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Course: Chemistry library > Unit 11
Lesson 3: Mixtures and solutionsBoiling point elevation and freezing point depression
Boiling point elevation is the raising of a solvent's boiling point due to the addition of a solute. Similarly, freezing point depression is the lowering of a solvent's freezing point due to the addition of a solute. In fact, as the boiling point of a solvent increases, its freezing point decreases. An example of this would be the addition of salt to an icy sidewalk. The solute (salt) reduces the freezing point of the ice, which allows the ice to melt at a lower temperature.. Created by Sal Khan.
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- Hello, I looked this up because I'm getting ready for a test that will most likely have this kind of stuff on it... It's a great video but I have one question.
My current book says that the formula for freezing point depression is ΔT=(-i)(Kf)(m), but I don't see where the (i) comes in for this video. My book isn't clear on the definition of (i) and I can't really find anything else on it.
I came to this video to figure out how to calculate (i), but how do you do without it?
Jaydo.(31 votes)- The value "i" is called the van't Hoff factor (named after the first person to win the Nobel Prize in Chemistry). It is a little weird at first, so I will show you some examples and then explain in a moment:
For sodium chloride, NaCl, i = 2
For magnesium chloride, MgCl2, i = 3
For glucose, C6H12O6, i = 1
For sodium sulfate, Na2SO4, i = 3
Essentially, the van't Hoff factor tells you how many "units" the compound in question will separate into when dissolved in water.
Now, for table salt, sodium chloride, it dissociates into a sodium cation and a chloride anion. The van't Hoff factor is two because there are two species.
For magnesium chloride, there are three species: one magnesium cation and two chloride anions. Therefore, i = 3.
For glucose, i = 1 because glucose is not an ionic compound like the others. When it dissolves in water, the covalent bonds holding a glucose molecule together are not broken. Sure, it will dissolve, but it just dissolves into glucose sub-units.
For sodium sulfate, i = 3 because it would break down into two sodium cations and one sulfate anion. Notice that the sulfate anion does not break up any further. The sulfur-oxygen bonds are covalent, whereas the bonds holding together the sodiums and the sulfate were ionic.
The beauty of "i" is that it doesn't depend on the identity of the substance. If you were to create a 1m solution of each of the four compounds mentioned above, the freezing point depression and boiling point elevation for Na2SO4 and MgCl2 would be the same. Sodium chloride would be your second most effective, and glucose would be the worst.(137 votes)
- Was that a mistake that he wrote solute lowers the boiling point at, it should be melting point right? 3:30(34 votes)
- It actually should have been the freezing point I believe, because athe says freezing point, and throughout the beginning of the video he talks about freezing the water. 0:04(6 votes)
- I don't understand why NaCl when dissociated in water is suddenly 2 moles rather than 1... Regardless of whether its a whole molecule or in anion and cation form, wouldn't we still have the same mass?(10 votes)
- Moles are not mass. Moles are counts of particles. Thus, when one particle breaks into two particles you double the number of particles. Likewise, when you have 1 mole of NaCl and the particles break apart, you now have 2 moles in total -- 1 mole of Na⁺ ions and 1 mole of Cl ⁻ ions.(25 votes)
- At, Sal, says that with NaCl breaking down, you get twice the moles of the solute that you have originally. His example being that two moles of NaCl being dissolved in water gives you four moles of the solute. SO my question is, does it always just double by two, or does it depend on the elements amu and/or how many elements are in the molecule? 10:20(5 votes)
- It depends on the molecules involved. For example, when is dissolve 1 mol of MgCl2 in water, I get three moles of solute, as I have 1 mole of Mg and 2 moles of Cl, making 3 moles overall. With AlCl3, I'd have 4 moles of solute for every 1 mole I dissolve.(7 votes)
- so does that mean adding solute will lower surface tension too? 6:47(5 votes)
- Surface tension is about strength of interactions between molecules AND mixing, while boiling point and freezing point changes are ONLY about mixing. So surface tension will be more complicated.(1 vote)
- Why do the Carbon-dioxide particles have some difficulty escaping through the water molecules when the is low pressure or low temperature and vice-versa is there is a high pressure or temperature??(3 votes)
- Because at low pressure/low temperature the CO2 particles have much less kinetic energy than at high pressure/high temperature. CO2 moving from being dissolved in water to free in the atmosphere is endothermic - it requires energy (heat!) input.
