crystallizes, it forms face-centered cubic cells. The unit cell
edge length is 4.087 Å. Calculate the density of silver.
Solution:
Step 3: Calculate the volume of the unit cell:
V   4.086 10 cm  6.822 10 23 cm3
8
3
Step 2: Determine the denisty.
7.167  10 22 g
3
d

10.51
g/cm
6.822  10 23 cm3
12.4
Types of Crystals
Ionic crystals are composed of charged ions that are held together by
Coulombic attraction.
The unit cell of an ionic compound can be defined be either the positions of
the anions or the positions of the cations.
Types of Crystals
Crystal structures of three ionic compounds:
CsCl
Simple cubic lattice
ZnS
Zincblende structure
(based on FCC)
CaF2
fluorite structure
(based on FCC)
Types of Crystals
In covalent crystals, atoms are held together in an extensive threedimensional network entirely by covalent bonds.
Types of Crystals
In molecular crystals, the lattice points are occupied by molecules; the
attractive forces between them are van der Waals forces and/or hydrogen
bonding.
Types of Crystals
In metallic crystals, every lattice point is occupied by an atom of the same
metal.
Electrons are delocalized over
the entire crystal.
Delocalized electrons make metals
good conductors.
Large cohesive force resulting from
delocalization makes metals strong.
Types of Crystals
12.5
Amorphous Solids
Amorphous solids lack a regular three-dimensional arrangement of
atoms.
Glass is an amorphous solid.
Glass is a fusion product.
SiO2 is the chief component.
Na2O and B2O3 are typically fused with molten SiO2 and allowed to cool
without crystallizing.
Amorphous Solids
Amorphous Solids
Crystalline quartz
Noncrystalline (amorphous)
quartz glass
12.6
Phase Changes
A phase is a homogeneous part of a system that is separated from the rest
of a the system by a well defined boundary.
When a substance goes from one phase to another phase, it has
undergone a phase change.
Example
Phase Change
Freezing of water
H2O(l) → H2O(s)
Evaporation (or vaporization) of water
H2O(l) → H2O(g)
Melting (fusion) of ice
H2O(s) → H2O(l)
Condensation of water vapor
H2O(g) → H2O(l)
Sublimation of dry ice
CO2(s) → CO2(g)
Deposition of iodine
I2(g) → I2(s)
Phase Changes
The six possible phase changes
Phase Changes
The boiling point of a substance is defined as the temperature at which
its vapor pressure equals the external atmospheric pressure.
The molar heat of vaporization (ΔHvap) is the amount of heat required to
vaporize a mole of substance at its boiling point.
Phase Changes
The transformation of a liquid to a solid is called freezing.
The reverse process is called melting, or fusion.
The melting point (freezing point) of a solid (or liquid) is the temperature
at which the solid and liquid phases coexist in equilibrium.
Ice ⇌ water
H2O(s) ⇌ H2O(l)
In dynamic equilibrium, the forward and reverse process are occurring at
the same rate.
Phase Changes
The molar heat of fusion (ΔHfus) is the energy required to melt 1 mol of a
solid.
Phase Changes
Heating curves:
Liquid and vapor in
equilibrium
Temperature
Boiling point
Melting point
Liquid
Solid and liquid in
equilibrium
Solid
Time
Vapor
Phase Changes
Sublimation is the process by which molecules go directly from the solid
phase to the vapor phase.
Deposition is reverse process of sublimation.
The molar enthalpy of sublimation (ΔHsub) of a substance is the energy
required to sublime 1 mole of a solid.
ΔHsub = ΔHfus + ΔHvap
Solid I2 in equilibrium with its vapor
Phase Changes
Calculate the amount of energy (in kJ) necessary to convert 346 g of liquid
water from 0°C to water vapor at 182°C.
Solution:
Step 1: Determine the different steps in the heating curve:
Temperature (°C)
182
Vapor (3)
100
Boiling (2)
Liquid (1)
0
Heat
Phase Changes
Calculate the amount of energy (in kJ) necessary to convert 346 g (19.20
mol) of liquid water from 0°C to water vapor at 182°C.
Solution:
Temperature
Step 2: Calculate the heat associated with each step:
Vapor (3)
Boiling (2)
Liquid (1)
Heat
(1) q = msΔT = (346 g)(4.184 J/g•°C)(100°C – 0°C) = 144,766 J
(2) q = nΔHvap= (19.20 mol)(40.79 kJ/mol) = 783.168 kJ or 783,168 J
(3) q = msΔT = (346 g)(1.99 J/g•°C)(182°C – 100°C) = 56,460 J
Phase Changes
Calculate the amount of energy (in kJ) necessary to convert 346 g (19.20
mol) of liquid water from 0°C to water vapor at 182°C.
Solution:
Temperature
Step 3: The total amount of energy required is the sum of steps 1, 2 and 3:
Vapor (3)
Boiling (2)
Liquid (1)
Heat
(1) q = msΔT = 144,766 J
(2) q = nΔHvap= 783.168 kJ or 783,168 J
(3) q = msΔT = 56,460 J
984,394 J or 984 kJ
12.6
Phase Diagrams
A phase diagram summarizes the conditions at which a substance exists
as a solid, liquid, or gas.
The triple point is the
only combination of
pressure and temperature
where three phases of a
substance exist in
equilibrium.
triple point
Phase Diagrams
The phase diagram of water:
12
Key Concepts
Intermolecular Forces
Dipole-Dipole Interactions
Hydrogen Bonding
Dispersion Forces
Ion-Dipole Interactions
Properties of Liquids
Surface Tension
Viscosity
Vapor Pressure
Crystal Structure
Unit Cells
Packing Spheres
Closest Packing
Types of Crystals
Ionic Crystals
Covalent Crystals
Molecular Crystals
Metallic Crystals
Amorphous Solids
Phase Changes
Liquid-Vapor Phase Transition
Solid-Liquid Phase Transition
Solid-Vapor Phase Transition
Phase Diagrams