Hi. It's Mr. Andersen and this is chemistry essentials video 54 which is on intermolecular
potential energy. This is a famous macromolecule right here. It's DNA. And every bond that
you see is a covalent bond where you're sharing electrons between atoms. But it's a double
helix. And what that really means is it's a double molecule. There are two molecules.
And in the middle they're held together with intermolecular forces. Those are going to
be hydrogen bonds or dipole-dipole forces which are going to hold those two chains together.
And so really it's easy to pull DNA apart and for it to zip right back together again.
And so we really want to make sure you understand the difference between an intermolecular and
intramolecular. An intermolecular is going to be between adjacent molecules. Between
the yellow and the yellow. And so that would be for example between water and another water
molecule. What are the intramolecular? Those are going to be the bonds. Those are going
to be those covalent or ionic bonds that are actually between the atoms within the molecule
itself. And so we're not dealing with bonds in this video. We're dealing with those forces
between molecules. And those forces are based on what those molecules are. We can have an
attraction between a dipole and a dipole. We can have an attraction between a dipole
and an induced dipole. And then also between induced dipoles. And between each of these
there's going to be potential energy in those intermolecular forces. It's going to take
energy to pull those apart. And so an example of a dipole-dipole would be a hydrogen bond.
And so what we're doing there is we're taking hydrogen atoms of one molecule and they're
being attracted to oxygen, nitrogen and fluorine atoms of another molecule. And those are both
dipoles. And so what we're getting is an attraction between those two. Those aren't of the same
molecule. They're just holding those two molecules together. If we look at between a dipole and
an induced dipole that could be for example between argon which normally doesn't have
a charge but can get a charge if we hold something like hydrochloric acid right next to it. And
then finally we could have two induced dipoles. And an example of the forces between those
are going to be dispersion forces. What are induced dipoles? They're everything. So there's
a force between everything and those are called dispersion forces. And they're based or the
strength of them is based on the number of electrons that you have. The more electrons,
the more dispersion forces we could have. And also the surface area and contact between
those two molecules. So again, in review, what is a dipole? It's simply a molecule that
has uneven distribution of electrons. And so if we're to look at water as an example,
the oxygen loves those electrons. It's highly electronegative so it's pulling it towards
it. And so there's going to be a slightly negative charge on this side. And a positive
charge on the other side of the water molecule. It's a dipole. If you look at hydrochloric
acid, the same thing here. The electrons are going to be pulled toward the chlorine atoms.
And so it's going to have a negative charge right here and a positive charge on the other
side. Now these two negatives are going to push on each other and so what would a dipole-dipole
bond look like? It's going to be looking like this. Or rather a force is going to look like
that. And it's going to hold those two molecules together. But again it's not a bond between
the two. Hydrogen bond is a great example of a dipole-dipole force. If we were to take
water molecules like this, you can see that all of their oxygens are going to have negative
charges. And so that's not how it's going to line up. In a hydrogen bond the way it
would line up would be like this. And so we now have an attraction between this hydrogen
and this oxygen. This would be a hydrogen bond right here. There would be another hydrogen
bond right here. Another hydrogen bond right here. Another hydrogen bond right here. Now
a point of confusion remember is that it's not in the same molecule. It's going to be
between a hydrogen of one molecule and then either nitrogen, oxygen or fluorine of another
molecule. Now this is a huge amount of energy that's held in this. And let me give you an
example of that. What we're looking at now is the boiling point of four different molecules.
And so we'll start right here with hydrogen telluride. That's H2Te. And if we look at
its molecular mass, it's over here. And then its temperature of boiling point is going
to be a little less than zero degrees. And so if I go to the next one up in that group,
we've got hydrogen selenide. And so we can see here that we have decrease in the molecular
mass. And we also have a decrease in the boiling point. If we go to the next one, hydrogen
sulfide, you can see that there's a really well established pattern right here. As we
move to smaller atoms we're getting less molecular mass. And we're also getting a lower boiling
point. So what's going to be the next one if we go through this pattern? It's going
to be H20. So that's going to be water. Could you predict where that's going to be? Well
you might think it's going to be something like this. But it actually is way up here.
Why is that? Because there's all of these hydrogen bonds between the water molecules.
And so it really takes a lot of energy to break those apart and to boil that water.
Thankfully that's why we're filled with water. So our temperature doesn't change that much.
So what's a dipole-induced dipole force? Well if we have hydrochloric acid and argon mixed
together what we'll find is that this argon doesn't have a charge. But if we pull it close
to a dipole what it will do is it will actually pull those electrons towards it. And so what
you get is an induced dipole. And so this is a force that's holding them together. Since
there are more electrons on this side it has a partial negative charge right here. Now
how are we going to measure the strength of that bond? It's based on two things. It's
based on the strength of that dipole and it's also based on the polarizability, in other
words the number of electrons and how those electrons can be pulled of that induced dipole.
And sometimes we'll have molecules that can both form induced dipoles. So right here we
have helium gas. As it moves close to another helium atom you can see that there's an attraction
between the electron and the protons of the adjacent atom. And there's also going to be
repulsion between these electrons. And so what we're creating is a temporary dipole
for just a fraction in time. But it's still going to hold those things together. And so
all molecules have these induced dipole to induced dipole forces. All molecules have
this attraction between them. What's going to increase that attraction? It's simply going
to be the number of electrons that we have and the surface area. So this is a picture
of a gecko that's climbing up glass. And if you look at their foot we're going to find
that there are folds. And then if you zoom in on those folds there's folds on the folds
on the folds. And so how are they magically able to walk up a wall like this? It's those
attractions. Those dispersion forces between them and the wall. And so as they increase
the surface area, increase the number electrons, and so we can have enough of an attraction
to actually hold them up. And so did you learn to make claims or predictions regarding these
intermolecular forces? Again it's based on what's holding them together. Is it dipole-dipole,
dipole-induced dipole or just induced dipoles. These are all intermolecular forces. And I
hope that was helpful.