Chapter 11: Intermolecular
Forces
To this point: we’ve focused on the nature of the forces
within molecules
E.g. H2O: we can describe the bonding, geometry,
and polarity using VB and VSEPR ideas
Intramolecular forces
forces within molecules, e.g.,
covalent/ionic bonds, etc.
influence molecular shape, bond
energies, polarity, etc.
Now: how do molecules interact with each other?
Intermolecular forces
forces between molecules
nature of intermolecular forces
determines physical properties of
liquids & solids, e.g., boiling &
melting points
What differentiates the gaseous state from the liquid &
solid states?
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How to model the three states of matter? We use the
kinetic – molecular theory
Kinetic-molecular description of gases, liquids & solids
Kinetic-molecular model of a gas:
Collection of widely separated,
noninteracting particles
Constant, random motion
Average kinetic energy of
particles >> attractive forces
between particles
Lack of strong attractive forces
between particles allows a gas to
fill its container
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Kinetic-molecular model of liquids
Intermolecular attractive forces
strong enough to hold particles close
together
Liquids are more dense and less
compressible than gases
Less empty space between
particles than in a gas
Clusters or groups of particles can still move past one
another
Average kinetic energy of particles  intermolecular
attractive forces
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Kinetic-molecular model of solids
Intermolecular attractive forces
strong enough to lock particles in
place
Less compressible than liquids –
less empty space between
particles
Often have highly ordered structures
(crystalline solids)
Particles are not free to move - they
can only vibrate relative to one
another
Average kinetic energy of particles
<< intermolecular attractive forces
Because of the close proximity of
particles in a solid or liquid (relative
to a gas), these phases are referred
to as condensed phases of matter
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How does a substance undergo a phase
change (e.g., sl, lg)?
How are the attractive intermolecular
forces overcome?
The physical state of a substance
depends on the balance between kinetic
energy and intermolecular attractive
forces
Since kinetic energy depends on T, we can initiate a phase
change in a substance by heating/cooling
What is the nature of the intermolecular forces that
determine the physical properties of liquids/solids?
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Intermolecular forces
Generally much weaker than intramolecular forces
(i.e., covalent/ionic bonds)
For example: ~ 16 kJ of energy is necessary to
overcome the intermolecular attractions in liquid HCl
in order to vaporize it (HCl(l)  HCl(g))
But… about 431 kJ of energy is required to break the
H – Cl polar covalent bond (HCl(g)  H(g) +Cl(g))
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In liquids: two properties in particular reflect the
strength of the intermolecular forces
-vapor pressure (later in this chapter)
-boiling point
In general, the stronger the IMF, the higher the boiling
point
In solids: the melting point increases as the strength
of the IMF increase
What types of intermolecular attractions exist between
electrically neutral molecules?
Dispersion forces
Dipole-dipole attractions
Hydrogen bonding
What kind of attractive force operates between a
permanently charged particle (i.e. an ion) and a
neutral molecule?
Ion-dipole force (important in solutions)
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Notice: all intermolecular interactions are
electrostatic in nature
They are therefore governed by a Coulombic force
law:
So why are IMF so much weaker than ionic (or
covalent) bonds?
General guidelines…
Ionic bond strengths: 700 – 3800 kJ (table 8.2)
Covalent bond strengths: 150-1100 kJ (table 8.4)
Dispersion force: ~ 2 – 20 kJ
Dipole – dipole force: ~ 2 – 20 kJ
Hydrogen bonding: ~ 5 – 25 kJ
Ion – dipole: ~ 15 kJ
Recall: from Coulomb’s law…
Attractive force F (between opposite charges):
Increases as Q1, Q2 increase
Decreases as d increases
(a) Charges responsible for IMF are generally MUCH
smaller than charges in ionic compounds
e.g. NaCl vs HCl
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and (b) distances between molecules are much larger
than distances between atoms (or ions) held together
by bonds
1. Dispersion Forces
Is there evidence for interactions between neutral
nonpolar atoms or molecules?
Gaseous helium can be liquified, as can N2(g) , O2(g),
etc.
At low T, high P (these cases) there must be some type of
interaction which overcomes the kinetic energy of the
particles and allows them to form liquids
These interactions between neutral, nonpolar species are
known as dispersion forces (or London dispserion forces)
Motion of e- in an atom or molecule gives rise to an
instantaneous dipole moment, e.g., 2 He atoms:
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The 'induced' dipole moment changes over time (as the emove);
The distortion of charge distribution sets up the attractive
interaction known as the dispersion force (LDF)
What determines the degree to which the charge
distribution will be distorted?
Polarizability: ease with which e- charge cloud is distorted
The greater the polarizability, easier e- cloud can be
distorted to set up an instantaneous dipole
Larger atoms/molecules: larger polarizability (why?)
Net result: strength of dispersion forces increases with
increasing MW
Substance
MW (amu)
Boiling point (K)
F2
38.0
85.1
Cl2
71.0
238.6
Br2
159.8
332.0
I2
253.8
457.6
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How do the shapes of molecules influence dispersion
forces?
E.g., which has the higher boiling point: CH3CH2CH2Cl
or (CH3)2CHCl? (These species are isomers – same
formula, different structure)
2. Dipole - dipole forces
Exist between neutral polar molecules
Result from electrostatic interaction between
positive/negative ends of dipoles; e.g., HCl
Dipole - dipole forces effective when molecules are very
close together – they are a balance between attractive and
repulsive interactions
Repulsive interactions force molecules apart – net result
is a weak attraction between molecules
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Note: for molecules of similar mass & size, strengths of
attractive interactions increase w/ increasing polarity
What effect would this have upon the boiling point of a
liquid?
Substance
MW (amu)
Dipole moment 
(D)
Boiling point (K)
CH3CH2CH3
44
0.0
231
CH3OCH3
46
1.3
248
CH3Cl
50
1.9
249
CH3CHO
44
2.7
294
CH3CN
41
3.9
355
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Problems du Jour
What is the nature of the major intermolecular attractive
forces in
Xe(l)
PCl3(l)
Br2(l)
Describe the intermolecular forces that must be overcome
to convert each of the following from a liquid to a gas:
Br2
H2S
CH3CH2CH3
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