>>Dr. Mark Morvant: In this video we're going to look at intermolecular forces. Intermolecular
forces are forces of attraction or repulsion, which act between neighboring atoms, molecules,
or ions. We can contrast intermolecular forces to intramolecular forces. Intramolecular forces
are forces within a molecule, such as a covalent bond. Therefore, the substantial difference
between intramolecular forces, within, and intermolecular forces, between, is whether
the force is holding the molecule together or if the force is attracting or repulsing
molecules in the bulk.
Intermolecular forces have two characteristics. One, they determine bulk properties: boiling
point, melting point, similar bulk physical properties. They are also electrostatic in
nature. As an electrostatic attraction or repulsion, intermolecular forces decrease
as distance increases. It's one of the reasons that in gases, when atoms' molecules are very
far apart, we can say that the attraction forces between them are very low.
There are several different types of intermolecular forces. There are ion-ion interactions. These
can also be considered ionic bonds, or also intramolecular forces. We're not going to
discuss that in detail in this video. There are ion-dipole interactions. These usually
occur in mixtures or solutions. Hydrogen bonding, dipole-dipole interactions, and London dispersion
forces—we're going to focus on these last three: hydrogen bonding, dipole-dipole interactions,
and London dispersion forces.
If we look at hydrogen bonding, hydrogen bonding is the attraction between the lone pair of
an electronegative atom and a hydrogen atom that is bonded to either nitrogen, oxygen,
or fluorine. Let's look at water, H2O. If we think about what's going on in the molecule,
we have an electronegative atom, oxygen, that is pulling electron density toward itself,
giving us two bond dipoles. These two bond dipoles are going to give us a partially negative
and a partially positive charge. Here's where we get our electrostatic attractions. Then
we will set up an interaction between the lone pair of the oxygen and the hydrogen.
Hydrogen bonding interactions are very strong. They're very strong because hydrogen's very
small, meaning that the lone pair of the nitrogen, oxygen, or fluorine is very close in distance.
If we consider our next intermolecular force, dipole-dipole interactions. Dipole-dipole
interactions are electrostatic between permanent dipoles in molecules. If we consider the molecule
hydrogen chloride, again, like in water, we're going to have a dipole moment going towards
the more electronegative atom. This is going to set up a partial negative on my chlorine
and a partial positive on my hydrogen. That will give us an electrostatic attraction between
the chlorine and the hydrogen, not as strong as hydrogen bonding, but still a significant
intermolecular force.
The last intermolecular force, London dispersion forces, or sometimes loosely called Van der
Waals forces, are forces that are very weak. London dispersion forces are weak interactions
caused by temporary fluctuations in the electron distributions within nonpolar molecules that
create instantaneous dipole moments. We have to remember that the electrons are not static
around a molecule. They're flowing in an electron cloud, and as such, it's possible for the
electrons to end up on one side of the molecule or the other. Let's consider bromine. Bromine
is a very electronegative atom. It likes to have a high electron density. So if we consider
two bromine atoms, Br2 and Br2, normally they are nonpolar. But if at any one time we end
up with a little more electron density on one side of the bromine than the other, we
can end up with a partial negative and a partial positive. This partial negative and this partial
positive can actually cause an induced instantaneous dipole in an adjacent bromine. Once we have
this instantaneous dipole and this induced dipole, we can have a London dispersion force
or an intermolecular force between these. Of course, this is instantaneous dipole. So
then as soon as that occurs, it could balance back out and be gone again in another instant.
So London dispersion forces are very weak. They only happen for a brief amount of time.
Let's consider a problem and how we might apply intermolecular forces to physical properties.
Rank the following molecules in order of increasing boiling point (low boiling point to high boiling
point). We have fluoromethane, methanol, and ethane. To figure out which of these has the
highest boiling point, we need to think about which of them has the strongest intermolecular
forces. If we think about kinetic molecular theory, we're going to need to have enough
energy in our system to overcome the intermolecular force. So the substances that have the greatest
intermolecular force will be the ones with the highest boiling point, the ones where
we have to put the most energy. Let's draw these three with their geometric structures.
So here we have fluoromethane. If we think about fluoromethane, it's a tetrahedral. We
have a bond dipole towards the fluorine. We also have very small dipoles towards the carbon.
So if we consider the fluoromethane, we have a dipole between the carbon and the fluorine,
and that's going to set up a molecular dipole towards the fluorine. That means that this
can have a dipole-dipole interaction.
If we look at the methanol, here again we have dipoles going towards the oxygen, but
we also have a hydrogen attached to an oxygen. This is the key that we have the possibility
of hydrogen bonding. So anytime we see a oxygen, nitrogen, or fluorine, we need to be looking
for a hydrogen. Because it has hydrogen and free lone pairs on our oxygen, this can do
hydrogen bonding.
In the last one, we notice here all we have is carbon and hydrogen. Carbon and hydrogen
have a very small difference in electronegativity. These bonds would be considered nonpolar.
I have nothing in here that would be considered a polar bond. Since the entire compound is
nonpolar, the only interaction that we can have here is not ionic-ionic, not dipole-dipole,
or hydrogen bonding, but just London dispersion forces.
Therefore, if we think about these three, which one has the strongest intermolecular
interaction? Well, that's going to be the hydrogen bonding. Well, which one has the
second? That's going to be dipole moment. And finally, the London dispersion forces.
So if we rank these, we'll see that we can have the ethane is going to be the lowest
boiling point, fluoromethane greater than that, and then methanol will have the most.
Now, the problem didn't say to look up the values, but we can look up the values for
these, and we'll see that we get a boiling point of -89 degrees Celsius, -78.2 degrees
Celsius, and 64.7 degrees Celsius. This is pretty common where you'll see hydrogen bonding
having a much higher boiling point or melting point than other substances of similar mass.
Intermolecular forces are very important. It turns out that hydrogen bonding is essential
to DNA and how it works. Dipole moments are also important to recognize so that we'll
know what types of interactions are occurring and what type of reactions can be occurring.
Practice determining intermolecular forces and become confident.