Why is vapor pressure lowering a colligative property? Why does vapor pressure increase with temperature? Why is vapor pressure independent of volume?
What is the boiling point of milk? Why does vapor pressure decrease when a solute is added? It doesn't have to be. So there's some air and the air has some kinetic energy over here. So, of course, do the water molecules.
And some of them start to evaporate. So some of the water molecules that are up here in the distribution, they have enough energy to escape, so they start hanging out with the air molecules, right? Now something interesting happens. This is the distribution of the molecules in the liquid state. Well, there's also a distribution of the kinetic energies of the molecules in the gaseous state. Just like different things are bumping into each other and gaining and losing kinetic energy down here, the same thing is happening up here.
So maybe this guy has a lot of kinetic energy, but he bumps into stuff and he loses it. And then he'll come back down. So there's some set of molecules. I'll do it in another set of blue. These are still the water-- or whatever the fluid we're talking about-- that come back from the vapor state back into the liquid state.
And so what happens is, there's always a bit of evaporation and there's always a bit of condensation because you always have this distribution of kinetic energies. At any given moment in time, out of the vapor above the liquid, some of the vapor loses its kinetic energy and then it goes back into the liquid state.
Some of the surface liquid gains kinetic energy by random bumps and whatever else and goes into the vapor state. And the vapor state will continue to happen until you get to some type of equilibrium. And when you get that equilibrium, we're at some pressure up here. So let me see, some pressure.
And the pressure is caused by these vapor particles over here, and that pressure is called the vapor pressure. I want to make sure you understand this. So the vapor pressure is the pressure created, and this is at a given temperature for a given molecule, right? Every molecule or every type of substance will have a different vapor pressure at different temperatures, and obviously every different type of substance will also have different vapor pressures. For a given temperature and a given molecule, it's the pressure at which you have a pressure created by the vapor molecules where you have an equilibrium.
Where you have just as many things vaporizing as things going back into the liquid state. And we learned before that the more pressure you have, the harder it is to vaporize even more, right? We learned in the phase state things that if you are at degrees at ultra-high pressure, and you were dealing with water, you would still be in the liquid state. So the vapor creates some pressure and it'll keep happening, depending on how badly this liquid wants to evaporate.
But it keeps vaporizing until the point that you have just as much-- I guess you could kind of view it as density up here, but I don't want to think-- you have just as many molecules here converting into this state as molecules here converting into this state. So just to get an intuition of what vapor pressure is or how it goes with different molecules, molecules that really want to evaporate-- and so why would a molecule want to evaporate? It could have high kinetic energy, so this would be at a high temperature.
It could have low intermolecular forces, right? It could be molecular. Obviously, the noble gases have very low molecular forces, but in general, most hydrocarbons or gasoline or methane or all of these things, they really want to evaporate because they have much lower intermolecular forces than, say, water.
Or they could just be light molecules. You could look at the physics lectures, but kinetic energy it's a function of mass and velocity. So you could have a pretty respectable kinetic energy because you have a high mass and a low velocity. So if you have a light mass and the same kinetic energy, you're more likely to have a higher velocity. You could watch the kinetic energy videos for that. But something that wants to evaporate, a lot of its molecules-- let me do it in a different color.
Something that wants to evaporate really bad, a lot more of its molecules will have to enter into this vapor state in order for the equilibrium to be reached.
Let me do it all in the same color. So the pressure created by its evaporated molecules is going to be higher for it to get to that equilibrium state, so it has high vapor pressure.
And on the other side, if you're at a low temperature or you have strong intermolecular forces or you have a heavy molecule, then you're going to have a low vapor pressure.
For example, iron has a very low vapor pressure because it's not vaporizing while-- let me think of something. Carbon dioxide has a relatively much higher vapor pressure. Much more of carbon dioxide is going to evaporate when you have it. Well, I really shouldn't use that because you're going straight from the liquid to the solid state, but I think you get the idea.
And something that has a high vapor pressure, that wants to evaporate really bad, we say it has a high volatility. To avoid this, divers rise from deep depths slowly so their bodies can expel the extraneous gas through the lungs and allow the amounts of gas in the blood to equilibrate with the external pressure.
This way the divers can avoid gas bubbles forming in blood vessels and therefore any personal injury. The vapor pressure of water at K is 9. Introduction When a liquid is in a confined, closed, container, an equilibrium exists between the liquid and its gaseous phase.
Some of these individual molecules have enough energy to break from the liquid and enter the gaseous phase. Boiling a elevated altitudes The fact that the vapor pressure is equal to the external pressure can become important when talking about boiling temperatures at various altitudes.
Characteristics of Vapor Pressure Vapor pressures are dependent only on temperature and nothing else. Solution Step 1: Once again this can be solved using the Clausius-Clapeyron equation. Vapor Pressure of Solutions: Raoult's Law While the Clausius-Clapeyron equation is useful for describing the vapor pressure behavior of a pure substance, it does not quite help us when we need to describe the vapor pressure of a solution comprised of two ore more different liquids with different vapor pressures, that is where Raoult's Law comes in.
Solution Step 1: Calculate the mole fractions moles of each substance divided by total moles of each substance in the solution. Solution Step 1: Since this solution is only comprised of water and ethylene glycol, we can easily calculate the mole fraction of ethylene glycol in this solution by subtracting water's mole fraction from 1.
As the concentration of the nonvolatile solute increases, the partial pressure of the solvent decreases. Problems The vapor pressure of water at K is 9. At K the vapor pressure of water is mmHg; what is the vapor pressure of water at K? A solution's partial pressure is This solution is comprised of Chemical A and Chemical B. The partial pressure of Chemical A is What is the partial pressure of Chemical B? Chemical A's partial pressure is Their respective mole fractions are 0.
What is the the partial pressure of a homogenous solution of Chemical A and Chemical B? Assume at K and an O 2 pressure of 4. Formaldehyde, with weaker IMF, should have a higher vapor pressure. Citronellal and citronellol are two related compounds found in citronella oils and used in perfumes and insect repellants.
Which compound would be expected to be more volatile? Citronellal would be more volatile due to the absence of hydrogen bonding. According to the kinetic molecular theory, the temperature reflects the average kinetic energy of the particles in a liquid. If you increase the temperature, you are increasing the average energy of the particles present. That means that more of them are likely to have enough energy to escape from the surface of the liquid.
That will tend to increase the vapor pressure. The increase in vapor pressure with temperature is not linear, however. As the temperature rises, the vapor pressure of water rises exponentially. In the next chapter, we will see how vapor pressure values can be predicted based on this relationship with temperature. The pressure scale the vertical one is measured in kilopascals kPa.
The vapor pressure of a liquid increases nonlinearly with temperature. In fact, this is always true at the normal boiling point of a liquid. A liquid boils when its equilibrium vapor pressure becomes equal to the external pressure on the liquid. When that happens, it enables bubbles of vapor to form throughout the liquid - those are the bubbles you see when a liquid boils.
If the external pressure is higher than the saturated vapor pressure, these bubbles are prevented from forming, and you just get evaporation at the surface of the liquid. If the liquid is in an open container and exposed to normal atmospheric pressure, the liquid boils when its saturated vapor pressure becomes equal to 1 atmosphere or Pa or At different external pressures, however, water will boil at different temperatures.
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