John C. Kotz
Paul M. Treichel
John Townsend
http://academic.cengage.com/kotz
Chapter 12
Intermolecular Forces and Liquids
John C. Kotz • State University of New York, College at Oneonta
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© 2009 Brooks/Cole - Cengage
WHY?
• Why is water usually a
liquid and not a gas?
• Why does liquid water boil
at such a high temperature
for such a small molecule?
• Why does ice float on
water?
• Why do snowflakes have 6
sides?
• Why is I2 a solid whereas
Cl2 is a gas?
• Why are NaCl crystals little
cubes?
© 2009 Brooks/Cole - Cengage
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Intermolecular Forces and Liquids
Chap. 12
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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.
© 2009 Brooks/Cole - Cengage
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
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Covalent Bonding Forces
for comparison of magnitude
C=C, 610 kJ/mol
C–C, 346 kJ/mol
C–H, 413 kJ/mol
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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.
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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.
© 2009 Brooks/Cole - Cengage
PLAY MOVIE
Attraction Between
Ions and Permanent
Dipoles
Many metal ions are hydrated.
This is the reason metal salts
dissolve in water.
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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 f [M(H2O)x]n+
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Dipole-Dipole Forces
Dipole-dipole forces bind molecules having
permanent dipoles to one another.
PLAY MOVIE
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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
© 2009 Brooks/Cole - Cengage
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
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H-Bonding Between
Methanol and Water
-
H-bond
+
-
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H-Bonding Between Two
Methanol Molecules
-
+
-
H-bond
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H-Bonding Between
Ammonia and Water
-
+
-
H-bond
This H-bond leads to the formation of
NH4+ and OH© 2009 Brooks/Cole - Cengage
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Hydrogen Bonding in H2O
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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.
© 2009 Brooks/Cole - Cengage
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
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Hydrogen Bonding in H2O
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Ice has open lattice-like structure.
Ice density is < liquid and so solid floats on water.
PLAY MOVIE
One of the VERY few
substances where
solid is LESS DENSE
than the liquid.
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A
consequence
of hydrogen
bonding
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Hydrogen Bonding in H2O
H bonds lead to 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.
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Hydrogen Bonding
H bonds leads to
abnormally high
boiling point of
water.
PLAY MOVIE
See Screen 13.7
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Boiling Points of
Simple HydrogenContaining Compounds
See Active Figure 12.8
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Methane Hydrate
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Methane Clathrate
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Hydrogen Bonding in Biology
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
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Double helix
of DNA
Portion of a
DNA chain
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Base-Pairing through H-Bonds
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Double Helix
of DNA
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Discovering the Double Helix
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Rosalind
Franklin, 19201958
James Watson
and Francis
Crick, 1953
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Maurice Wilkins,
1916 - 2004
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Hydrogen Bonding in Biology
Hydrogen bonding and base pairing in DNA.
PLAY MOVIE
See ChemistryNow, Chapter 12
© 2009 Brooks/Cole - Cengage
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.
PLAY MOVIE
Dipole-induced
dipole
© 2009 Brooks/Cole - Cengage
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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.
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IM FORCES — INDUCED DIPOLES
Consider I2
dissolving
in ethanol,
CH3CH2OH.
-
I-I
- O
R
H
+
© 2009 Brooks/Cole - Cengage
I-I
The alcohol
temporarily
creates or
INDUCES a
dipole in I2.
+
- O
R
H
+
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FORCES INVOLVING
INDUCED DIPOLES
Formation of a dipole in two nonpolar I2
molecules.
Induced dipoleinduced dipole
PLAY MOVIE
© 2009 Brooks/Cole - Cengage
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FORCES INVOLVING
INDUCED DIPOLES
The induced forces between I2 molecules are
very weak, so solid I2 sublimes (goes from a
solid to gaseous molecules).
PLAY MOVIE
© 2009 Brooks/Cole - Cengage
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FORCES INVOLVING
INDUCED DIPOLES
The magnitude of the induced dipole depends
on the tendency to be distorted.
Higher molec. weight f larger induced
dipoles.
Molecule
Boiling Point (oC)
CH4 (methane)
- 161.5
C2H6 (ethane)
- 88.6
C3H8 (propane)
- 42.1
C4H10 (butane)
- 0.5
© 2009 Brooks/Cole - Cengage
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Boiling Points of Hydrocarbons
C4H10
C3H8
C2H6
CH4
Note linear relation between bp and molar mass.
