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SCH 3U
Intermolecular forces and Physical Properties
Recall: All bonds made between atoms are electrostatic attractions between positively charged nuclei and
negatively charged electrons.
To determine the physical properties of substances you must first identify the type of compound that you are
observing. Non-polar covalent, polar covalent, or ionic. The type of compound is determined by the
intramolecular bonds.
Intramolecular bonds – exist within one compound, these include ionic and covalent bonds. (sharing or
transfer of electrons)
Bonding Continuum: The
involved in the bond.
0.0
0.4/0.5
Non-polar
(EN ) is determined by taking the difference of the EN values of each element
0.8/0.9
slightly polar
1.6/1.7
very polar covalent
3.3
ionic
However, to compare substances, one must examine their physical properties such as melting point, boiling
point, vapour pressure, evaporation, viscosity, surface tension and solubility which are related to the strength
of attractive forces between molecules. Thus, the physical properties of a substance depend upon the type of
intermolecular forces present between its particles…not on the type of bonds within molecules (so not on the
electronegativity differences)
Intermolecular forces – attractive forces that exist between two or more compounds. These forces are
much weaker than bonds.
When a substance evaporates, melts, or sublimates, the molecules gain kinetic energy from
an outside source. This energy allows them to overcome the intermolecular forces holding
them close together.
**There are no intermolecular forces in ionic compounds because ionic compounds do not
contain molecules. Ionic compounds are always solids at room temperature. In an ionic
crystal, ionic bonds hold all the ions together. Chemical bonds, both ionic and covalent are
considerably stronger than intermolecular forces.
There are three types of intermolecular forces.
1. London dispersion or Van der Waals forces:
(found between all molecules)
 Weakest type of intermolecular forces, important in non-polar covalent molecules
 Because electrons are moving around in atoms there will be instants when the charge around an atom is not
symmetrical. Resulting in an attraction between the nuclei of one compound and the electrons of another
compound.
 May also be an attraction between temporary dipoles formed by the continuous movement of electrons
 All molecules experience VDW forces, however they may be masked by stronger forces.
Temporary dipoles
E.g. Hydrogen gas: the bonds within each hydrogen molecule are strong
covalent intramolecular bonds but between each hydrogen molecule, there are
weak forces of attraction because of temporary polarity even though the
molecules themselves are non-polar.
E.g. Instantaneous dipoles :
Induced Dipoles :
Eventually electrons are situated so that tiny dipoles form
inducing a dipole in the other
A dipole forms in one atom or molecule,
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

at low temperatures, these attractive forces between molecules are significant—resulting in changes in
state to liquid and solid as temperature drops;
thus, non-polar covalent solids and liquids have relatively low mp and bp and tend to be quite volatile;
(odour producing)
non-polar solutes tend to be soluble in non-polar solvents
Permanent Dipoles
2. Dipole-Dipole forces
 Can only exist between polar molecules
 In addition to the London dispersion forces, there now exists an attraction
between permanent (constant) dipoles (oppositely charged)
 The positive end of one molecule is attracted to the negative end of another.
E.g.
Since this dipole is permanent the attraction is much stronger. Clearly, the strength
of this attraction increases with the increasing polarity of the molecules.


thus, polar covalent solids and liquids have higher mp and bp because it
requires more energy to separate them from each other., they have little to no odour
polar solutes tend to be soluble in polar solvents
3. Hydrogen bonding forces (pg. 86)
 Can only exist between polar molecules that contain H-F, H-O, H-N bonds, these bonds create extreme
dipoles
 Dipoles are ‘extreme’ due to large END.
 They are about five times stronger than a regular dipole-dipole bond.
 Due to the size of the hydrogen, the positive charge can get very close to the negative dipole of another
molecule.
E.g.

H-bonds exist between water molecules and are very influential on
the physical properties of water. i.e. causing a high mp and bp;
allowing water to have a high heat capacity; making water a good
polar solvent “universal solvent”

Life on this planet depends on the properties of water –both as a
solvent (e.g. cytoplasmic water, urine, blood, plasma etc.) and as a
heat absorbing substance thus preventing our blood from literally
boiling when we exert ourselves;

H-bonds are also on the most important intermolecular forces at
work biochemically in living cells; e.g. enzymes bind very specifically
to substances based on H-bonds in specific patterns; the DNA double
helix is held together in two linear twisted halves by H-bonds between the bases A &T, G&C; proteins
hold their shape and functions by H-bonding between their constituent amino acids; cellulose in plant
cells makes the cell wall stronger due to H-bonding between cellulose strands

