12.1-12.2 Physical States &
Phase Changes
Chap. 12 INTERMOLECULAR
FORCES
PROPERTY
• Know how energy determines physical properties
and how phase changes occur as a result of
heat flow.
SOLID
LIQUID
GAS
high
moderate
to
high
low
definite
indefinite
(fluid)
indefinite
(fluid)
small
small
large
very
small
small
moderate
DENSITY
mass ÷ volume
• Distinguish between bonding (intermolecular) and
nonbonding (intermolecular) forces.
SHAPE
• Know how interparticle forces contribute to
physical properties such as surface tension,
viscosity, capillarity, and crystal structure.
COMPRESSIBILITY
∆V as ƒ(pressure)
• Understand how the macroscopic properties of
water arise from its molecular structure.
THERMAL
EXPANSION
∆V as ƒ(temperature)
vaporization
SOLIDS
• Particles vibrate around fixed positions
• Strong cohesive (attractive) forces
∆Hvap = 40.7 kJ/mol
100
H2 O(g
)
Types of Phase Changes
A Kinetic-Molecular View
H2O(ℓ) + H2O(g)
• Particles in constant, random motion
T, °C
LIQUIDS • Cohesive forces dominate, particles
“see” each other
50
25
• Particles in constant, random motion
• Disruptive (kinetic energy) forces
dominate, particles move
independently of each other
0
melting ∆Hfus = 6.02 kJ/mol
H2 O(s)
GASES
H2 O(ℓ
)
75
H2O(s) + H2O(ℓ)
4
8
12
16
20
24
Heat Added, kJ
Phase Changes as Equilibria
Gas
• Some of the molecules in a
liquid may contain sufficient
energy to escape its surface
CONDENSATION
Liquid
FREEZING
MELTING
DEPOSITION
Esys
SUBLIMATION
VAPORIZATION
• As a result, some of the
liquid vaporizes
• Some of the vapor loses
energy and condenses
• In a closed system, the
pressure exerted by the
vapor in equilibrium with its
liquid is the VAPOR
PRESSURE
Solid
endothermic
exothermic
Page 12-1
28
1.0
0.8
EK for escape
T1
Mol
Fraction
n-Heptane
Pvap,
atm
0.6
n-Pentane
n-Hexane
72
100
86
0.4
T2 > T1
H2O
18
0.2
1-Butanol
74
−50
EK
−25
0
25
50
75
100
T, °C
NORMAL
NORMAL
BOILING
BOILING POINT
POINT
TT where
= 1 atm
where PPvap
vap = 1 atm
H2 O
(g)
Clausius–Clapeyron Equation
1.75
1.50
0.020
Pvap,
atm
Pvap, atm
1.25
1.00
0
bp:
Pvap = 1 atm
−1
−2
0.010
ln(Pvap)
0.75
H2O(s)
ℓ
(
mp
n-Pentane
−3
−4
)
10
2O
0
T, °C
H
−10
0.50
0.25
ln(P) = −∆Hvap/RT + C
1-Butanol
−6
0
20
40
60
PP2
−∆H
11
11
−∆Hvap
2
vap
ln
ln ——
—— == ———
——— ——
—— −− ——
——
RR
TT2
TT1
PP1
1
2
1
H2O
−5
80
100
T, °C
2.5
3.0
3.5
4.0
4.5
5.0
1/T, 10−3K−1
Phase Change Summary
New York
1.0
0.9
0
P,
atm
– P when ℓ ↔ g
2
La Paz
0.7
0.6
0.5
• VAPOR PRESSURE
1
Denver
0.8
3
– Increases with temperature
Altitude,
km
– Decreases with mass (homologous series)
– Increases with weaker intermolecular forces
4
Mount
Everest
5
0.4
• VAPORIZATION/CONDENSATION
6
75
80
85
90
95
Boiling Point, °C
– Significant ƒ(T)
– Boiling point = T where Pvap = Pexternal
100
BOILING
BOILING POINT
POINT
PPvap == PPatm
vap
atm
– Normal bp observed at 1 std atm (760 torr, etc)
liquid
liquid ↔
↔ gas
gas
Page 12-2
Phase Change Summary
Phase Diagrams
• MELTING (FUSION)/FREEZING
• Graphical representations of phases as ƒ(P,T)
– Melting point = T where s ↔ ℓ
• Phase transitions shown as lines between regions
– Little or no ƒ(P)
• Identify
– CRITICAL POINT: Highest T (critical temperature) at
which a substance can exist as a liquid; P (critical
pressure) necessary to effect liquefaction at critical
point
• SUBLIMATION/DEPOSITION
– Weak intermolecular forces
• CO2
– TRIPLE POINT: T, P where 3 phases are in
equilibrium
• Naphthalene (C10H8)
– Significant ƒ(T)
218
73
H2O(ℓ)
CO2(ℓ)
CRITICAL
POINT
CO2(s)
VAPORIZATION
P, atm
1.00
P, atm
H2O(s)
0.006
TRIPLE
POINT
H2O(g)
DEPOSITION
0
100
a
b
5.11
1.00
SUBLIMATION
POINT
374
−78
T, °C
CO2(g)
−56
31
T, °C
12.3 Intermolecular Forces
Intermolecular Forces and Distance
• Physical properties depend on physical state
Bond Length
• Physical properties of CONDENSED PHASES (s
and ℓ) depend on particle-particle interactions.
