1
Chapter 12 — IM Forces and Liquids
Why?
Chapter 12
Intermolecular
Forces and Liquids
• 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?
All of these questions may be answered by
“Intermolecular Forces”
Jeffrey Mack
California State University,
Sacramento
Intermolecular Forces
• The forces holding solids and liquids together are
called intermolecular forces.
• Intermolecular Forces are the attractions and
repulsions between molecules.
• They are NOT chemical bonds.
• The intermolecular forces of a substance may
exhibit are a function of:
1. charge (ions vs. neutrals)
2. polarity (molecular shape, dipoles)
3. molar mass
Covalent Bonding Forces for
Comparison of Magnitude
Intermolecular Forces
Intermolecular forces influence chemistry in many
ways:
• They are directly related to properties such as
melting point, boiling point, and the energy
needed to convert a solid to a liquid or a liquid to
a vapor.
• They are important in determining the solubility
of gases, liquids, and solids in various solvents.
• They are crucial in determining the structures of
biologically important molecules such as DNA
and proteins.
Covalent Bonding Forces for
Comparison of Magnitude
20 to 30 kJ/mol
C=C (610 kJ/mol)
C–C (346 kJ/mol)
D(H-Cl) = 432 kJ/mol
C–H (413 kJ/mol)
CN (887 kJ/mol)
Intermolecular forces are much weaker than the bonds that
make up compounds.
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Chapter 12 — IM Forces and Liquids
Ion-Ion Forces: Formal Charges
The forces that govern charged particles are defined by
Coulomb’s law.
Q  Q
Fk 
r2
Q = the charges on the cation and
anion
r = the distance between
k = a constant
Greater charge = stronger attraction
Greater distance = weaker attraction
These are the strong forces that lead to salts with high melting
temperatures.
H2O, mp = 0 °C NaCl, mp = 800
°C
MgO, mp = 2800 °C
Solvation of Ions
Attractions Between Ions &
Permanent Dipoles
The polar nature of water
provides for attractive
forces between ions and
water.
Enthalpies of Hydration: A Measure
of Ion-Dipole Forces
+
When a cation exists in solution, it is surrounded by the
negative dipole ends of water molecules.
When as anion exists in solution, it is surrounded by
the positive dipole ends of water molecules.
Enthalpies of Hydration: A Measure
of Ion-Dipole Forces
As the size of the ion increases, the
exothermicity of the process decreases.
This is due to the weaker ion-dipole forces.
As the size of the ion increases, the
exothermicity of the process decreases.
This is due to the weaker ion-dipole forces.
Attraction Between Ions &
Permanent Dipoles
Water is highly polar
and can interact with
positive ions to give
hydrated ions in
water.
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Chapter 12 — IM Forces and Liquids
Molecular Polarity
Dipole-Dipole Forces
Molecular Geometry
Non-Polar Molecule
Linear
Both atoms the same (outer the same for
linear tri-atomic)
Trigonal Planar
All bonding groups the same
Tetrahedral
All bonding groups the same
Trigonal bipyramidal
Octahedral
Dipole-dipole forces bind molecules having
permanent dipoles to one another.
All bonding groups the same or both axial
groups the same and all three equatorial
groups the same,
All bonding groups the same or all groups
trans to one another the same.
Any deviations of symmetry yield a polar molecule.
Dipole-Dipole Forces
As the polarity for a
given set of molecules
with similar molar
masses increases, the
boiling point increases.
Hydrogen Bonding
Molar Mass
(amu)
Dipole
Moment
(D)
BP
(K)
CH3CH2CH3
44.1
0.1
231
CH3OCH3
46.07
1.3
248
CH3Cl
40.49
1.9
249
CH3CN
41.05
3.9
355
Compound
A special form of dipole-dipole attraction, which
enhances dipole-dipole attractions.
400
350
300
250
Boiling point
200
150
100
50
BP
Dipole
0
Dipole Moment
1
H-bonding is strongest when X and Y are N, O, or F
molar Mass
2
3
Molar Mass
4
Hydrogen Bonding
The water molecules
network with one
another.
