-
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Let's think about what might
happen to the boiling point or
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the freezing point of any
solution if we start adding
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particles, or we start
adding solute to it.
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For our visualization, let's
just think about water again.
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It doesn't have to be water.
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It can be any solvent, but let's
just think about water
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in its liquid state.
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The particles are reasonably
disorganized because of their
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kinetic energy, but they still
have that hydrogen bonds that
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wants to make them be
near each other.
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So this is in the liquid
state, and they have a
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reasonable amount of
kinetic energy.
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You know, each of these
particles is moving in some
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direction, rubbing against each
other, bouncing off of
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each other.
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Now, to move it into the solid
state, or to freeze it, what
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has to happen?
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The ice has to enter kind of
a crystalline structure.
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It has to get pretty organized,
so let's say it has
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to look something like this.
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The water molecules are going
to have a regular structure
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where the hydrogen bonds
dominate any kind of kinetic
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movement they want to do, and
all the kinetic movement,
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they're just vibrating
in place.
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So you have to get a
little bit orderly
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right there, right?
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And then, obviously, this
lattice structure goes on and
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on with a gazillion
water molecules.
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But the interesting
thing is that this
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somehow has to get organized.
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And what happens if we start
introducing molecules into
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this water?
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Let's say the example of
sodium-- actually, I won't do
-
any example.
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Let's just say some arbitrary
molecule, if I were to
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introduce it there, if I
were to put something--
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let me draw it again.
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So now I'll just use that same--
I'll introduce some
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molecules, and let's say they're
pretty large, so they
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push all of these water
molecules out of the way.
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So the water molecules are now
on the outside of that, and
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let's have another one that's
over here, some relatively
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large molecules of solute
relative to water, and this is
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because a water molecule
really isn't that big.
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Now, do you think it's going
to be easier or harder to
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freeze this?
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Are you going to have to remove
more or less energy to
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get to a frozen state?
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Well, because these molecules,
they're not going to be part
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of this lattice structure
because frankly, they wouldn't
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even fit into it.
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They're actually going to make
it harder for these water
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molecules to get organized
because to get organized, they
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have to get at the right
distance for the hydrogen
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bonds to form.
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But in this case, even as you
start removing heat from the
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system, maybe the ones that
aren't near the solute
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particles, they'll start to
organize with each other.
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But then when you introduce a
solute particle, let's say a
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solute particle is sitting
right here.
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It's going to be very hard for
someone to organize with this
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guy, to get near enough for
the hydrogen bond to start
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taking hold.
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This distance would make
it very difficult.
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And so the way I think about
it is that these solute
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particles make the structure
irregular, or they add more
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disorder, and we'll eventually
talk about
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entropy and all of that.
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But they make it more irregular,
and it's making it
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harder to get into
a regular form.
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And so the intuition is is that
this should lower the
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boiling point or make
it-- oh, sorry,
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lower the melting point.
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So solute particles make you
have a lower boiling point.
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Let's say if we're talking
about water at standard
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temperature and pressure or at
one atmosphere then instead of
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going to 0 degrees, you might
have to go to negative 1 or
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negative 2 degrees, and we're
going to talk a little bit
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about what that is.
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Now, what's the intuition of
what this will do when you
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want to go into a gaseous
state, when you
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want to boil it?
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So my initial gut was, hey, I'm
already in a disordered
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state, which is closer to what
a gas is, so wouldn't that
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make it easier to boil?
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But it turns out it also makes
it harder to boil, and this is
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how I think about it.
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Remember, everything with
boiling deals with what's
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happening at the surface, and
we talked about that in our
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vapor pressure.
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So at the surface, we said if
I have a bunch of water
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molecules in the liquid state,
we knew that although the
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average temperature might not
be high enough for the water
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molecules to evaporate, that
there's a distribution of
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kinetic energies.
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And some of these water
molecules on the surface
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because the surface ones
might be going
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fast enough to escape.
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And when they escape into vapor,
then they create a
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vapor pressure above here.
