Intermolecular forces and vapor pressure | Intermolecular forces | AP Chemistry | Khan Academy

So we have four different molecules here, and what I want you to think about is if you had a pure sample of each, which of those pure samples would have the highest boiling point, second highest, third highest, and fourth highest? Pause this video and try to figure that out.

All right, now to figure that out, it really just boils down to which of these has the highest intermolecular forces when they're in a liquid state. Because if you have high intermolecular forces, it would take a lot of energy or a higher boiling point to really overcome those intermolecular forces and get to a gas state. So let's think about the intermolecular forces that we have studied.

I will start with hydrogen bonds. Hydrogen bonds, because you could really view those, are the strongest of the dipole-dipole interactions, and they're going to be stronger than your London dispersion forces. We can see that diethyl ether won't form hydrogen bonds; we don't see any bonds between hydrogen and an oxygen, a nitrogen, or a fluorine. Ethanol has one oxygen-hydrogen bond, methanol also has one oxygen-hydrogen bond, and water has two oxygen-hydrogen bonds.

So if I had to rank the hydrogen bond contribution to the intermolecular forces, I would put water as number one because it can form the most hydrogen bonds. I would put methanol and ethanol as a tie for second, and then I would put diethyl ether last because it can't form hydrogen bonds. So just looking at this, I know that water is going to have the highest boiling point, and diethyl ether is going to have the lowest boiling point.

But what about the difference between methanol and ethanol? We could think about other types of dipole forces, but not a lot that you could intuit just by eyeballing them. They might actually have similar dipole moments on a molecular basis, but we can think about London dispersion forces. I'll do this in a different color: London dispersion forces, and if we're just trying to actually, I'll rank all of them.

So London dispersion forces are proportional to how polarizable a molecule is, which is proportional to how large its electron cloud is, which is proportional to its molar mass. It's clear that diethyl ether has the highest molar mass, followed by ethanol, followed by methanol, and followed by water. How did I know that? Well, you literally can take atoms away from the diethyl ether to get to an ethanol, and you can literally take atoms away from that to get to a methanol, and you can literally take atoms away from that to get to a water.

So we know that this is the order of molar mass, and so London dispersion forces, I wouldn't make that change the ranking between water or diethyl ether because these are going to be a lot weaker than those hydrogen bonds. But they can be useful for the tiebreaker between ethanol and methanol.

So my overall ranking on boiling point, the highest boiling point I would put would be water, followed by, since ethanol won the tiebreaker, followed by ethanol, followed by methanol, and then the lowest boiling point would be diethyl ether. If we look at the actual data, it's consistent with what we just talked about. We can see very clearly that water has the highest boiling point, ethanol is second, methanol is third, and diethyl ether was fourth. Completely consistent with our intuition.

Now what's also interesting here, you might have noticed, is this thing called vapor pressure. You might have also noticed that vapor pressure seems to trend the opposite way of boiling point. The things that have the high boiling point have the low vapor pressure, and the things that have the low boiling point have a high vapor pressure.

So what are we talking about? Why about vapor pressure, and why do we see this relationship? I'm not going to go deep into vapor pressure; there will be other videos on that on Khan Academy. But just to get you a sense, imagine a closed container here, and I put one of these samples of one of these molecules in a liquid state. I'm going to draw the molecules clearly, not drawn to scale, as these little circles, and the temperature matters.

So let's say that this is at 20 degrees Celsius. Now you might notice at 20 degrees Celsius it's lower than the boiling point of all of these characters, so for the most part, they're going to be in a liquid state. But we know that not every one of these molecules is moving with the exact same kinetic energy. The temperature, you could view as a measure of the average kinetic energy of the molecules, but they're all bumping around into each other in different positions with different amounts of velocities and therefore different kinetic energies.

And so every now and then, you're going to have a molecule that has the right position and the right kinetic energy to escape and get into the vapor state, into a gaseous state. And so that's going to keep happening. But then the things that are in the gaseous state, every now and then, they're bumping into each other and they're bumping into the sides of the container, and every now and then they might approach the surface with the right kinetic energy and the right position so that they get recaptured by the intermolecular forces and enter a liquid state.

And so you can imagine this will keep happening where things go from liquid and then they go to vapor, but then when that vapor gets high enough, or when you could say the vapor pressure gets high enough — remember that pressure is just from the vapor molecules bouncing around — then you will get to some form of an equilibrium.

You could imagine the things that have a lower boiling point, that means they have lower intermolecular forces; more of the vapor is going to form, and so you're going to have a higher vapor pressure before you get to equilibrium. On the other hand, things with high intermolecular forces, fewer of those molecules are going to break away, and so you're going to have a lower vapor pressure when you get to that equilibrium.

You can see that very clearly here. So I will leave you there. We got a little bit of practice seeing everything we've seen so far, and we learned a little bit about vapor pressure and how that relates to intermolecular forces and boiling point.