O
– INTRAMOLECULAR (bonding)
– INTERMOLECULAR (nonbonding, interparticle)
F
Cl
• BONDING FORCES are relatively strong
– Charges operating at short distances
van der Waals Radius
½d between adjacent nonbonded atoms
• INTERMOLECULAR FORCES are relatively weak
– Low-strength charges operating at greater distances
Covalent
Radius
½ distance between bonded atoms
(= ½ bond length)
Page 12-3
Br
I
Comparison of Bonding Forces
Comparison of Nonbonding Forces
Force
Force
Basis of
Attraction
Energy,
kJ/mol
Example
Ionic
Cation ↔
Anion
400-4000
NaCl
Covalent
Nuclei ↔
Shared e− pair
15-1100
H2
Metallic
Cation ↔
Delocalized e−
75-1000
Ion charge ↔
Dipole charge
X-H bond ↔
H bond
Dipole charge
Dipole charge ↔
Dipole-Dipole
Dipole charge
IonIon charge ↔
Induced dipole Polarizable e− cloud
DipoleDipole charge ↔
Induced dipole Polarizable e− cloud
Ion-Dipole
Hg
Dispersion
DIPOLE INTERACTIONS
δ− δ+ H
δδ−
Br
Cl
Br
δ−
Na+
δδ+
δ−
O
Example
40-600
Na+•••OH2
10-40
HO-H•••OH2
5-25
I-Cl•••I-Cl
3-15
Fe2+•••O2
2-10
H2O•••I-I
0.05-40
Br-Br•••Br-Br
LiCl
1250
• Charged ends attract other charged species or
species in which charges can be induced
Cl
Polarizable e− cloud ↔
Polarizable e− cloud
Energy,
kJ/mol
Example: Polarity and Boiling Point
• Electrons that make up some bonds are not
shared equally (∆EN)
δ+ H
Basis of
Attraction
δ+
H
δ−
O
§13.1
1000
δ+
H
bp, °C
750
500
H
δ+
NCNH2
250
HCONH2
CH3CH2OH
H
δ+
0
CH3OCH3
CH3CH2CH3
• These interactions can be moderately strong in
certain cases
1
2
3
4
5
6
7
μ, D
SPECIAL CASE:
HYDROGEN BONDING
Hydrogen Bonding: Implications
• Bonds between H and electronegative elements
(X = N, O, F, Cl) are particularly strong dipoles
Responsible for certain
characteristics of
H2O, a small molecule
which has relatively
• The attraction between the H atom of one dipole
and the X atom of another is called a
HYDROGEN BOND
– High surface tension,
capillarity
– High heat capacity,
heat of vaporization
– Great solvent power
δ−
δ+
δ+
– Complex density profile
(max at 4°C)
Page 12-4
50
0
DISPERSION (LONDON) FORCES
H2O
100
Group
Group7A
7AHydrides
Hydrides
Group
Group6A
6AHydrides
Hydrides
Group
Group5A
5AHydrides
Hydrides
Group
4A
Hydrides
Group 4A Hydrides
HF
• There is a small chance that the electrons in
any bond will be unevenly distributed for a
short period of time (INSTANTANEOUS
DIPOLES)
NH3
bp, °C
• In these instants, a covalent bond would have
dipole character and attract other temporary
dipoles
−50
−100
−150
• The averages of many such weak, short-lived
attractions are called DISPERSION FORCES
CH4
20
40
60
80
100
δδ+
120
Cl
Cl
δδ−
↔
Cl
↔
Cl
δδ−
Cl
Cl
δδ+
Molecular Mass
H H H H H
• The ease with which a molecule’s charge
distribution can be distorted is its polarizability
H C
C
C
C
C
H H H H H
• Larger molecules have greater polarizabilities
than small molecules
• The magnitude of dispersion forces increases
with molecular weight [this is why polymers have
unique solution properties]
H
Molecules of linear molecules
have greater interparticle
contact than spherical
molecules of equal mass
PENTANE
bp 36.1°C
• The magnitude of dispersion forces depends in
molecular shape
NO
YES
NO
IONIC
IONIC
BONDING
BONDING
H-X
Bonding?
NO
DIPOLEDIPOLEDIPOLE
DIPOLE
H
H
H
NEOPENTANE
bp 9.5°C
• SURFACE TENSION
– Energy required to increase a liquid’s surface area
(J/m2)
– Minimizes surface area and net intermolecular forces
– Balance between cohesive and adhesive forces
Polar
Molecules
+ Ions?
