INTERMOLECULAR
FORCES
A Quick Introduction
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Intermolecular forces exist everywhere
Short-range attractive forces operating between
the particles that make up the units of a liquid
or solid substance
Intermolecular forces also cause gases to
liquefy or solidify at low temperatures and high
pressures
Did you know…
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Intermolecular forces are generally weaker than
ionic or covalent bonds
Many properties such as boiling point reflect
the strengths of the intermolecular forces
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The stronger the attractive force, higher the temp.
when liquid boils
Melting points of solids increase as strengths of
IF increase
An Attraction Force:
Ion-dipole Forces
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Exist between an ion and the particle charge on the
end of a polar molecule
Polar molecules = dipoles (has both poles + and -)
+ ion are attractive to – end of dipole…and vice
versa
Magnitude of attraction increases as the magnitude
or ion charge of the dipole moment increases
Types of Intermolecular
Forces
Dipole-dipole forces
 Ion – Dipole forces
 London Dispersion forces
 Hydrogen-bonding forces
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Dipole-dipole forces
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Exist between neutral polar molecules
Weaker than ion-dipole forces
Molecules with equal mass or size, strengths of
intermolecular attraction increases with
increasing polarity
A Few Queries…
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What about non-polar molecules?
So how does a non-polar gas liquefy?
Because you need some sort of attraction
between particles
…well…
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1930 – Fritz London proposed origin of this
type of attraction
Recognized that motion of electrons in
atom/molecule can create an instantaneous dipole
moment
Example
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Helium is spherically symmetrical
Molecules -> non-polar; and possess no permanent
dipole moment
However, instantaneous distribution of electrons is diff.
from avg. distribution
 If we freeze the electrons in helium atom, both
electrons could be on same side of nucleus; giving it
instantaneous dipole moment
Example
London Dispersion Forces
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Electrons repel one another, motion of
electrons on one atom influence the neighboring
atoms.
So temporary dipole moment on one atom can
induces a similar dipole on a adjacent atom
This is significant only when molecules are close
together.
Polarizability
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Ease with which the charge distribution can be
distorted by an external electric field
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Think of it as “squashiness” of an electron cloud
The greater the polarizability of molecule, the
more easily its electron cloud can be distorted to
a momentary dipole
So…
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More polarizable molecules have stronger
London dispersion forces
larger molecules have greater polarizabilitymore electrons farther from the nuclei
So London dispersion forces increase with
increasing size and molecular weight
Other factors that determine
strength:
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Shape
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Ex. N-pentane and neopentane
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N-pentane attraction is higher because more molecules come in
contact- because it is more cylindrically shaped
Summary of: Comparing Strengths
of Intermolecular Forces
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Molecules that have comparable eights and
shapes have dispersion forces that are approx.
equal
When molecules differ greatly in weights,
dispersion forces are accordingly different
Hydrogen Bonding
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Hydrogen bonding is a special type of
intermolecular attraction that exists
between the hydrogen atom in a polar
bond and an unshared electron pair on a
nearby small electronegative ion or atom
Tend to be the strongest type of
intermolecular forces
More About Hydrogen Bonding
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Can be considered unique dipole-dipole
attractions
F, O, and N are so electronegative, that a bond
between them and hydrogen is quite polar, with
hydrogen on positive end
The End
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http://chemed.chem.purdue.edu/genchem/topi
creview/bp/intermol/intermol.html
http://www.chem.unsw.edu.au/UGNotes/haine
sIMF/contents.html
http://scidiv.bcc.ctc.edu/wv/08/0008-0012interforce.html