Video transcript
- [Instructor] So we havefour different molecules here. And what I want you to think about, 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, andtry to figure that out. All right, now to figure that out, it really just boils down to which of these has thehighest intermolecular forces when they're in a liquid state? Because if you have highintermolecular forces, it would take a lot of energyor a higher boiling point to really overcome thoseintermolecular forces and get to a gas state. So let's think about theintermolecular forces that we have studied. So I will start with hydrogen bonds, hydrogen bonds. 'Cause you could really view those, those are the strongest of thedipole-dipole interactions, and they're going to be stronger than your London dispersion forces. We can see that diethyl etherwon'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 oneoxygen-hydrogen bond. Water has two oxygen-hydrogen bonds. So if I had to rank thehydrogen bond contribution to the intermolecular forces, I would put water as number one 'cause it can form themost hydrogen bonds. I would put methanol andethanol as a tie for second. And then I would put diethyl ether last 'cause it can't form hydrogen bonds. So just looking at this, I know that water's going tohave the highest boiling point. Diethyl ether is going tohave the lowest boiling point. But what about the differencebetween methanol and ethanol? And we could think aboutother types of dipole forces, but not a lot that you couldintuit just by eyeballing them. They might actually havesimilar dipole moments on a molecular basis. But we can think aboutLondon dispersion forces. I'll do this in a different color. So London dispersion forces. And if we're just trying to, actually I'll rank all of them. So London dispersionforces are proportional to how polarizable a molecule is, which is proportional to howlarge its electron cloud is, which is proportional to its molar mass. And it's clear that diethyl ether has the highest molar mass, followed by ethanol, followed by methanol, followed by water. How did I know that? Well, you literally can take atoms away from the diethyl etherto 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 isthe order of molar mass. And so London dispersion forces, I wouldn't make that change the ranking between water or diethylether because these are going to be a lot weaker thanthose hydrogen bonds. But they can be useful for the tiebreaker between ethanol and methanol. And so my overall rankingon boiling points, the highest boiling pointI would put would be water, followed by, since ethanolwon the tiebreaker, followed by ethanol, followed by methanol, and then the lowest boilingpoint would be diethyl ether. And if we look at the actual data, it's consistent with whatwe 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 thisthing called vapor pressure. And you might have also noticedthat vapor pressure seems to trend the oppositeway as boiling point. The things that havethe high boiling point have the low vapor pressure, and the things that havethe low boiling point have a high vapor pressure. So what are we talking about, why, about vapor pressure, and whydo we see this relationship? And I'm not going to godeep into vapor pressure. There'll be other videoson that on Khan Academy. But just to get you a sense, imagine a closed container here. And I put one of these, a sample of one of thesemolecules in a liquid state, and I'm gonna just draw the molecules, clearly not drawn to scale,as these little circles. And the temperature matters, so let's say that thisis 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'regoing to be in a liquid state, but we know that not every oneof these molecules is moving with the exact same kinetic energy. The temperature, youcould view as a measure of the average kineticenergy of the molecules, but they're all bumpingaround into each other, in different positions, withdifferent 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 positionand 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 thatare in the gaseous state, every now and then they'rebumping into each other, and they're bumping intothe sides of the container. And every now and then, they might approach the surface with the right kinetic energy,with the right position, so that they get recapturedby 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 vaporpressure gets high enough, remember, that pressure's just from the vapor molecules bouncing around, then you will get to someform of an equilibrium. And you could imagine, the things that havea lower boiling point, that means they have lowerintermolecular forces, more of the vapor is going to form, and so you're going to havea higher vapor pressure before you get to equilibrium. On the other hand, things withhigh intermolecular forces, fewer of those moleculesare going to break away, and so you're going tohave a lower vapor pressure when you get to that equilibrium. And 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 littlebit about vapor pressure and how that relates tointermolecular forces and boiling point.
