Intermolecular Forces
and
Liquids and Solids
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Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Intermolecular vs Intramolecular
•
40-44 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: boiling point, melting
point, viscosity, vapor
Generally,
pressure, DHvap, DHfus and
intermolecular forces
DHsub
are much weaker than
intramolecular forces.
11.2
Summarizing Intermolecular Forces
Types of Bonding in Crystalline
Solids
Intermolecular Forces Affect
Many Physical Properties
The strength of the
attractions between
particles can greatly
affect the properties of
a substance or
solution.
Some Illustrative Data for Molecular Compounds
DHvap (kJ/mole) at 25ºC
CS2
27.51
CH3OH
37.43
CH3CH2OH
42.32
H2O
43.98
C6H5NH2
55.83
Normal Tbp (ºC)
46.3
64.6
78.29
100.0
184.17
Surface Tension (J/m2)
0.03158
0.02207
0.02197
0.07199
0.04212
Viscosity (cP) at 25ºC
0.352
0.544
1.074
0.890
3.847
Normal Tmp (ºC)
-110.8
-97.6
-114.1
0.0
-6.0
DHfus (kJ/mole) at 25ºC
4.40
3.18
5.02
6.01
10.56
at 25ºC
Viscosity
• Resistance of a liquid to
flow is called viscosity.
• It is related to the ease
with which molecules
can move past each
other.
• Viscosity increases with
stronger intermolecular
forces and decreases with
higher temperature.
Surface Tension
Surface tension results
from the net inward
force experienced by
the molecules on the
surface of a liquid.
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
High
viscosity
11.3
Water is a Unique Substance
Maximum Density
40C
Density of Water
Ice is less dense than water
11.3
T2 > T1
Condensation
Evaporation
Least
Order
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.
Clausius-Clapeyron Equation
DHvap
ln P = +C
RT
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
11.8
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
Molar heat of fusion (DHfus) is the energy required to melt
1 mole of a solid substance.
11.8
States of Matter
The fundamental difference between states of
matter is the distance between particles.
Because in the solid and liquid states particles are
closer together, we refer to them as condensed
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.
Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2
Dipole-Dipole Interactions
The more polar the molecule, the higher is its
boiling point.
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
Factors Affecting London Forces
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
11.2
Factors Affecting London Forces
• The shape of the molecule affects
the strength of dispersion forces:
long, skinny molecules (like npentane tend to have stronger
dispersion forces than short, fat ones
(like neopentane).
• This is due to the increased surface
area in n-pentane.
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
Ion-Dipole Interactions
• A fourth type of force, ion-dipole interactions are
an important force in solutions of ions.
• The strength of these forces are what make it
possible for ionic substances to dissolve in polar
solvents.
Covalent-Network and
Molecular Solids
• Diamonds are an example of a covalent-network
solid in which atoms are covalently bonded to
each other.
– They tend to be hard and have high melting points.
Allotropes of carbon: diamond and graphite
carbon
atoms
154.5 pm
diamond
graphite
Covalent-Network and
Molecular Solids
• Graphite is an example of a molecular solid in
which atoms are held together with weaker van
der Waals forces.
– They tend to be softer and have lower melting points.
A crystalline solid possesses rigid and long-range order. In a
crystalline solid, atoms, molecules or ions occupy specific
(predictable) positions.
A unit cell is the basic repeating structural unit of a crystalline
solid.
lattice
point
Unit Cell
At lattice points:
Unit cells in 3 dimensions
•
Atoms
•
Molecules
•
Ions
11.4
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
Crystalline Solids
Because of the order
in a crystal, we can
focus on the repeating
pattern of
arrangement called
the unit cell.
Molecular Crystals
• Lattice points occupied by molecules
• Held together by intermolecular forces
• Soft, low melting point
• Poor conductor of heat and electricity
11.6
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
Metallic Solids
• Metals are not covalently
bonded, but the attractions
between atoms are too
strong to be van der Waals
forces.
• In metals, valence
electrons are delocalized
throughout the solid.
Attractions in Ionic Crystals
In ionic crystals, ions
pack themselves so as to
maximize the attractions
and minimize repulsions
between the ions.
11.4
11.4
11.4
Shared by 8
unit cells
Shared by 2
unit cells
11.4
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
Crystalline Solids
There are several types of basic arrangements in
crystals, such as the ones shown above.
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
11.4
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 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
11.8
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
A phase diagram summarizes the conditions at which a
substance exists as a solid, liquid, or gas.
Phase Diagram of Water
11.9
11.9