Have studied INTRAmolecular forces—
the forces holding atoms together to
form molecules.
Now turn to forces between molecules —
INTERmolecular forces.
Forces between molecules, between ions,
or between molecules and ions.
© 2006 Brooks/Cole - Thomson
Intermolecular
Forces
Ion-Ion Forces
for comparison of magnitude
Na+—Cl- in salt
These are the strongest
forces.
Lead to solids with high
melting temperatures.
NaCl, mp = 800 oC
MgO, mp = 2800 oC
© 2006 Brooks/Cole - Thomson
Covalent Bonding Forces
for comparison of magnitude
C=C, 610 kJ/mol
C–C, 346 kJ/mol
C–H, 413 kJ/mol
© 2006 Brooks/Cole - Thomson
CN, 887 kJ/mol
Attraction Between Ions
and Permanent Dipoles
••
••
water
-
dipole
O
H
H +
Water is highly polar and can
interact with positive ions
to give hydrated ions in
water.
© 2006 Brooks/Cole - Thomson
Attraction Between Ions
and Permanent Dipoles
••
••
water
-
dipole
O
H
H +
Water is highly polar and can
interact with positive ions
to give hydrated ions in
water.
© 2006 Brooks/Cole - Thomson
Attraction Between Ions
and Permanent Dipoles
Many metal ions are hydrated. This is the reason
metal salts dissolve in water.
© 2006 Brooks/Cole - Thomson
Attraction Between Ions
and Permanent Dipoles
Attraction between ions and dipole depends on ion charge
and ion-dipole distance.
Measured by ∆H for Mn+ + H2O --> [M(H2O)x]n+
- H
O
H
+
•••
Mg2+
-1922 kJ/mol
© 2006 Brooks/Cole - Thomson
- H
O
H
+
- H
O
H
+
•••
Na +
-405 kJ/mol
•••
Cs+
-263 kJ/mol
Dipole-Dipole Forces
Such forces bind molecules having permanent dipoles
to one another.
© 2006 Brooks/Cole - Thomson
Dipole-Dipole Forces
Influence of dipole-dipole forces is seen in the boiling
points of simple molecules.
Compd
Mol. Wt.
Boil Point
N2
28
-196 oC
CO
28
-192 oC
Br2
160
59 oC
ICl
162
97 oC
© 2006 Brooks/Cole - Thomson
Hydrogen Bonding
A special form of dipole-dipole attraction, which
enhances dipole-dipole attractions.
H-bonding is strongest when X and Y are N, O, or F
© 2006 Brooks/Cole - Thomson
Hydrogen Bonding in H2O
H-bonding is especially strong
in water because
• the O—H bond is very polar
• there are 2 lone pairs on the
O atom
Accounts for many of water’s
unique properties.
© 2006 Brooks/Cole - Thomson
Hydrogen Bonding in H2O
Ice has open
lattice-like
structure.
Ice density is
< liquid.
And so solid floats
on water.
Snow flake: www.snowcrystals.com
© 2006 Brooks/Cole - Thomson
Hydrogen Bonding in H2O
Ice has open lattice-like structure.
Ice density is < liquid and so solid floats on water.
One of the VERY few
substances where solid is
LESS DENSE than the
liquid.
© 2006 Brooks/Cole - Thomson
Hydrogen Bonding in H2O
H bonds ---> abnormally high specific heat capacity of water (4.184 J/g•K)
This is the reason water is used to put out fires, it is the reason lakes/oceans
control climate, and is the reason thunderstorms release huge energy.
© 2006 Brooks/Cole - Thomson
Boiling Points of Simple
Hydrogen-Containing
Compounds
Active Figure 13.8
© 2006 Brooks/Cole - Thomson
Double helix
of DNA
Portion of a
DNA chain
H-bonding is especially strong in biological systems — such as DNA.
DNA — helical chains of phosphate groups and sugar molecules. Chains are helical
because of tetrahedral geometry of P, C, and O.
Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases.
—adenine with thymine
—guanine with cytosine
© 2006 Brooks/Cole - Thomson
Base-Pairing through H-Bonds
© 2006 Brooks/Cole - Thomson
Base-Pairing through H-Bonds
© 2006 Brooks/Cole - Thomson
FORCES INVOLVING
INDUCED DIPOLES
How can non-polar molecules such as O2 and I2 dissolve in
water?
The water dipole INDUCES a dipole in
the O2 electric cloud.
Dipole-induced
dipole
© 2006 Brooks/Cole - Thomson
FORCES INVOLVING
INDUCED DIPOLES
Solubility increases with mass the gas
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FORCES INVOLVING
INDUCED DIPOLES
• Process of inducing a
dipole is polarization
• Degree to which electron
cloud of an atom or
molecule can be
distorted in its
polarizability.
