Intermolecular Forces
Interesting Questions to answer
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The boiling point of the noble gas helium is
-268.7oC, whereas the boiling point of the
noble gas Neon is -245.9oC. What accounts
for this difference?
The boiling point of H2O is 100oC, whereas
the boiling point of H2S is -59oC. What
accounts for this difference?
Carbon dioxide (CO2) is a gas at SATP,
whereas Silicon dioxide (SiO2) is a solid.
What accounts for this difference?
What are Intermolecular Forces?
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Forces that bind atoms within a
molecule are due to chemical
bonding (i.e. sharing electrons)
These forces are referred to as
intramolecular forces
Forces that hold molecules together
are generally called intermolecular
forces.
What are Intermolecular Forces?
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Intermolecular forces are much
weaker than the forces involved in
chemical bonds
However, intermolecular forces play
a large role in the physical
properties of substances, such as:
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State (solid, liquid, gas)
Volatility (ability to vaporize)
Melting/boiling point (self-explanatory)
Types of Intermolecular Forces
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1.
We will discuss the following
intermolecular forces:
van der Waal’s forces
1.
2.
2.
Dipole-dipole
London forces (Dispersion)
Hydrogen bonds
van der Waal’s Forces
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Johannes van der Waals (1837 –
1923) was the first to suggest the
importance of intermolecular forces.
van der Waals forces is a general
term for those intermolecular forces
that include dipole-dipole and
London forces.
van der Waals forces is a very weak
attractive force between many
substances.
Dipole-dipole
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Polar molecules can attract one
another through dipole-dipole
forces.
A molecule is polar if it possesses
an asymmetrical charge
distribution.
Molecules that possess this are said
to have a dipole moment.
Example of a dipole moment
Dipole-dipole
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Dipole-dipole forces is an attractive
intermolecular force resulting from
the tendency of polar molecules to
align themselves such that the
positive end of one molecules is
near the negative end of another.
London Forces (Dispersion)
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Neon becomes a liquid at -2460C
Since it forms a liquid, there must
be forces adhering the molecules
together
Even substances that possess no
dipole moments at all still possess
intermolecular forces.
This force is named after Fritz
London
London Forces (Dispersion)
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The electrons in an atom form a
spherical distribution over time
However, at any instant, more
electrons may be present on one
side of the nucleus than the other
This creates a momentary dipole
moment
Formation of a dipole moment
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+
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Neon atom without a
dipole moment
+
-
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-
-
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-
Neon atom with a
dipole moment
London Forces (Dispersion)
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If the neon atom with a dipole
moment encounters another neon
atom, it will induce a dipole
moment in that neon atom
This creates two dipoles that are
momentarily attracted to each
other.
Creating a dispersion force
London Forces (Dispersion)
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London forces (also called
dispersion forces) are the weak
attractive forces between molecules
resulting from the small,
instantaneous dipoles that occur
because of the varying positions of
the electrons during their motion
about nuclei.
London Forces (Dispersion)
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ALL substances possess dispersion
forces
The greater the number of
electrons, the greater the dispersion
force
Examples of substances that ONLY
possess dispersion forces:
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F2, Cl2, He, Ar, Kr, CH3, C2H6
Example Question
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Arrange the following substance in
order of increasing magnitude of
the London forces: SiCl4, CCl4,
GeCl4.
Answer –
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London forces increase with increasing
number of electrons:
CCl4 (74e-), SiCl4(82e-), GeCl4(100e-).
Methanol vs. Fluoromethane
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Consider fluoromethane (CH3F)and
methanol (CH3OH)
Both have identical numbers of
electrons (16 e-) and similar dipole
moments
However, CH3F boils at -78oC, and
CH3OH boils at 65oC
What accounts for this difference?
Hydrogen Bonds
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Clearly methanol with its much
higher boiling point has much
stronger intermolecular forces than
fluoromethane does.
This introduces a new type of
intermolecular force: Hydrogen
bonding.
Hydrogen Bonds
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Hydrogen bonding is a weak to
moderate attractive force that
exists between a hydrogen atom
covalently bonded to a very
electronegative atom, X, and a lone
pair of electrons on another small,
electronegative atom, Y.
Hydrogen Bonds in Water
Hydrogen Bonds
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Consider the boiling point of
substances that contain hydrogen
bonded to group VIA elements:
Substance
Boiling point
H2O
100oC
H2S
-60oC
H2Se
-40oC
H2Te
0oC
Hydrogen Bonds
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Hydrogen bonding is present in
water, but absent in H2S, H2Se, and
H2Te.
This is because the electronegativity
difference between H and S, Se and
Te is not great enough.
Only molecules containing N-H, OH, and F-H bonds can form
hydrogen bonds
Hydrogen Bonds
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When water freezes, the molecules
of water form hexagon-shaped
crystals.
This is due to the hydrogen bonds
that form between water molecules.
Hexagonal crystal lattice of H2O
Example Question
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1.
2.
3.
What kinds of intermolecular
forces are expected in the
following substances?
Methane
Trichloromethane (chloroform)
butanol
Answers
1.
2.
3.
Methane is nonpolar. Therefore only
London (dispersion) forces are present.
Chloroform is an unsymmetrical
molecule with polar bonds. We would
expect dipole-dipole forces and London
forces
Butanol has a hydrogen atom attached
to an oxygen atom. Therefore, you
expect hydrogen bonding. You would
also expect dipole-dipole and London
forces
Homework
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Problems
Page 266 #1-7