Intermolecular Forces
Intra- vs. Inter
Intra-: inward
– Ex. Intradermal, Intravenous

Inter-: between or among
– Ex. Interstate, International

Intramolecular forces act within a
molecule.
– Ionic, covalent, and metallic bonding

Intermolecular forces act between
molecules.
– London dispersion, dipole-dipole interactions,
ion-dipole interactions, and Hydrogen bonding
Intramolecular vs. Intermolecular
Forces - Similarities
 Attractive
forces
 Force due to electron sharing
(charge)
 Affect spatial arrangements of atoms
and molecules, respectively
Intramolecular vs. Intermolecular
Forces - Differences
Intramolecular
Forces
Intermolecular
Forces
Strong
Weak
Act within molecules
Act between molecules
Persist for life of
molecule
More brief in life of
molecule
Not strongly effected by
physical changes
Stabilize individual
molecules
Strongly effected by
physical changes
Responsible for bulk
properties of matter
Lava Lamp Experiment
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Each pair of students should write their names on the top of 1
sheet of paper.
One student should obtain a 250 mL beaker and fill halfway
with water.
A second student should add about 1 cm of oil to top (between
¼ and ½ inch or about the width of pinky finger).
On your piece of paper, draw and describe the beaker and label
it as “Drawing 1”.
When Drawing 1 is complete, raise your hand for addition of 1
drop of food coloring.
Draw the beaker and its contents immediately after the addition
of food coloring. Label this “Drawing 2”.
Wait 1-2 minutes and draw the beaker again. This is “Drawing
3”.
When Drawings 2 and 3 are complete, raise your hand for
addition of sugar.
Draw and describe what happens right after the sugar is added
as “Drawing 4”.
Draw and describe what happens about one minute after the
sugar is added as “Drawing 5”.
Lava Lamp Experiment
1.
2.
3.
4.
5.
What materials will mix together?
What happened to food coloring in oil? How
would you describe the color, shape, size, and
movement of the food coloring?
What happened to food coloring in water? How
would you describe the color, shape, size, and
movement of the food coloring?
What do you think the phrase “like dissolves
like” means?
How would you apply “like dissolves like” to the
materials used in the beaker?
Intermolecular Forces




London dispersion forces
Dipole-dipole forces
Hydrogen bonding
Ion-dipole forces
Name
of
force
Rank of
strength
Ion
involved?
Polar or
nonpolar
molecules?
Is H
involved?
Example
Why don’t oil and water mix?
London dispersion forces




Weakest
intermolecular force
Only attractive force
between non-polar
molecules
Created from
temporary fluctuations
in electron density
around atoms
The larger the
molecule, the greater
the dispersion force.
https://www.chem.unsw.edu.au/coursenotes/CHEM1/nonunipass
/hainesIMF/dispersion.html
London dispersion forces
Why don’t oil and water mix?
Lava Lamp Experiment
Which materials exhibit London
dispersion forces?
 What observations can be explained
by London dispersion forces?

Name of
force
London
dispersion
forces
Rank of
strength
4
Ion
involved?
No
Polar or
nonpolar
molecules?
Both (strongest
for nonpolar)
Is H
involved?
No
Example
Oil and water
Dipole
In polar molecules electrons are not
equally shared between atoms.
 In areas of the electron cloud where
electrons are more likely to be found, a
“dipole” is formed. This end of the
molecular has a partial negative charge.
The opposing side of the molecule will
have a partial positive charge.
 These molecules are “polar”.

Dipole
Cl
δ
-
H δ+
Example: H2O
δ
δ+
-
H
O
Example: HCl
δ
-
H+
δ
Images modified from: http://employees.csbsju.edu/hjakubowski/classes/ch123/ch123ch2mcmfay5th.htm
Dipole-Dipole Forces
Attractive force
between neutral,
polar molecules
(molecules that
possess a dipole).
 The larger the
dipole, the greater
the force.
 Animation

Image modified from: http://www.chem.ufl.edu/~itl/2041_u01/lectures/lec_g.html
Lava Lamp Experiment
What materials exhibit dipole-dipole
interactions?
 What observations can be explained
by dipole-dipole interactions?

Name of
force
Rank of
strength
Ion
involved?
Polar or
nonpolar
molecules?
Is H
involved?
Example
London
dispersion
forces
4
No
Both (strongest
for nonpolar)
No
Oil and water
Dipoledipole
forces
3
No
Polar
No
Alcohol in
water
Why does salt dissolve?
Ion-Dipole Forces


Interaction between
charged molecule
(ion) and polar
molecule (dipole).
Strength depends on
charge and size of ion
and magnitude and
size of dipole
– Cations interact more
strongly with dipoles
than anions.
Image from: http://www.chem.purdue.edu/gchelp/liquids/iondip.html
Why does salt dissolve?
NaCl in Water
How would heating affect solubility? Stirring?
Lava Lamp Experiment
Were there any ion-dipole forces in
the lava lamp experiment?
 Can any observations be explained
by ion-dipole forces?

Name of
force
Rank of
strength
Ion
involved?
Polar or
nonpolar
molecules?
Is H
involved?
Example
London
dispersion
forces
4
No
Both (strongest
for nonpolar)
No
Oil and water
Dipoledipole
forces
3
No
Polar
No
Alcohol in
water
Ion-dipole
forces
1
Yes
Polar
No
Salt in water
Hydrogen Bonding
 Permanent
A
H
B
Where A and B are F, N, or O
dipole-dipole
interaction
 Only occurs in
molecules
containing H-F,
H-N, or H-O
bonds
Hydrogen Bonding of Water
Hydrogen Bonding of Water
Snowflakes
Hydrogen Bonding & Boiling Point
Image from: http://faculty.ycp.edu/~peterman/chm136/chm136s07ex1a.htm
Lava Lamp Experiment
Were there any hydrogen bonds in
the lava lamp experiment?
 Can any observations be explained
by hydrogen bonding?

Name of
force
Rank of
strength
Ion
involved?
Polar or
nonpolar
molecules?
Is H
involved?
Example
London
dispersion
forces
4
No
Both (strongest
for nonpolar)
No
Oil and water
Dipoledipole
forces
3
No
Polar
No
Alcohol in
water
Ion-dipole
forces
1
Yes
Polar
No
Salt in water
Hydrogen
bonding
2
No
Polar
Yes
Water
“Like dissolves like”

To be soluble a compound must
interact with the solute by:
– Dipole-dipole forces
– Ion-dipole forces
– London dispersion forces
Polar solutes dissolve in polar
solutions
 Non-polar solutes dissolve in nonpolar solutions

Summary
Image from: http://www.chem.ufl.edu/~itl/2041_u01/lectures/lec_g.html