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Chemistry: Boiling Pt Elev, Freezing Pt Depression


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  • Type: Video Tutorial
  • Length: 9:50
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 106 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Final Exam Test Prep and Review (49 lessons, $64.35)
Chemistry: Physical Properties of Solutions (14 lessons, $22.77)
Chemistry: Colligative Properties (5 lessons, $9.90)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/chemistry. The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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Raoult's Law tells us that if we add a solute to a solvent that the vapor pressure of that solvent will decrease and it depends on how much solute you add. The more solute you add, the lower the vapor pressure goes. And what we've got to ask ourselves at this point is, who the heck cares? There's got to be some reason for us to talk about this because I mean we're not going to in general care about very small changes in vapor pressure. But what we're going to see is that small changes in vapor pressure have very profound consequences with things as common as boiling points and freezing points. So we're going to make that connection now.
First of all, let's review what Raoult's Law is telling us. Mathematically, it says again that as we increase concentration of solute, no matter what the solute is, remember this is a colligative property, that just depending upon how much we have dissolved in the liquid, that that will lower the vapor pressure of the solvent, not of the solute now, of the solvent. So, we increase amount of solute, we see a decrease, a steady decrease in the vapor pressure. Now what does that have to do with something like boiling for instance?
Well, let's take a look. Now this is a diagram we've seen before, but it can be a little intimidating so let's walk through this. First of all, I want you to ignore the pink line for a moment. Just look at these other three lines. This is a phase diagram and let's suppose that it was the phase diagram for water although, in fact, it isn't. In water, we would have this leaning this way. But this in general a generic phase diagram. And so this is the liquid gas interface here in green, for the pure liquid. This is the solid and liquid interface and this is the solid gas interface in red here. Remember, this is the triple point, the point where all three phases coexist. Now, remember that the normal boiling point, the point at which we get boiling at one atmosphere is the point at which the vapor pressure curve intersections with one atmosphere. At that point, we get boiling, as long as we're at one atmosphere, and this is the temperature at which that occurs. Now, look what happens when we lower the vapor pressure by adding a solute to the liquid. The pink line again shows the lower vapor pressure at a given temperature, and look what happens. This now intersects the one atmosphere line at a higher temperature. That insists that the boiling point, the normal boiling point is now higher than it was for the pure solvent. This is what we refer to as boiling point elevation. Another colligative property, a property that does not depend on the chemical nature of the solute, but it does depend on the chemical nature of the solvent. It depends what we're talking about boiling. But not about what we put in it, only about how much we put in it. The more we dissolve, the lower this drops, the further we push the boiling point out, the higher the boiling point becomes. So an example of that that you're all familiar with is antifreeze. Well, this also could be called anti-inflammatory-boil. It has the exact same effect in the engine. We dissolve some solute, the antifreeze in this case, into boiling water and we're going to change the boiling point of that solution. We're going to lower it. So let's do that right now. In fact, let me read the temperature. On this thermometer, it's about 98 degrees right now. And so we know that that would be 100 if we were at exactly one atmosphere. And so, we'll go ahead and pour in a little bit of this and you'll notice the boiling stopped and I can look at the temperature. Now, initially, my temperature drops a little bit because I'm adding something that is going to absorb some of the heat, but when this thing comes back to boiling again, we will look at the temperature and we'll see that in fact the boiling point has, in fact, increased. So, we'll let this thing increase while we keep talking here and we'll come back to it. But, again, the notion of antifreeze for an engine is simply, again, an example of using or exploiting a colligative property. What we're doing is taking a solute, putting it in water in the radiator and that increases the boiling point of the water. We can do the same thing with any number of different substances. Anything, as long as it dissolves in that water, it's going to change the vapor pressure of water. And, in fact, a number of different types of substances have been used in the past in engines in this way, by raising the boiling point. Now, I mentioned that this also has to do with freezing. After all, this is called "antifreeze." We're getting back to our boiling point again and now our temperature is just about 105 degrees, so we've raised our temperature a fair bit in this process by adding again a little bit of a solute.
Now, again, about freezing. Same idea here. If we dissolve something in solution, what we're going to do, just like with boiling, is we're going to stabilize the solution. Remember back to Raoult's Law, due to entropy, we stabilize the liquid phase; that makes it a little more stable than the vapor phase that caused us to have to work a little harder to get it to boil. The same thing is going to be true about freezing. It's going to be a little bit more difficult for us to freeze it. We're going to have to go to a little lower temperature. Let me show you how that works.
Once again, same diagram as before, our phase diagram here. The pink line, once again, indicates the ideal solution whereas the green line indicates our pure solvent. When the vapor pressure drops, we said that the boiling point increases, and also notice what happens to the triple point, down here. Notice that the triple point actually decreases in temperature. Again, simply because we're lowering this curve. As the triple point drops down, that ends up causing this solid liquid interface also to shift over and as a result the point at which we get freezing, at one atmosphere, has shifted to a lower temperature now. Bottom line is this, as we make the solution more stable by adding our solute, we make it more difficult to boil and more difficult to freeze. In other words, we have to remove more of the thermal energy before we can finally get it to freeze. Once again, because we are making the solution more stable by increasing its entropy or the entropy of the system at least. So we describe that in terms of a mathematical relationship for both boiling and freezing. They look almost identical. Delta T refers to the change in boiling temperature in this case, T, the change in the boiling point, that means the difference between the boiling point of the solution and the boiling point of the pure liquid that that is equal to a constant. This is called the boiling point elevation constant times concentration. Now, just to cause you grief, we're using concentration units of molal in this equation, not molar. So, unfortunately, this is not moles per liter, this is moles per kilogram of solvent. But, again, the important thing to see in this is that the change of boiling point is directly proportional to the amount. Again, that tells you it's a colligative property. Likewise, the freezing point depression is a constant times, again, molal. The higher the concentration, the lower the freezing point drops. Notice the minus signs. So our freezing point is decreasing whereas our boiling point is increasing.
Now, let's go to a table that describes various different boiling points and freezing point constants as a function of solvent. Now, remember a colligative property is independent of solute. But, it's going to depend on the chemical nature of the solvent. Different solvents boil at different points and as you might expect, their constants describing how much their boiling points change are also going to be different depending upon what the solvent is. So, for instance, water down here at the bottom, we have a boiling point elevation constant of 0.51. Notice that that's only about half of the size of ethanol, for instance, which has a much more pronounced effect. In other words, we're going to see a more sensitive change of boiling point for ethanol than we did of water. Our boiling point constant, again, that describes that relationship between concentration and change in temperature. Likewise, we can talk about the freezing point changers that we're going to see for such a system for a range of different solvents in this case. And listed in red here are just the different boiling points associated with these different solvents. So, again, it's these numbers that we're going to look up that we're going to use to make this connection between how much stuff we have in solution and what happens to the boiling point. So, in a moment we will go ahead and do an example of a problem where we make the connection between boiling point or freezing point change and concentration. And, in fact, what we're going to do is use that information to determine a molecular weight of a substance.
Physical Properties of Solutions
Colligative Properties
Boiling Point Elevation and Freezing Point Depression Page [1 of 2]

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