H–bonding in water
brings about a
network of
interactions which
explain phenomena
such as:
capillary action
surface tension
why ice floats
Surface Tension
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
reqired to break the surface.
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Chapter 12 — IM Forces and Liquids
Surface Tension
Capillary Action
IMF’s also lead to CAPILLARY action and to the
existence of a concave meniscus for a water column.
concave
meniscus
ATTRACTIVE FORCES
between water
and glass
SURFACE TENSION also leads to spherical
liquid droplets.
Capillary Action
H2O in
glass
tube
COHESIVE FORCES
between water
molecules
• Ice, H2O(s) floats because it is less dense than
water, H2O(l).
• The H–bonds allow the molecules in the liquid phase
to to approach closer than normal for non H–bonding
liquids.
• This is why water has its maximum density at 4 °C.
Movement of water up a piece of paper is a
result of H-bonds between H2O and the OH
groups of the cellulose in the paper.
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.
The Consequences of Hydrogen
Bonding
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Chapter 12 — IM Forces and Liquids
Boiling Points of Simple HydrogenContaining Compounds
18 g/mol
Hydrogen Bonding
Notice that water has an
unusually high bp for its Mwt...
20 g/mol
H-bonding leads to
abnormally high
boiling point of water.
17 g/mol
16 g/mol
This is a result of
hydrogen bonding!
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
electron cloud.
Induced Dipole Forces
How can non-polar molecules such as O2 and I2
dissolve in water?
The water dipole INDUCES a dipole in the O2
electron cloud.
Dipole-induced
dipole
Induced Dipole Forces
Once polarized, the O2 is attracted to additional
water molecules.
Forces Involving Induced Dipoles
Formation of a dipole in two nonpolar I2
molecules.
Induced dipoleinduced dipole
The degree to which electron cloud of an atom or molecule can
be distorted is measured by its polarizability.
The larger the molecule, the more easily it is polarized.
As the electrons in a molecule become more loosely held and
more spread out, the greater the degree of polarizibility in
the molecule.
The explains the trend we see in solubility.
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Chapter 12 — IM Forces and Liquids
London Dispersion Forces
London dispersion forces exist between all
molecules.
London dispersion forces are a function of molecular
polarizability.
The Polarizability of a molecule is measured by the
ease with which an electron cloud can be distorted.
The larger the molecule (the greater the number of
electrons) the greater polarizability. The greater the
surface area available for contact, the greater the
dispersion forces.
London dispersion forces therefore increase as
molecular weight increases.
Forces Involving Induced Dipoles
London Dispersion Forces
For molecules with the same relative polarizability, the forces
scale with molar mass:
Higher Mwt. =
Molecule
CH4 (methane)
C2H6 (ethane)
C3H8 (propane)
C4H10 (butane)
larger induced dipoles.
C4H10
BP (oC)
- 161.5
- 88.6
- 42.1
- 0.5
Note the linear relation
between bp and molar
mass.
C 3H 8
C 2H 6
CH4
Intermolecular Forces Summary
The induced forces between I2 molecules are
very weak, so solid I2 sublimes (goes from a
solid to gaseous molecules).
Intermolecular Forces
Properties of Liquids
• Of the three states of matter, liquids are the
most difficult to describe precisely.
• Under ideal conditions the molecules in a gas
are far apart and are considered to be
independent of one another.
• The structures of solids can be described easily
because the particles that make up solids are
usually in an orderly arrangement.
• The particles of a liquid interact with their
neighbors, like the particles in a solid, but, unlike
in solids, there is little long-range order.
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Chapter 12 — IM Forces and Liquids
Properties of Liquids
Liquids: Vaporization
Liquids
• Particles are in constant
motion.
• Particles are in close
contact.
• Liquids are almost
incompressible
• Liquids do not fill the
container.
• Intermolecular forces are
relevant.
In order for a
liquid to vaporize,
sufficient energy
must be available
to overcome the
intermolecular
forces.
Breaking IM forces requires energy.
The process of vaporization is
therefore endothermic.
Liquids: Enthalpy of Vaporization
Liquids: Enthalpy of Vaporization
The HEAT OF VAPORIZATION is the heat required to
vaporize the liquid at constant P.
