Hi. It's Mr. Andersen and this chemistry essentials video 18 on intermolecular
forces, which are simply the forces between molecules. And so when we were putting drops
of water on this penny, we were able to get 43 drops on there because there are high intermolecular
forces between the water molecules. And so intermolecular forces are important because
they give us the properties of solids and liquids and gases. And so some of the properties
would be the boiling or melting point. Surface tension. Capillary action. Vapor pressure.
And miscibility. And we'll talk about each of these. And in gases there's not too many
intermolecular forces because they're moving around really really quickly. But when there
are intermolecular forces they explain the difference between an ideal gas, which can
be explained using the ideal gas law and real gases. And this usually occurs towards
condensation. We can use a PV relationship to find it. And then finally intermolecular
forces are incredibly important in biology. In both enzymes and proteins. These forces
between molecules are incredibly important. So let's make sure you know the difference
between inter and intra molecular. Intermolecular are going to be the forces between molecules.
So between this one and this one. Intramolecular are going to be within the molecule itself.
And so we'll get to those when we start talking about covalent, ionic and metallic bonding.
But intermolecular forces are simply the forces between these molecules. And so let's take
a look at them. Let's say we vary the strength. Let's say we make those forces weaker. What
happens? They'll spread apart. What happens if we make them greater? They're going to
come together. What happens if we make them even greater? They're going to come together
and form real nice structures. So something like a solid. And so what are some properties
that are affected by intermolecular forces? Well let's start with the melting point or
the boiling point. And so melting point is when you go from a solid to a liquid. And
so if there are greater forces, it's hard to get liquids to start to form. And boiling
points is going to be these molecules actually jumping off into a gas. And so the greater
those intermolecular forces are, the harder it is for it to move off. Surface tension
is going to be like a skin over a liquid. And so imagine down here that we're looking
down at three drops of three different liquids. And those liquids are mercury, alcohol and
water. Could you match them appropriately? Well mercury is going to have the greatest
intermolecular force. And so those are going to be right here. And water is going to be
next. And so it's going to form this nice bubble due to hydrogen bonding. And then alcohol
is going to spread out. And so we can look at intermolecular forces. It's going to tell
us how much of that skin on the surface is going to form. Capillary action is going to
be movement of a liquid up a tube. And so if we're looking at water, not only is water
attracted to itself, we call that cohesion. But it's also going to be attracted to the
tube that it's in. We call that adhesion. And the thinner we make that tube, the more
the water is going to move up. And that's how water gets up a tree. There are really
tiny tubes called xylem that allow water to move up it. But if we were to look at something
like mercury, it's attracted to itself, but it's not attracted to its surroundings. And
so it's actually going to move down in a capillary tube. Next is vapor pressure. And vapor pressure
is measured in a closed system. So if we put a liquid in a closed system, it's going to
be the amount of pressure exerted by the gases. And so how are we going to get more gases
there? They're going to jump from liquid to a gas stage. And so the higher that vapor
pressure is, the more volatile that liquid is going to be. So alcohol for example is
going to have high volatility. And that's because a lot of it is jumping out of this
liquid phase into the gas phase because there a low intermolecular forces. Miscibility you
can think of that as like mixability. How easy is it for two different materials to
mix together? Well you know that water and gasoline don't mix well. And the reason why
is that water is going to be polar and this gasoline is not. But water does mix readily
with hydrochloric acid. And that's because the hydrochloric acid molecule is going to
be polar. And so these like charges are able to dissolve like charges and vice versa. Also
this explains why water is really really good at breaking down ionic compounds. And so this
is salt. And salt is really high bonds in an ionic compound. But if you add water to
it it readily breaks apart. And the reason why is those chloride ions or the chlorine
ions are going to have a negative charge. And the positive parts of the water are able
to surround it. And if I had a sodium ion here, which is positive, then negative parts
of the water are going to surround it. And that allows it to breakdown. When we get to
gases remember we have ideal gases which can be explained using the ideal gas law where
PV=nRT. And then we have real gases. Now an ideal gas is an imaginary model. And it assumes
that there's no attraction between these molecules. That there's an infinite amount of space for
these molecules to move around. But as we start to cool down those gases towards a liquid
phase we start to get attraction, intermolecular attraction between these molecules. And also
we have a finite kind of a space. And so this law starts to fall apart. So if we were actually
to graph it, PV over RT, for an ideal gas, if we graph it will always be one. But if
we were to vary the temperature for a real gas, let's try nitrogen, at 1000 kelvin it
would look like that. 500 it would look like that. And 200 it would look like that. Now
the farther you are off that blue line, the farther you are off of an ideal gas. And so
you can see that as we cool nitrogen down, as we get it closer towards that condensation
point, that model of an ideal gas is falling apart. We could look at different gases at
313 K as well. Let's look at hydrogen versus nitrogen. You can see hydrogen is going to
be more like an ideal gas. And that has to do, that it's a smaller mass and it's going
to move more readily. It's also incredibly important in these intermolecular forces in
biological systems. And so this is an enzyme. An enzyme is basically a catalyst that breaks
down a molecule. In this case this could be sucrase, or excuse me, sucrose which is sugar.
And it's broken down by sucrase. These intermolecular forces are incredibly important. It almost
has to fit in like a key inside a lock. And enzymes and almost everything in life that's
important are made of proteins. And there are a huge intermolecular forces within this
giant macromolecule that hold it together and give it its characteristic space. And
so did you learn the following two things. To explain observations regarding solubility
of ionic compounds? Remember something like water, since it's polar can break apart those
ions and kind of usher them away from the ionic compound. And then finally can you explain
terms like vapor pressure, viscosity in terms of strength and type of intermolecular forces?
We went through this list and it's all based on those attractions between the molecules.
And I hope that was helpful.