Intermolecular Forces
The fantastic four!
ION-DIPOLE FORCE
Why does salt dissolve in water?
Using the phrase “like dissolves like” means that
salt (NaCl) must have something in common
with water (H2O).
They have like poles (as in charges, though not
always full charges).
The dissolving process is allowed by the
development of intermolecular forces between
the Na+ and water molecules.
As the salt is dissolved by the water, it breaks
into its anion (Cl-) and cation (Na+)
http://www.youtube.com/watch?v=xdedxfhcpW
o
Forms between:
An ion-dipole force requires ions (generally a
soluble salt such as NaCl or CaCl2) and a polar
solvent (like water or rubbing alcohol).
DIPOLE-DIPOLE FORCE
Why does rubbing alcohol dissolve in
water?
For these two substances to mix or for
the rubbing alcohol (isopropyl alcohol)
they must be alike.
And they are due to existing partial
charges!
The force that allows these two molecules to
interact is the dipole-dipole force.
Forms between:
A dipole-dipole force occurs between two
substances which have unequal sharing of
electrons which creates small partial charges.
Partial negative
Partial positive
LONDON DISPERSION FORCE
Why will oil and water not mix?
Again, it’s the like dissolves like.
Water has permanent dipoles, oil does not.
Oil may have temporary poles that are induced
by outside forces, but in general there is an even
distribution of electrons.
What will mix with oil?
It needs to be another molecule which does not
have dipoles or at least part of a larger molecule
does not have dipoles.
Acetone will dissolve in cooking oil.
Why?
Acetone does not have an overall polarity (poles).
Forms between
London dispersion forces are present in all
interactions, but it is dominant in non-polar
molecules because no other intermolecular
force is present.
It will have temporary (induced) poles.
HYDROGEN BONDING
Why will ice float in water?
Oh, its hydrogen bonding and the fact that the
water molecules (liquid) are actually closer
together in the liquid then in the solid.
In the solid, the hydrogen bonding is rigidly
holding the molecules apart (not any extra
kinetic energy to move them closer).
And the Snow Flake?
The shape of snow flakes are actually
determined by the shape of water molecules
and the intermolecular forces between them.
Water molecules use hydrogen bonding to stick
to each other (not to from the bonds inside the
water molecule itself).
Requirements to be present
1. It must have H in the molecule
2. It must have N, O or F present in the
molecule
Hydrogen Bonding Video
http://www.youtube.com/watch?v=lkl5cbfqFRM
Strongest to Weakest
Strongest intermolecular force
Ion-Dipole (full charges on the ions)
Hydrogen Bonding
Dipole-Dipole
London Dispersion Forces (no permanent
charges partial or complete)
Impact of Strength of Intermolecular
Forces
The stronger the intermolecular force (within a
series of like elements) the higher the melting
and boiling points will be.
So by looking at the boiling points for a series of
molecules, the one with the highest value also
has the strongest intermolecular forces.
Impact of Strength of Intermolecular
Forces
London Dispersion Force
For a pair of molecules which have London
Dispersion Force, to determine which one has the
strongest, you look at which one has the largest
number of electrons.
This is generally going to be the molecule which has
the largest total mass.
The more electrons, the more the electron cloud
could be deformed.