How does intermolecular forces affect evaporation

What are intermolecular forces? Why do biological systems need enzymes? Why do biological systems need water? How do intermolecular forces affect vapor pressure? How do intermolecular forces affect viscosity?

How do intermolecular forces affect solubility? How do intermolecular forces affect capillary action? How do typical dipole-dipole forces differ from hydrogen bonding interactions? But they can be useful for the tiebreaker between ethanol and methanol. And so my overall ranking on boiling points, 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.

And 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.

And you might have also noticed that vapor pressure seems to trend the opposite way as 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? And I'm not going to go deep into vapor pressure.

There'll 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, a sample of one of these molecules 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 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, with 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's just from the vapor molecules bouncing around, then you will get to some form of an equilibrium. And 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. The forward direction represents the evaporation process, while the reverse direction represents the condensation process. Because they cannot escape the container, the vapor molecules above the surface of the liquid exert a pressure on the walls of the container.

The vapor pressure is a measure of the presure force per unit area exerted by a gas above a liquid in a sealed container. Vapor pressure is a property of a liquid based on the strength of its intermolecular forces. A liquid with weak intermolecular forces evaporates more easily and has a high vapor pressure. A liquid with stronger intermolecular forces does not evaporate easily and thus has a lower vapor pressure. For example, diethyl ether is a nonpolar liquid with weak dispersion forces.

Water is a polar liquid whose molecules are attracted to one another by relatively strong hydrogen bonding. Vapor pressure is dependent upon temperature.

When the liquid in a closed container is heated, more molecules escape the liquid phase and evaporate. The greater number of vapor molecules strike the container walls more frequently, resulting in an increase in pressure.

The Table below shows the temperature dependence of the vapor pressure of three liquids. Notice that the temperature dependence of the vapor pressure is not linear. Use the link below to answer the following questions:.