Intermolecular Forces:
relationships between
molecules
Polarity and Shape


Polar bonds do not necessarily always create
a polar molecule.
The shape of the molecule and the polarity of
each bond is considered before stating if a
molecule is considered POLAR or NONPOLAR


We will watch a 5 minute explanation to help us understand this
difficult concept
major intermolecular forces
Intramolecular forces
intra = inside

Intramolecular forces: forces that bond the
atoms to each other within the molecule.

COVALENT BONDS!!!
Intermolecular Forces
inter = between

Forces affecting the relationships between
molecules




London Forces (Dispersion Forces) AKA van
der Waals forces
Dipole-Dipole Interactions
Ion-dipole forces
Hydrogen Bonding
London (dispersion) Forces
aka van Der Waals forces



weakest intermolecular
force
occurs between all
types of molecules
It is a temporary
attractive force that
results when the
electrons in two
adjacent atoms occupy
positions that make the
atoms form temporary
dipoles
London Dispersion forces in a
molecule
These instantaneous dipoles may be induced and stabilized
as an ion or a polar molecule approaches the non-polar
molecule.
London Dispersion Forces



Present in all molecules
Increase in strength as the number of
electrons in a compound increases,
eg. Cl2 (14 electrons) vs. CCl4 (32 electrons)
both molecules are nonpolar, but the larger
number of electrons in CCl4 affords a greater
chance of charge imbalance and therefore
stronger London forces.
Dipole-Dipole Forces



Dipole = polar molecule
Molecules with dipoles
will change their
direction so that their
oppositely charged
ends are near to one
another.
The electrostatic
attraction between the
ends is dipole-dipole
force
Dipole-Dipole Forces


These forces occur
between polar molecules
The larger the ΔEN, in the
polar bonds, and the
greater the number of polar
bonds in a molecule, the
stronger the forces holding
molecules together
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2
Dipole - Dipole Interactions
Ion Dipole Forces
(the reason why ionic compounds dissolve
in water)


The force of attraction
between an ion and a
polar molecule.
NaCl breaks up
because the ion dipole
with water is stronger
than the attraction of
Na+ to Cl-
Polar molecules can interact
with ions:

Ion - Dipole Interactions
The Hydrogen Bond
• A hydrogen bond is the strongest form
of dipole-dipole interaction.
• A hydrogen bond is formed between
polar molecules that contain hydrogen
covalently bonded to the small, highly
electronegative atoms, F, O, or N.
F—H
O—H
N—H
• A molecule containing this combination
of atoms, will be attracted to another
molecule with the same atom
combination.
• A strong attractive force between the
two molecules which is called a
hydrogen bond is formed.
hydrogen
bond
covalent
bond
covalent
bond
water
waterhas
has
water
water
thehashas
water
the
has
thehighest
lowest
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highest
highest
the
heat
highest
of
molarofmass
fusion
melting
vaporization
point
boiling point
The melting point, boiling point, heat of fusion and heat of
vaporization of water are extremely high and do not fit the
trend of properties relative to molar mass within Group 17.
Hydrogen bonding in water
animation
• hydrogen bonding animation
Water exhibits these unusual properties
because of hydrogen bonding between
water molecules.
Which intermolecular forces
would act on this molecule?
Which intermolecular forces
would act on CH2Cl2 compared
to CCl4?
Which intermolecular forces
would act on this molecule?
Which intermolecular forces
would act on this molecule?
Why do we care about
intermolecular forces?
• The forces that act on molecules determine
their physical properties
• Strong intermolecular forces between
molecules increase melting and boiling points
because it is these forces that are broken when
substances change state.
Melting point
The temperature at which a solid, given
sufficient heat, becomes a liquid. For a
given substance, the melting point of its
solid form is the same as the
freezing point of its liquid form
Boiling Point
The temperature at which a liquid boils at a
fixed pressure, especially under standard a
tmospheric conditions.
Viscosity
A measure of a fluid's
resistance to flow. It
describes the internal
friction of a moving fluid.
A fluid with
high viscosity resists
motion because its
intermolecular forces
hold the molecules
together.
Surface Tension
The energy, or work, required to increase
the surface area of a liquid due to
intermolecular forces.
Polarity and Boiling Point:
The polarity of the molecules determines the
forces of attraction between the molecules in the
liquid state.
Polar molecules are attracted by the opposite
charge effect (the positive end of one molecule
is attracted to the negative end of another
molecule).
Molecules have different degrees of polarity as
determined by the functional group present.
The greater the forces of attraction the higher the
boiling point or the greater the polarity the higher
the boiling point.
What is boiling point?
Boiling point is the
temperature at which a
liquid turns into a gas (or
vapour).
For a liquid to turn into a
gas, intermolecular
forces must be broken.
The stronger the
intermolecular forces, the
higher the boiling point of
a substance.
The evidence for hydrogen bonding
Many elements form compounds with
hydrogen - referred to as "hydrides".
If you plot the boiling points of the hydrides
of the Group 4 elements, you find that the
boiling points increase as you go down the
group.
The increase in boiling point happens because the molecules are
getting larger with more electrons, and so van der Waals dispersion
forces become greater.
The evidence for hydrogen bonding
If you repeat this exercise with the
hydrides of elements in Groups 5, 6 and
7, something odd happens.
Solubility and
Intermolecular Forces
Like dissolves like
– Polar solutes dissolve in polar solvents
– Nonpolar solutes dissolve in nonpolar
solvents
Molecules with similar intermolecular
forces will mix freely
Ionic Solute with
Polar Solvent
Ionic Solute with
Nonpolar Solvent
Nonpolar Solute with
Nonpolar Solvent
Nonpolar Solute
with Polar Solvent
Comparison of the Properties of Substances with Ionic,
Covalent, Metallic or Intermolecular Bonds
Ionic
Covalent
Metallic
Intermolecular
Bond
strength
Strong
Very strong
Variable strength,
generally
moderate
Weak
Hardness
Moderate to high
Insulators in
solids and liquid
states
Low to moderate;
ductile, malleable
Crystal soft and
somewhat
plastic
Electrical
conductivity
Conducts by ion
transport, but only
when liquid or
dissociated
Low
Good conductors;
conducts by
electron transport
Insulators in
both solid and
liquid states
Melting
point
Moderate to high
Very high
Generally high
Low
Solubility
Soluble in polar
solvents
Very low
solubility
Insoluble except
in acids or alkalis
by chemical
reaction
Soluble in
organic
solvents
Examples
Most minerals
Diamond, oxygen,
hydrogen, organic
molecules
Cu, Ag, Au, other
metals
Organic
compounds
Boiling Point of Various Material (˚C)
Noble gas
Helium
neon
argon
He
Ne
Ar
-269
-246
-186
Nonpolar covalent
hydrogen
oxygen
methane
chlorine
H2
O2
CH4
Cl2
-253
-183
-164
-34
polar covalent
ammonia
hydrogen
fluoride
water
NH3
-33
19.5
HF
100
H2O
ionic
metallic
potassium
chloride
sodium
chloride
magnesium
oxide
KCl
771
NaCl
1413
MgO
2826
copper
iron
tungsten
Cu
Fe
W
2567
2750
5660