Intermolecular forces and Physical Properties
SCH 3U
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 (EN ) is determined by taking the difference of the EN values of each element involved in
the bond.
0.0
0.4
Non-polar
slightly polar
0.8
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.
There are four types of intermolecular forces.
1.
London dispersion or Van der Waals forces:
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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.


(found between all molecules)
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 :
Eventually electrons are situated so that tiny dipoles form
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Induced Dipoles :
A dipole forms in one atom or molecule, inducing a
dipole in the other
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
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

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
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E.g.
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.

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.
Video: On Water its unique properties (World of Chemistry)
http://www.learner.org/resources/series61.html?pop=yes&pid=804
4. Ionic bonding forces
(EN ) is 1.7 or greater.
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Can only exist if the
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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 < ion-ion (strong ionic attractions)
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.
Animation of boiling points of different IMF forces: http://www.chm.davidson.edu/ronutt/che115/Phase/Phase.htm
In summary
Covalent the intermolecular forces present in these types of substances depend upon whether or not a molecule is polar!
Non-polar molecules experience only Van der Waals attractive forces (protons of one molecule to the electrons of another
molecule)
Polar molecules experience Van der Walls forces and dipole-dipole attractive forces which are stronger (these are the attractive
forces between the negative dipole of one molecule and the positive dipole of another molecule)
**Certain molecules are more polar than others (based on the strength of the END). The more polar a molecule, the stronger the
dipole-dipole attractive force will be!
Polar molecules containing H-O, H-N or H-F bonds will experience VDW forces, and hydrogen bonding forces which are the
strongest intermolecular force, an extreme case of dipole-dipole force (the hydrogen nucleus is attracted to the negative dipole of
another molecule)
Ionic have ionic bonds present between positive particles & negative particles (there technically aren’t any ‘IMFs’ other than
ionic bonding)
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)
Strong ionic attractions (ion to ion)
Highest melting point / boiling point
Harder
(stronger intermolecular forces require more energy to
overcome)
Not really smelly at all!!
(strong IMFs are very hard to overcome, molecules
cannot escape easily)