Pirate Chemistry
Intermolecular Forces
We have now learned that covalent molecules can be polar; one side of the molecule is partially
negative while its opposite side is partially positive. So what? Big deal! Why should we care?
We should care because whether or not a molecule is polar or not determines most of the physical and some of the chemical properties of that substance. The reason is due to what are called
intermolecular forces– interactions between one molecule and another.
Let me give you an example. In this first picture I have created a wall from children’s building
blocks. If I come along and give it a good whack, the wall falls over and the blocks scatter.
The next picture shows the same thing but this time the wall is made from Lego’s blocks; the
kind that snap into each other. When I give this wall some energy, it still breaks apart but
you’ll notice that the pieces have not scattered nearly as much as the first case. Why? It should
be obvious that in the first case there was nothing holding the blocks together. There were no
forces which kept one block attached to another so they easily fell apart. However in the second case, the Lego’s blocks snap together. There is a force or a strength from block to block.
Thus, the wall held together much better.
Intermolecular forces are like the bumps and holes in the Lego’s bricks. They are forces between molecules that keep the molecules attracted to each other. How do they arise? Let’s
look at two circumstances:
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In our first case we will look at carbon dioxide, CO2. Remember that CO2 is a non-polar molecule because it’s polar bonds are equal and opposite and so they cancel out:
= 3.5
= 3.5
= 2.5
O C O
O C O
O C O
O C O
Non-polar
Consequently, there is no major force of attraction between the molecules of CO2. What do we
know about CO2? It is a gas. The particles are free to roam about freely in any container as
there is nothing holding them together.
The gas particles of CO2 fill up the space of this container as they bounce around off the walls
of the container and each other. If the container was opened, the gas would immediately escape.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
Pirate Chemistry
Now let’s look at water, H2O. Remember that water is very polar molecule due to its bent
shape and high difference in electronegativity:
= 2.1
H O
H
= 3.5
= 2.1
—
H O H O
+
H
H
Consequently, each water molecule acts as a little magnet for another. The positive side of one
water molecule is attracted to the negative side of another.
This attraction spreads out through all the
molecules and keeps the molecules close together through these intermolecular forces.
Consequently, water is a liquid rather than a
gas like CO2
The water molecules attract to each other which keeps the molecules close together in the liquid
state.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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Types of Intermolecular Forces (IMFs)
What we have seen in the previous two examples two extreme examples of intermolecular
forces (IMFs). Let’s define exactly what types they are and how to identify them.
London Forces
The first type of intermolecular force we’ll discuss is the weakest. It is known as London
Forces or Induced Dipole interactions. They occur mainly in molecules that are non-polar.
Let’s look at an example: oxygen, O2
O O
http://www.globalwarmingart.com/images/a/a4/Oxygen_Molecule_VdW.png
Oxygen is completely non-polar. Each atom pulls the bonded electrons equally at itself so all
vectors are canceled. This is seen here in picture 1:
However, the electrons around the molecule are moving around the atom at near the speed of
light. Just by chance, the electrons may end up more on one side of the atom than the other. If
this happens, the molecule obtains a partial negative charge on that side of the molecule. This
is picture 2:
—
+
This now polar molecule induces (causes) polarity in the molecule next to it as the electrons of
that molecule are repelled by the like charges. This is picture 3. The molecules are now attracted to each other as one side is negative and the other side is positive.
—
+
—
+
This is the London Force. It is a
temporary attraction between
molecules that have been induced
to polarity.
This doesn’t last very long, though as a microsecond later the electrons move back to their
original position which causes the temporary polarity to go away. Thus, the molecule returns to
is non-polar state.
4
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Pirate Chemistry
This process is easier to do with an atom such as Iodine rather than oxygen. Iodine is much larger and its outer electrons are much further from the nucleus. They are also shielded by five
complete inner shells of electrons which makes the valence electrons easier to manipulate.
Thus, the larger the atom, the more London Forces it tends to have.
I
I
O
O
Iodine is larger and thus easier to influence their electrons than oxygen.
As the atom gets bigger, the London Forces increase.
Dipole Forces (polar)
The next type of intermolecular force is that molecules that have dipoles– a fancy term for
molecules that are polar. Polar molecules as we said before behave like little magnets, keeping
the molecules closer together. A molecule like acetone or ether is polar due to its shape and the
presence of the highly electronegative oxygen atom at the top. Molecules of these atoms that
have permanent dipoles tend to be more attracted to each other than molecules that only have
temporary dipoles (London Forces). Thus the intermolecular forces are stronger.
—
O
H
C
H H
C
+
—
H
C
H
H
Dipole
Force
—
O
H
C
H H
C
+
C
H
H
H
Acetone and ether
are both polar molecules that have permanent dipoles
which causes molecules of the compound to attract to
each other.
H
C
H H
O
+
C
H
H
H
Dipole
Force
—
H
C
H H
O
+
C
H
H
H
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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Hydrogen Bonds
The last type of intermolecular force is that of hydrogen bonding. Hydrogen bonding is really
no different than the previous IMF of Dipole attractions except that it is much, much stronger.
Hydrogen bonding occurs when an atom of hydrogen is bonded to a very small, highly electronegative atom. There are only three such atoms: F, O, and N. When this occurs, the atom of F,
O, or N is able to pull the electrons in the bond so close to itself that it makes for an extremely
polar situation.
Water is a prime example of hydrogen bonding. Water has H directly bonded to O so the intermolecular force that arises is that of hydrogen bonding or H-bonds. Please note that the term
“hydrogen bonding” is often misunderstood for the bond itself. H-bonds are intermolecular
forces; they are the attractions between molecules.
H
+
H
O
H
+
—
This is the H-Bond. It is an
intermolecular force; not a
bond.
H
O
—
This is a bond that happens to
have hydrogen in it. It is NOT
a “hydrogen bond”.
Also please note that for H-bond to arise, the H atom has to be directly bonded to the F, O, or N
atom itself. It is not enough for the molecule simple to have F, O, or N as well as H. They have
to be attached to each other. All of the molecules below are polar but not all of them have hydrogen bonds as their strongest intermolecular force.
—
—
O
H
C
H H
C
+
H
C
H
H
+
C F
—
H H
H
H
C C
H H
H
O
+
H
H
+
N
—
H H
H
Polar but will not make
hydrogen bonds. The H
atoms are not directly
attached to the O atom.
Polar but will not make
hydrogen bonds. The H
atoms are not directly
attached to the F atom.
This molecule will make
hydrogen bonds. The
molecule has H directly
bonded to the O atom.
This molecule will make
hydrogen bonds. The
molecule has H directly
bonded to the N atom.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
Pirate Chemistry
Intermolecular Forces Summary
Type of IMF
When Present
Strength
London Forces
Non-polar molecules
Weakest
Dipole Forces
Polar molecules
H-Bonds
H directly bonded to N, O, or F
Strongest
Questions
1.
2.
3.
4.
5.
6.
What are intermolecular forces?
What are the three types of intermolecular forces?
What type of molecules have London Forces as their strongest IMF?
What type of molecules have Dipole Forces as their strongest IMF?
How can you tell if a molecule will form H-bonds?
In the picture below, circle the IMF:
7. For each of the molecules below, decide what is the strongest IMF present: London
Forces, Dipole Forces, or H-bonds. You will have to look up electronegativity values and
apply the vectors to determine if the molecule is non-polar (London Forces), polar (Dipole
Forces), or polar with H-bonds (H directly attached to an atom of F, O, or N).
N
H
H
H
N
Cl
Cl Cl
S
H
C
C
S
N
H C
H H
O
H
C
H H
H H
H C C O
H H H
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org