Vapour Pressure and Boiling Point

Okay, this is a topic that I'm REALLY unsure about. So feel free to help me on this one.

First we're going to talk about a process which I'm sure you're all familiar with: evaporation. In evaporation, liquid turns into a gas below its boiling point, but it's a pretty slow process. How does it work?

Well, you see, not every particle in a liquid has exactly the same kinetic energy. The kinetic energy of all of these particles roughly fits a normal distribution. Most particles have a kinetic energy close to the mean, but there will be some with lower kinetic energy and some with higher kinetic energy. Generally the liquid particles with higher kinetic energy sit closer to the top of the liquid while those with a lower kinetic energy sit nearer to the bottom. Those particles with a high enough kinetic energy will be able to break free of the attractive forces between particles in the liquid and escape into the atmosphere. Since it's the particles with the highest kinetic energy that leave the liquid, the average kinetic energy drops. This is why the temperature of a liquid decreases as the liquid evaporates.

Condensation is the opposite process. Not every particle in a gas has the same kinetic energy either, you see. Those particles with lower kinetic energy will be more likely to condense back into a liquid, especially if they collide with particles of a liquid.

In a sealed container containing some liquid, at first the liquid will start to evaporate and exert some pressure on the walls of the container. As the liquid evaporates and more and more liquid particles become gaseous, there is a higher likelihood of the gas particles colliding with the liquid and becoming liquid particles again (i.e. condensing). So you have two processes here: evaporation and condensation. Eventually they'll reach a point where the amount of liquid evaporating equals the amount of gas condensing, and the system is said to be in a state of dynamic equilibrium. The pressure that the evaporating particles exert at this point is said to be the vapour pressure of the liquid... I think. No, wait, I just looked at my textbook and it says that the vapour pressure is the pressure between the gas and the liquid in the closed container. Are they more or less the same thing? I don't know. Bleh. I hate blogging about something I don't know, but I guess now I know what I don't know!

Here's another term from my textbook: volatile liquid. Liquids with high vapour pressure are considered to be volatile and evaporate rapidly if not in a sealed container. I think that this is because if the vapour pressure is high it means that there are lots of particles evaporating and condensing at the same time (according to my mind, anyway). For lots of particles to be evaporating in the first place, the intermolecular bonds would have to be relatively weak and easy to break.

An increase in temperature also increases vapour pressure because an increase in temperature means that more particles would have enough energy to break free of the liquid and evaporate.

Boiling is a different process to evaporating. While evaporating only takes place at the surface of the liquid and can take place well below a substance's boiling point, boiling only takes place at or near the boiling point and occurs throughout the whole liquid. Boiling occurs when the vapour pressure is equal to the atmospheric pressure.

You know those bubbles that appear when something's boiling? Well, they're made out of gas. The reason they stay all bubbly is because the gas pushes outwards while the liquid pushes inwards. I think that the reason why the liquid pushes inwards is because of the atmospheric pressure pushing down on the surface of the liquid. (Y'know, eventually, push is going to come to shove and I'm going to shove all these vapour pressure bits off my test and into oblivion... never mind.)

So what are some factors that affect boiling point? Well, different substances have different boiling points, due to the differing strengths of intermolecular bonds. Ethanol, for example, has a lower boiling point than water because its intermolecular bonds are weaker. Also, changing the atmospheric pressure also changes the vapour pressure required for water to boil. Boiling water at higher altitudes, where the atmospheric pressure is lower, requires less heat than boiling water at sea level.

I think that's just about it. If I've got something wrong, or if there's something else I need to add, please tell me. It's not a topic I'm confident with as I said at the start of this post.