© 2006 Brooks/Cole - Thomson
IM FORCES — INDUCED DIPOLES
Consider I2
dissolving
in ethanol,
CH3CH2OH.
-
I-I
- O
R
H
+
© 2006 Brooks/Cole - Thomson
I-I
The alcohol
temporarily
creates or
INDUCES a
dipole in I2.
+
- O
R
H
+
FORCES INVOLVING
INDUCED DIPOLES
Formation of a dipole in two nonpolar I2 molecules.
Induced dipole-induced dipole
The induced forces between I2 molecules are very weak, so solid I2 sublimes
(goes from a solid to gaseous molecules).
© 2006 Brooks/Cole - Thomson
FORCES INVOLVING
INDUCED DIPOLES
The magnitude of the induced dipole depends on the
tendency to be distorted.
Higher molec. weight ---> larger induced dipoles.
Molecule
Boiling Point (oC)
CH4 (methane)
- 161.5
C2H6 (ethane)
- 88.6
C3H8 (propane) - 42.1
C4H10 (butane)
- 0.5
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Boiling Points of Hydrocarbons
C4H10
C3H8
C2H6
CH4
Note linear relation between bp and molar mass.
© 2006 Brooks/Cole - Thomson
Intermolecular Forces Summary
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Equilibrium Vapor Pressure
Active Figure 13.18
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Liquids Equilibrium Vapor Pressure
FIGURE 13.18: VP as a function of T.
1. The curves show all conditions of P and T where LIQ and VAP are in
EQUILIBRIUM
2. The VP rises with T.
3. When VP = external P, the liquid boils.
This means that BP’s of liquids change with altitude.
4. If external P = 760 mm Hg, T of boiling is the NORMAL BOILING
POINT
5. VP of a given molecule at a given T depends on IM forces. Here the VP’s
are in the order
ether
O
C 2H 5
H 5C 2
dipoledipole
alcohol
O
H 5C 2
H
H-bonds
increasing strength of IM interactions
© 2006 Brooks/Cole - Thomson
water
O
H
H
extensive
H-bonds
Liquids
HEAT OF VAPORIZATION is the heat req’d (at
constant P) to vaporize the liquid.
LIQ + heat ---> VAP
Compd.
∆Hvap (kJ/mol)
H2O
40.7 (100 oC) H-bonds
SO2
26.8 (-47 oC) dipole
Xe
12.6 (-107 oC) induced dipole
© 2006 Brooks/Cole - Thomson
IM Force
Phase Diagrams
Lines connect all conditions of T and P where EQUILIBRIUM exists between the phases
on either side of the line. (At equilibrium particles move from liquid to gas as fast as they
move from gas to liquid, for example.)
© 2006 Brooks/Cole - Thomson
Phase Equilibria — Water
Solid-liquid
Gas-Liquid
Gas-Solid
© 2006 Brooks/Cole - Thomson
Triple Point —
Water
At the TRIPLE POINT all
three phases are in equilibrium.
© 2006 Brooks/Cole - Thomson
Phases Diagrams—
Important Points for Water
T(˚C)
P(mmHg)
Normal boil point
100
760
Normal freeze point
0
760
Triple point
0.0098
© 2006 Brooks/Cole - Thomson
4.58
Critical T and P
As P and T increase, you finally reach the CRITICAL T and P
Above critical T
no liquid exists
no matter how
high the pressure.
At P < 4.58 mmHg and T < 0.0098 ˚C solid H2O can go directly to
vapor. This process is called SUBLIMATION
This is how a frost-free refrigerator works.
© 2006 Brooks/Cole - Thomson
Critical T and P
COMPD
Tc(oC)
Pc(atm)
H2O
374
218
CO2
31
73
CH4
-82
46
Freon-12
(CCl2F2)
112
41
Notice that Tc and Pc depend on intermolecular
forces.
© 2006 Brooks/Cole - Thomson
Solid-Liquid Equilibria
P
Solid
H 2O
Liquid
H 2O
Normal
freezing
point
760
mmHg
Raising the pressure at constant T
causes water to melt.
The NEGATIVE SLOPE of the S/L
line is unique to H2O. Almost
everything else has positive slope.
0 °C
T
The behavior of water under pressure is an example of LE CHATELIER’S
PRINCIPLE At Solid/Liquid equilibrium, raising P squeezes the solid.
It responds by going to phase with greater density, i.e., the liquid phase.
In any system, if you increase P the DENSITY will go up.
Therefore — as P goes up, equilibrium favors phase with the larger density
(or SMALLER volume/gram).
© 2006 Brooks/Cole - Thomson