Intermolecular Forces
Section 6.5
Introduction
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We will consider ionic and covalent bonds
between atoms
If there are no attractive forces between
molecules, then all substances would be gases
There must be some force that attracts
molecules to other molecules that does not
create a real bond
These are known collectively as
intermolecular forces
Types of Intermolecular Forces
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In order of increasing strength:
van der Waals' forces (AKA London forces or
dispersion forces)
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Dipole-dipole forces
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Hydrogen bonding
van der Waals' Forces
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Exist between all species
Is the only intermolecular force between nonpolar species
Result of temporary (instantaneous) dipoles
when one side of the molecule becomes
partially negative because of the random
motion of electrons
Continued
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This results in a weak dipole moment that then
attracts other molecules
Strength of this force increases with molar
mass as more electrons are available for
temporary dipoles
Effect of van der Waals' Forces on
Boiling Point
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Higher boiling point means more energy is
needed to break the intermolecular forces
As the molar mass increases, the boiling point
increases
As the surface area increases, the boiling point
increases (again, more electrons available)
More elongated the molecule, the stronger the
van der Waals' forces, so the higher the boiling
point
Continued
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Sometimes the van der Waals' force can be
quite strong as in the case of some polymers
that have high mass and are very long
molecules
Dipole-dipole forces
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Dipole moment: a measure of the polarity of a
molecule
Arrows are used to represent the polarity of the
bond (heading toward the partially negative
part)
The permanent dipoles formed cause
electrostatic attraction between molecules that
have the permanent dipoles
Stronger than van der Waals' forces in
molecules of similar size
Continued
Effect of Dipole-dipole Forces on
Boiling Point
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Compared to molecules of similar mass with
only van der Waals' forces, much higher
boiling point
Polar molecules have van der Waals' forces
and dipole-dipole forces
Stronger intermolecular force: higher boiling
point
Hydrogen Bonding
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Not a “true” bond
Occurs when hydrogen is bonded to highly
electronegative, small atoms, like N, O, or F
Creates a very high dipole moment as the more
electronegative atom attracts the electrons,
leaving the hydrogen very partially positive
Can be thought of as partway between a
dipole-dipole force and a dative covalent bond
Continued
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For maximum strength, the 2 atoms and the
hydrogen should be in a straight line
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When X and Y are N, O, or F:
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Xδ-: hydrogen bond Hδ+ - Yδ-
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Usually much stronger than other
intermolecular forces
H-bonding
Effect of H-bonding on Boiling Point
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Consider HF and HCl:
HF has van der Waals' forces, dipole-dipole
forces, and H-bonding
HCl has van der Waals' forces and dipoledipole forces
Boiling point of HF is higher than HCl
More energy is needed to break the
intermolecular forces
Consider H2O and H2S
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H-bonding present in H2O, but not in H2S
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Boiling point of water is higher than H2S
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H-bonding allows water to form tetrahedral
shapes when it is a liquid
H-bonding allows water to form hexagons when
it freezes, so ice is less dense than liquid water
H-bonding in water also forms temporary
hexagon arrays on the surface of water, giving it
a high surface tension
Consider NH3 and PH3
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NH3 can hydrogen bond with itself and with
the water when the ammonia is aqueous
PH3 can only hydrogen bond with water, not
with itself
Ammonia has the higher boiling point
Biological Importance
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The weak bond between the nitrogenous bases
in nucleic acids is a H-bond
Occurs between thymine and adenine as well
as between cytosine and guanine
Easily broken by enzymes