YES
C
H
H C
12.4-5 The Liquid State
Polar
Molecules
Only?
DISPERSION
DISPERSION
FORCES
FORCES
H
H
C
Branching
Branching reduces
reduces
dispersion-force
dispersion-force interactions
interactions
⇒
⇒ lower
lower bp
bp
INTERACTING
INTERACTING
PARTICLES
PARTICLES
Ions?
H C
H
• Dispersion forces operate between all molecules
NO
H
H C
H
YES
• CAPILLARITY (CAPILLARY ACTION)
– Flow arising from an imbalance of adhesive and
cohesive forces
– Magnitude depends on the relation between liquid and
container surface areas
IONIONDIPOLE
DIPOLE
YES
• VISCOSITY
– Resistance to flow (N·s/m2)
– Magnitude depends on strength of intermolecular
forces
– Decreases with increasing temperature
H-BONDING
H-BONDING
Page 12-5
12.6 The Solid State
ADHESIVE FORCES
COHESIVE FORCES
Between particles of different
substances
Among particles of a
substance
Glass
Si-O bonds
broken
X
H2O
7.3×10−2 J/m2
Quartz (SiO2)
• CRYSTALLINE
– Ordered, repeating
structural unit
– Highly regular shapes
• QUASICRYSTALLINE
– Multiple structural
units with long-range
repetition
CH3CH2OCH2CH3
1.7×10−2 J/m2
Cubic Packing
Unit Cells
• The 3-D system of
points representing the
centers of the elements
(atoms, molecules) in a
crystal is its LATTICE
• AMORPHOUS
– No orderly structure
– Solids with structural
constraints to ordering
– Macromolecules
corner:
1/8 Na
When crystals form, atoms
or molecules are arranged in
a way that maximizes interparticle attractions.
center:
1 Na
• The UNIT CELL is the
smallest repeating unit
of the lattice
• The total of all
elements in the unit
cell should equal the
cation–anion ratio
face:
1/2 Cl
edge:
1/4 Cl
Elements touch 4 others in
each layer; 2nd layer lies
directly over 1st layer:
SIMPLE CUBIC
(52% OCCUPIED)
NaCl unit cell
Close Packing (74% OCCUPIED)
Elements touch 4 others in
each layer; layers fit into gaps
(depressions) in lower layers:
BODY-CENTERED CUBIC
(68% OCCUPIED)
2nd layer fits into
depressions of 1st layer
• An arrangement that
minimizes the empty space
between elements is called
CLOSE PACKING
HEXAGONAL
CLOSE PACKING
ABAB…
• In a closed-packed layer,
elements touch 6 others.
3rd layer can fit into
2nd layer depressions
directly above 1st
layer
• Where elements (usually
ions as in Li2O) are of
unequal size, the larger
element assumes this
arrangement
3rd layer can also
fit into 2nd layer
depressions directly
above gaps between
1st layer elements
Page 12-6
CUBIC
CLOSE PACKING
(FACE-CENTERED CUBIC)
ABCABC…
Covalent Network Solids
Molecular/Atomic Solids
Repeating units of atoms held together by
COVALENT BONDS
Repeating units made up of MOLECULES (atoms if
solid noble gases)
carbon
Water, 0°C
Diamond
QUESTION:
What holds the molecules together?
Repeating lattice of ions held together by
IONIC BONDS
Metal atoms with inner electrons in a “sea” of
outer electrons that flow from one atom to
another (METALLIC BONDS)
CsCl
E
cesium chloride
Conduction
Band
(unoccupied)
Metallic Solids
Conduction
Band
(unoccupied)
Ionic Solids
Valence
Band
(occupied)
Valence
Band
(occupied)
conductor
+
+
+
+
+
+
+
+
+
+
+
+
semiconductor
face-centered cubic
contiguous
Characteristics of Crystalline Solids
4000
Solid
Molecular/
Atomic
Network
Covalent
Ionic
Metallic
Unit
Particles
Interparticle
Forces
Properties
Examples
Molecules or
atoms
Dispersion
Dipole-dipole
H-bond
Soft
Lower mp
Poor conductor
Ar(s), CO2(s),
cane sugar
[C12H22O11]
Atoms
Covalent
bonds
Hard
High mp
Poor conductor
Diamond [C],
quartz [SiO2]
Anions +
cations
Ionic bonds
Hard, brittle
High mp
Good conductor (ℓ)
NaCl
Metallic
bonds
Soft to hard
Malleable, Ductile
Varying mp
Good conductor
Cu, Fe
Atoms
3500
3000
E,
kJ/mol
2500
2000
1500
1000
500
Ionic
Bonding
Page 12-7
Covalent
Bonding
Metallic
Bonding
600
500
E,
kJ/mol
400
300
200
100
IonDipole
H
Bond
Other Dispersion
Dipole
Page 12-8