CO2 (aq) + heat ---> CO2 (g)(2 votes)
- delta t(v) = 2 degrees what?
is it Celsius or Fahrenheit?(3 votes)- In science, temperature is measured in Kelvin or Celcius.
For this example, since an increase in temperature of 2 degree Kelvin or 2 degree Celcius is the same, it doesn't matter you use. Note that delta T is the change in temperature.(2 votes)
- I'm confused a solute makes the boiling point lower? I thought it made the boiling point higher.(3 votes)
- So what is clear to me is that since it is even harder for the molecules of a liquid like water to bond when they are mixed with some solute molecules, it requires less kinetic energy for the molecules to bond and create a solid. (Lower temperature decreases the kinetic energy.).
But what happens if we put a solute into an amorphus liquid? Would decrease its solidification temperature too, or because the amorphus compound doesn't want to create a geometric and clear structure it doesn't affect its freezing point?(2 votes)- The relationships between solvent and solute in terms of boiling point elevation and freezing point depression are not as simple, in reality, as they speak of at this introductory level, though it is true that (at dilute levels of the solute) it is just what they tell you here. However, there are a variety of reasons why solutes increase the tendency of the solvent to be in liquid form beyond what you've mentioned -- but this gets into some advanced Chemistry topics (and very advanced mathematics).
As far as nonaqueous solvents, I haven't studied that particular issue very closely. However, I do know that chloroform also exhibits the same characteristic: solutes make it be more likely to be in the liquid phase, thus increasing its boiling point and decreasing its freezing point.
Whether aqueous solution or some other solute, the use of a volatile or liquid solute involves a very complex relationship with the solvent in terms of changing boiling points and freezing points. This is especially the case when the solvent and solute form an azeotrope.(2 votes)
- hello, i am having trouble interpretting the formula deltaTv=k.mi, so the temperature of vaprisation changes according to the concentration of solutes or number of particles of solutes in the solvent, but what about the type of solute? shouldn't a glucose molecule affect the boiling point more than a sodium molecule would? wouldn't glucose take up more space and prevent water molecules escaping into the air more?(2 votes)
Video transcript
Let's think about what might
happen to the boiling point or the freezing point of any
solution if we start adding particles, or we start
adding solute to it. For our visualization, let's
just think about water again. It doesn't have to be water. It can be any solvent, but let's
just think about water in its liquid state. The particles are reasonably
disorganized because of their kinetic energy, but they still
have that hydrogen bonds that wants to make them be
near each other. So this is in the liquid
state, and they have a reasonable amount of
kinetic energy. You know, each of these
particles is moving in some direction, rubbing against each
other, bouncing off of each other. Now, to move it into the solid
state, or to freeze it, what has to happen? The ice has to enter kind of
a crystalline structure. It has to get pretty organized,
so let's say it has to look something like this. The water molecules are going
to have a regular structure where the hydrogen bonds
dominate any kind of kinetic movement they want to do, and
all the kinetic movement, they're just vibrating
in place. So you have to get a
little bit orderly right there, right? And then, obviously, this
lattice structure goes on and on with a gazillion
water molecules. But the interesting
thing is that this somehow has to get organized. And what happens if we start
introducing molecules into this water? Let's say the example of
sodium-- actually, I won't do any example. Let's just say some arbitrary
molecule, if I were to introduce it there, if I
were to put something-- let me draw it again. So now I'll just use that same--
I'll introduce some molecules, and let's say they're
pretty large, so they push all of these water
molecules out of the way. So the water molecules are now
on the outside of that, and let's have another one that's
over here, some relatively large molecules of solute
relative to water, and this is because a water molecule
really isn't that big. Now, do you think it's going
to be easier or harder to freeze this? Are you going to have to remove
more or less energy to get to a frozen state? Well, because these molecules,
they're not going to be part of this lattice structure
because frankly, they wouldn't even fit into it. They're actually going to make
it harder for these water molecules to get organized
because to get organized, they have to get at the right
distance for the hydrogen bonds to form. But in this case, even as you
start removing heat from the system, maybe the ones that