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Summary of
Intermolecular Forces
• Ion-dipole forces
• Dipole-dipole forces
–Special dipole-dipole force:
hydrogen bonds
• Forces involving nonpolar
molecules: induced forces
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Intermolecular Forces
Summary
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Intermolecular Forces
See Figure 12.12
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Liquids
Section 12.4
PLAY MOVIE
© 2009 Brooks/Cole - Cengage
In a liquid
• molecules are in
constant motion
• there are appreciable
intermolec. forces
• molecules close
together
• Liquids are almost
incompressible
• Liquids do not fill the
container
Liquids
The two key properties we need to describe
are EVAPORATION and its opposite—
CONDENSATION
Evaporation f
LIQUID
Add energy
VAPOR
break IM bonds
make IM bonds
Remove energy
r condensation
© 2009 Brooks/Cole - Cengage
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Liquids—Evaporation
To evaporate,
molecules must
have sufficient
energy to break IM
forces.
PLAY MOVIE
© 2009 Brooks/Cole - Cengage
Breaking IM forces
requires energy. The
process of
evaporation is
endothermic.
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Liquids—
Distribution of Energies
Number of molecules
lower T
higher T
See Figure 12.13
0
Molecular energy
Minimum energy req’d to break IM
forces and evaporate
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Distribution of
molecular
energies in a
liquid.
KE is proportional to T.
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Distribution of Energy in a Liquid
See Figure 12.13
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Liquids
49
At higher T a much
larger number of
molecules has high
enough energy to
break IM forces and
move from liquid to
vapor state.
High E molecules carry
away E. You cool
down when sweating
or after swimming.
© 2009 Brooks/Cole - Cengage
When molecules of liquid
are in the vapor state,
they exert a VAPOR
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Liquids
PRESSURE
EQUILIBRIUM
VAPOR PRESSURE is
the pressure exerted by
a vapor over a liquid in
a closed container
when the rate of
evaporation = the rate
of condensation.
PLAY MOVIE
© 2009 Brooks/Cole - Cengage
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Measuring Equilibrium
Vapor Pressure
Liquid in flask evaporates and exerts pressure on manometer.
See Active Figure 12.16
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Vapor Pressure
See ChemistryNow, Chapter 12
PLAY MOVIE
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Equilibrium Vapor Pressure
See Active Figure 12.17
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Liquids
Equilibrium Vapor Pressure
FIGURE 12.17: 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.
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Boiling Liquids
PLAY MOVIE
PLAY MOVIE
Please open both movies at the same time.
© 2009 Brooks/Cole - Cengage
Liquid boils when its
vapor pressure
equals atmospheric
pressure.
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Boiling Point
at Lower Pressure
PLAY MOVIE
When pressure is lowered, the vapor
pressure can equal the external pressure at
a lower temperature.
© 2009 Brooks/Cole - Cengage
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Consequences of Vapor
Pressure Changes
PLAY MOVIE
When can cools, vp of water drops.
Pressure in the can is less than that of
atmosphere, so can is crushed.
© 2009 Brooks/Cole - Cengage
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Liquids
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See Figure 12.17: VP versus T
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
C2H5
H5C2
dipoledipole
alcohol
O
H5C2
H
H-bonds
water
O
H
H
extensive
H-bonds
increasing strength of IM interactions
© 2009 Brooks/Cole - Cengage
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Liquids
HEAT OF VAPORIZATION is the heat
req’d (at constant P) to vaporize the liquid.
LIQ + heat f VAP
Compd.
∆vapH (kJ/mol)
IM Force
H2O
40.7 (100 oC)
H-bonds
SO2
26.8 (-47 oC)
dipole
Xe
12.6 (-107 oC)
induced dipole
© 2009 Brooks/Cole - Cengage
Equilibrium Vapor Pressure & the
Clausius-Clapeyron Equation
• Clausius-Clapeyron equation —
used to find ∆vapH˚.
• The logarithm of the vapor
pressure P is proportional to ∆vapH
and to 1/T.
• ln P = –(∆vapH˚/RT) + C
ĘvapH  1
1
ln
=


P1
R  T1 T2 
P2
© 2009 Brooks/Cole - Cengage
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Liquids
Molecules at surface behave differently than those in
the interior.
Molecules at surface experience net INWARD force of
attraction.
This leads to SURFACE TENSION — the energy req’d
to break the surface.
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Surface Tension
PLAY MOVIE
SURFACE TENSION also leads to
spherical liquid droplets.
© 2009 Brooks/Cole - Cengage
Liquids
Intermolec. forces also lead to CAPILLARY
action and to the existence of a concave
meniscus for a water column.
concave
meniscus
H2 O in
glass
tube
© 2009 Brooks/Cole - Cengage
ADHESIVE FORCES
between water
and glass
COHESIVE FORCES
between water
molecules
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Capillary Action
PLAY MOVIE
Movement of water up a piece of paper
depends on H-bonds between H2O and
the OH groups of the cellulose in the
paper.
© 2009 Brooks/Cole - Cengage