03/03/2017
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
Intermolecular Forces
aka van Der Waals forces
inter = between

Forces affecting the relationships between
molecules



London Forces (Dispersion Forces
Dipole-Dipole Interactions
Hydrogen Bonding

Intramolecular forces: forces that bond the
atoms to each other within the molecule.

COVALENT BONDS!!!
London Dispersion 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
1
03/03/2017
London Dispersion forces in a
molecule
London Dispersion Forces



These instantaneous dipoles may be induced and stabilized
as an ion or a polar molecule approaches the non-polar
molecule.
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
The Hydrogen Bond
Present in all molecules
Increase in strength as the number of
electrons in a compound increases,
eg. Cl2 (14 total electrons) vs. CCl4 (32 total
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



These forces occur
between polar molecules
The larger the ∆EN, in the
polar bonds, and the
greater the number of polar
bonds in a molecule, the
stronger the forces holding
molecules together
Stronger forces = higher
melting and boiling points
• 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
2
03/03/2017
• 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
Hydrogen bonding in water
animation
water
waterhas
has
water
water
thehaswater
has the
has
thehighest
lowest
theheat
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.
Which intermolecular forces
would act on this molecule?
• 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 this molecule?
3
03/03/2017
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
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.
4
03/03/2017
Surface Tension
• Molecules with strong intermolecular forces
will have higher surface tension due to the
attraction of the molecules to each other.
Solubility
• 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
5
03/03/2017
Boiling Point
What is boiling point?
• The temperature at which a liquid boils at a fix
ed pressure, especially under standard atmosph
eric conditions.
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).
• Nonpolar molecules have very little attraction to each
other, therefore they will have low boiling and
melting points.
The greater the forces of attraction the higher the
boiling point or the greater the polarity the higher
the 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.
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
6