aren't near the solute particles, they'll start to
organize with each other. But then when you introduce a
solute particle, let's say a solute particle is sitting
right here. It's going to be very hard for
someone to organize with this guy, to get near enough for
the hydrogen bond to start taking hold. This distance would make
it very difficult. And so the way I think about
it is that these solute particles make the structure
irregular, or they add more disorder, and we'll eventually
talk about entropy and all of that. But they make it more irregular,
and it's making it harder to get into
a regular form. And so the intuition is is that
this should lower the boiling point or make
it-- oh, sorry, lower the melting point. So solute particles make you
have a lower boiling point. Let's say if we're talking
about water at standard temperature and pressure or at
one atmosphere then instead of going to 0 degrees, you might
have to go to negative 1 or negative 2 degrees, and we're
going to talk a little bit about what that is. Now, what's the intuition of
what this will do when you want to go into a gaseous
state, when you want to boil it? So my initial gut was, hey, I'm
already in a disordered state, which is closer to what
a gas is, so wouldn't that make it easier to boil? But it turns out it also makes
it harder to boil, and this is how I think about it. Remember, everything with
boiling deals with what's happening at the surface, and
we talked about that in our vapor pressure. So at the surface, we said if
I have a bunch of water molecules in the liquid state,
we knew that although the average temperature might not
be high enough for the water molecules to evaporate, that
there's a distribution of kinetic energies. And some of these water
molecules on the surface because the surface ones
might be going fast enough to escape. And when they escape into vapor,
then they create a vapor pressure above here. And if that vapor pressure is
high enough, you can almost view them as linemen blocking
the way for more molecules to kind of run behind them as they
block all of the other ambient air pressure
above them. So if there's enough of them and
they have enough energy, they can start to push back or
to push outward is the way I think about it, so that more
guys can come in behind them. So I hope that lineman analogy
doesn't completely lose you. Now, what happens if you were
to introduce solute into it? Some of the solute particle
might be down here. It probably doesn't have much
of an effect down here, but some of it's going to be
bouncing on the surface, so they're going to be taking up
some of the surface area. And because, and this is at
least how I think of it, since they're going to be taking up
some of the surface area, you're going to have less
surface area exposed to the solvent particle or to the
solution or the stuff that'll actually vaporize. You're going to have a
lower vapor pressure. And remember, your boiling
point is when the vapor pressure, when you have enough
particles with enough kinetic energy out here to start
pushing against the atmospheric pressure, when the
vapor pressure is equal to the atmospheric pressure,
you start boiling. But because of these guys, I
have a lower vapor pressure. So I'm going to have to add even
more kinetic energy, more heat to the system in order to
get enough vapor pressure up here to start pushing back
the atmospheric pressure. So solute also raises
the boiling point. So the way that you can think
about it is solute, when you add something to a solution,
it's going to make it want to be in the liquid state more. Whether you lower the
temperature, it's going to want to stay in liquid as
opposed to ice, and if you raise the temperature, it's
going to want to stay in liquid as opposed to gas. I found this neat-- hopefully,
it shows up well on this video. I have to give due credit, this
is from chem.purdue.edu/ gchelp/solutions/eboil.html, but
I thought it was a pretty neat graphic, or at least
a visualization. This is just the surface of
water molecules, and it gives you a sense of just how things
vaporize as well. There's some things on the
surface that just bounce off. And here's an example where
they visualized sodium chloride at the surface. And because the sodium chloride
is kind of bouncing around on the surface with the
water molecules, fewer of those water molecules kind of
have the room to escape, so the boiling point
gets elevated. Now, the question is by how
much does it get elevated? And this is one of the neat
things in life is that the answer is actually
quite simple. The change in boiling or
freezing point, so the change in temperature of vaporization,
is equal to some constant times the number of
moles, or at least the mole concentration, the molality,
times the molality of the solute that you're putting
into your solution. So, for example, let's say I
have 1 kilogram of-- so let's say my solvent is water. I'll switch colors. And I have 1 kilogram of water,