When molecules of liquid are in the vapor state, they
exert a VAPOR PRESSURE.
vapH
Liquid + energy = Vapor
Notice how the types of forces greatly affects the Hvap and boiling point.
IMF
∆vapH (kJ/mol)
BP
H2O
H-bonds
40.7
100 °C
SO2
Dipole
26.8
47 °C
Xe
London
12.6
107 °C
Compound
Vapor Pressure
When molecules of liquid
are in the vapor state, they
exert a VAPOR 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.
The EQUILIBRIUM VAPOR
PRESSURE is the pressure
exerted by a vapor over a liquid
in a closed container.
At equilibrium,
rate of evaporation = the rate of condensation.
Vapor Pressure
Recall from kinetic molecular theory…
As Temp increases, so does the average KE of the
particles.
This means that there are more particles that can
escape into the gas phase!
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Chapter 12 — IM Forces and Liquids
Boiling Point
Liquid boil when Pvap = Patm
(Vapor pressure equals
atmospheric pressure.
Consequences of Vapor Pressure
Changes
When can cools, vapor pressure of water
drops. Pressure inside of the can is less than
that of atmosphere, which collapses the can.
Measuring Equilibrium Vapor
Pressure
Boiling Point at Reduced Pressure
As the external pressure is lowered, the vapor
pressure equals the external pressure at a
lower temperature. Boiling therefore occurs at
a reduced temperature.
Equilibrium Vapor Pressure
The vapor pressure of a liquid is seen to increase
exponentially with temperature.
The Temperature Dependence of
Vapor Pressure Goes As:
lnPvap  
 vapH
C
RT
slope :
-
D vapH
R
A plot of lnPvap vs. 1 yields a
T
slope of:
Liquid in flask evaporates and exerts pressure on
manometer.
∆vapH° is related to T and P by
the Clausius-Clapeyron
equation
y-intercept
=C
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Chapter 12 — IM Forces and Liquids
Rather than plot the data, it is convenient to arrange
the equation in terms of two temperatures:
 vapH
lnPvap  
lnP(T2)  lnP(T1) = 
æ P(T2 ) ö
ln ç
÷=
è P(T1 ) ø

æ P(T2 ) ö
ln ç
÷=
è P(T1 ) ø
RT
 vap H
RT2
 vap H

RT2
Problem:
Determine the vapor pressure of water at 50.0 °C given that
the Hvap = 40.7 kJ/mol and the vapor pressure at 20.0 °C
is 17.54 torr.
C
  vap H

+C 
 RT1

æ P(T2 ) ö  vap H é 1 1 ù
ln ç
ê - ú
÷=
R
è P(T1 ) ø
ë T1 T2 û
+ C  
 vap H
 vap H é 1 1 ù
ê - ú
R
ë T1 T2 û
Where P = vapor pressure, T = temp (K)
æ P(T2 ) ö
 vap H é 1 1 ù
ê - ú
ç P(T ) ÷ = exp
è
R
1 ø
ë T1 T2 û
solving:
RT1
P(T2 ) = P(T1 )´ exp
R = 8.314
 vap H é 1 1 ù
ê - ú
R
ë T1 T2 û
J
mol × K
Liquids: IMF’s Summary
Problem:
Determine the vapor pressure of water at 50.0 °C given that
the Hvap = 40.7 kJ/mol and the vapor pressure at 20.0 °C
is 17.54 torr.
P(T2 ) = P(T1 )´ exp
 vap H é 1 1 ù
ê - ú
R
ë T1 T2 û
kJ 103 J
´
1
1
ù
mol 1kJ ´ é
ëê 293.15K 323.15K ûú
J
8.341
mol × K
40.7
P(50.0 °C) =17.54 torr  exp
= 82.7 Torr
Molecules in
the Liquid
State
vapH
Volatility
Equilibrium
Vapor
Pressure
Boiling
Point
Strong
IMF’s
More
Endothermic
Low
Low
High
Weak IMF’s
Less
Endothermic
High
High
Low