From an environmental viewpoint, water molecules H-bonded to one another cause water to freeze
from the top down in our lakes –ice is less dense than liquid water because of the crystal lattice
structure that forms from rigid H-bonded water molecules; thus fish and other aquatic life forms can
survive the winter at the bottom of the lake where the water is still in liquid form.
Ionic bonding forces (Intramolecular forces)
 Can only exist if the (EN ) is 1.7 or greater.
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
They are the strongest.
They are the forces holding ions together in ionic solids. They hold many ions in a crystal lattice structure.
When dissolved in water the ions will separate, allowing the solution to conduct
electricity.
Animation of dissolving NaCl in water
http://preparatorychemistry.com/NaCl_flash.htm
While the intramolecular forces keep the atoms in a molecule together and are the basis for the chemical
properties, the intermolecular forces are those that keep the molecules themselves together and are virtually
responsible for all the physical properties of a material.
The intermolecular forces increase in strength according to the following:
London dispersion < dipole-dipole < H-bonding
Now, as these things increase in strength it becomes harder to remove the molecules from each other.
Therefore, one would expect the melting and boiling points to be higher for those substances which have
strong intermolecular forces. We know that it takes energy to go from a solid
to a liquid to a gas.
This energy is directly related to the strength of attraction between molecules
in the condensed phases. Since energy is directly proportional to the
temperature, the above trends ought to hold true.
In addition, there are energies associated with making these phase transitions:
Each of these processes are considered to be endothermic, and scale with the magnitude of the intermolecular
forces. Thus, as these intermolecular forces increase, so do the energies that require substances to melt,
vapourize, or sublime (go from solid to a gas).
Every substance also has an associated vapour pressure with it. The vapour pressure is defined to be the
amount of gas of a compound that is in equilibrium with the liquid or solid. If the intermolecular forces are
weak, then molecules can break out of the solid or liquid more easily into the gas phase. Consider two different
liquids, one polar one not, contained in two separate boxes. We would expect the molecules to more easily
break away from the bulk for the non-polar case. This would mean that, proportionately, there are more
molecules in the gas phase for the non-polar liquid. This would increase the vapor pressure. Thus, unlike the
physical properties listed above, the vapour pressure of a substance decreases with increasing intermolecular
forces.
Now, as an example, we will plot vapor pressure as a function of temperature for three compounds:
Which molecule corresponds to which curve?
Let us rank the species in order of increasing IM forces:
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C4H10O has only dipole-dipole attractions and L-D forces
H2O and CH3OH both have H-bonding, as well as dipole-dipole and L-D forces. However, the CH3OH
has but one hydrogen to use in H-bonding, where H2O has two.
The relative strengths are: C4H10O < CH3OH < H2O.
The top curve has the highest vapor pressure, and ought to correspond to the species with the least amount of
IM forces, or C4H10O. The middle curve is CH3OH, and the bottom curve is H2O.
Review Support Materials
Student tutorial: https://www.wisc-online.com/learn/naturalscience/chemistry/gch6804/intermolecular-forces
Intermolecular forces:
https://www.youtube.com/watch?v=-QqTwJzi7Wo
Intermolecular attractions:
https://www.youtube.com/watch?v=dQ33TVQCfyY
Dipole-Dipole video:
https://www.youtube.com/watch?v=cERb1d6J4-M
London dispersion forces:
https://www.youtube.com/watch?v=1iYKajMsYPY
How do intermolecular forces match up with properties??
(melting point, boiling point, vapour pressure, solubility, electrical conductivity)
Only London Dispersion (VDW)
Lowest melting point / boiling point
Softest
(weak intermolecular forces require little energy (in the form of heat)
Smelliest
(weak IMF are easily overcome, molecules can escape)
London Dispersion (VDW) & weak dipole-dipole
Higher melting point / boiling point
Harder
(stronger intermolecular forces require more energy to overcome)
Less smelly
(stronger IMFs are harder to overcome, cannot escape easily)
London Dispersion (VDW) & stronger dipole-dipole
Higher melting point / boiling point
Harder
(stronger intermolecular forces require more energy to
overcome)
Less smelly
(stronger IMFs are harder to overcome, molecules cannot escape
easily)
(VDW) Dipole-Dipole & hydrogen-bonding
Higher melting point / boiling point
Harder
(stronger intermolecular forces require more energy to overcome)
Less smelly
(stronger IMFs are harder to overcome, molecules
cannot escape easily)
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Strong ionic attractions (ion to ion)
Highest melting point / boiling point
Intramolecular forces
Hardest
Crystals, soluble in water, conductive in water
(stronger intramolecular forces require more energy to
overcome)
Not really smelly at all!!