and let's say we're just at atmospheric pressure. And let's say I have some
sodium chloride, NaCl. And let's say I have
2 moles of NaCl. I'll have 2 moles. The question is how much will
this raise the boiling point of this water? So first of all, you just have
to figure out the molality, which is just equal to the
number of moles of solute, this 2 moles, divided
by the number of kilograms of solvent. So let's say we have 1
kilogram of solvent. This was, of course, moles. So our molality is 2
moles per kilogram. So we just have to figure out
what this constant is, and then we'll know the temperature
elevation. And actually, that same
Purdue site, they gave a list of tables. I haven't run the experiments
myself. They have some neat
charts here. But they say, OK water, normal
boiling point is 100 degrees Celsius at standard atmospheric
pressure. And then they say that the
constant is 0.512 Celsius degrees per mole. So let's just say 0.5. So it equals 0.5. So k is equal to 0.5. And I want to be very clear here
because this is a very-- I won't say a subtle point, but
it's an interesting point. So I said that there's 2-- the
molality of-- I just realized I made a mistake. I said the molality of
sodium chloride is 2. 2 moles per kilograms. But
that would be if sodium chloride stayed in this
molecular state, if it stayed together, right? But what happens is that the
sodium chloride actually disassociates, and we learned
all about it in that previous video. Each molecule or each sodium
chloride pair disassociates into two molecules,
into a sodium ion and a chlorine anion. And because of that, because
this disassociates into two, the molality is actually going
to be two times the number of moles of sodium chloride I have.
So it's going to be two times this. So my molality will
actually be 4. And this is an interesting
point. If I was dealing with--
and I wrote it here. So this right here is glucose,
and this is sodium chloride, or at least sodium chloride
in its crystal form. One molecule, I guess you can
view it, or one salt of it. I guess you could just view it
as one of these little pairs right here. But the interesting thing is
is you could have the same number of moles of sodium
chloride when you view it as a compound and glucose. But glucose, when it goes into
water, it just stays as one molecule of glucose. So a mole of glucose will
disassociate into a mole of glucose in water. Well, I guess it won't
disassociate. It'll just stay as one mole,
while a mole of sodium chloride will turn into
two moles because it disassociates. It turns into two separate
particles. So in my example, when I start
with a mole of this, I end up-- actually, once I dissolve
it in water, I ended up with 4 moles per kilogram of molality,
because this turns into two particles. So given that the molality
is 4 moles. 2 moles of sodium, 2 moles
of chloride per kilogram. So I just use that constant that
I just got from Purdue. And I get the change in
temperature is equal to that constant, 0.5, times 4, which
is equal to 2 degrees. So my boiling point will be
elevated by 2 degrees. Now, if I had the same number of
moles, if I had 2 moles of glucose dissolved into my water,
I'd only get half as much, half as much
of an increase. Because the molality would
be half as much. Because it doesn't turn
into two particles. In some textbooks, you'll
actually see it written like this. You'll actually see the same
formula written like change in boiling temperature, or vapor
temperature, or whatever you want to think, is equal to k
times m times i, where they'll say this is the molality
of the compound you're talking about. In this case, this number
would be 2, and i is the number of molecules or the
number of things that it disassociates into. So in this case, this
would have been 2. And that's where we would have
gotten 4 times k, which is 0.5, which is 2. In the case of water, this would
be-- oh, sorry, in the case of the glucose, this
would still be 2. But it only turns into one
particle when it goes in the water, so that would be 1. So you would only have a 1
degree increase in the boiling point of water. Now, freezing point
is the same thing. Change in freezing
point is also proportional to the molality. And you can either say the
molality of the original non-in-water compound times
the number of compounds it disassociates into, although
this k is going to be different for freezing than
it is for boiling. Of course, this k changes at
different pressures and for different elements. But the really big takeaway is
just to realize that even if you have a mole of this and a
mole of that, and they're going to be dissolved into the
same amount of water, because this dissociates into two
particles and this disassociates into only one
for every-- or this disassociates into two moles for
every mole of the crystal you have-- this doesn't
disassociate; it just stays as one-- this'll have twice as
large of an effect on the freezing point change or on the
boiling point elevation than the glucose will.