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And if that vapor pressure is
high enough, you can almost
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view them as linemen blocking
the way for more molecules to
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kind of run behind them as they
block all of the other
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ambient air pressure
above them.
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So if there's enough of them and
they have enough energy,
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they can start to push back or
to push outward is the way I
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think about it, so that more
guys can come in behind them.
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So I hope that lineman analogy
doesn't completely lose you.
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Now, what happens if you were
to introduce solute into it?
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Some of the solute particle
might be down here.
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It probably doesn't have much
of an effect down here, but
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some of it's going to be
bouncing on the surface, so
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they're going to be taking up
some of the surface area.
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And because, and this is at
least how I think of it, since
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they're going to be taking up
some of the surface area,
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you're going to have less
surface area exposed to the
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solvent particle or to the
solution or the stuff that'll
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actually vaporize.
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You're going to have a
lower vapor pressure.
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And remember, your boiling
point is when the vapor
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pressure, when you have enough
particles with enough kinetic
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energy out here to start
pushing against the
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atmospheric pressure, when the
vapor pressure is equal to the
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atmospheric pressure,
you start boiling.
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But because of these guys, I
have a lower vapor pressure.
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So I'm going to have to add even
more kinetic energy, more
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heat to the system in order to
get enough vapor pressure up
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here to start pushing back
the atmospheric pressure.
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So solute also raises
the boiling point.
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So the way that you can think
about it is solute, when you
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add something to a solution,
it's going to make it want to
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be in the liquid state more.
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Whether you lower the
temperature, it's going to
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want to stay in liquid as
opposed to ice, and if you
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raise the temperature, it's
going to want to stay in
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liquid as opposed to gas.
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I found this neat-- hopefully,
it shows up
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well on this video.
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I have to give due credit, this
is from chem.purdue.edu/
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gchelp/solutions/eboil.html, but
I thought it was a pretty
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neat graphic, or at least
a visualization.
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This is just the surface of
water molecules, and it gives
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you a sense of just how things
vaporize as well.
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There's some things on the
surface that just bounce off.
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And here's an example where
they visualized sodium
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chloride at the surface.
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And because the sodium chloride
is kind of bouncing
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around on the surface with the
water molecules, fewer of
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those water molecules kind of
have the room to escape, so
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the boiling point
gets elevated.
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Now, the question is by how
much does it get elevated?
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And this is one of the neat
things in life is that the
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answer is actually
quite simple.
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The change in boiling or
freezing point, so the change
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in temperature of vaporization,
is equal to some
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constant times the number of
moles, or at least the mole
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concentration, the molality,
times the molality of the
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solute that you're putting
into your solution.
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So, for example, let's say I
have 1 kilogram of-- so let's
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say my solvent is water.
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I'll switch colors.
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And I have 1 kilogram of water,
and let's say we're
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just at atmospheric pressure.
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And let's say I have some
sodium chloride, NaCl.
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And let's say I have
2 moles of NaCl.
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I'll have 2 moles.
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The question is how much will
this raise the boiling point
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of this water?
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So first of all, you just have
to figure out the molality,
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which is just equal to the
number of moles of solute,
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this 2 moles, divided
by the number of
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kilograms of solvent.
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So let's say we have 1
kilogram of solvent.
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This was, of course, moles.
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So our molality is 2
moles per kilogram.
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So we just have to figure out
what this constant is, and
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then we'll know the temperature
elevation.
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And actually, that same
Purdue site, they
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gave a list of tables.
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I haven't run the experiments
myself.
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They have some neat
charts here.
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But they say, OK water, normal
boiling point is 100 degrees
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Celsius at standard atmospheric
pressure.
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And then they say that the
constant is 0.512 Celsius
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degrees per mole.
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So let's just say 0.5.
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So it equals 0.5.
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So k is equal to 0.5.
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And I want to be very clear here
because this is a very--
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I won't say a subtle point, but
it's an interesting point.
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So I said that there's 2-- the
molality of-- I just realized
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I made a mistake.
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I said the molality of
sodium chloride is 2.
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2 moles per kilograms. But
that would be if sodium
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chloride stayed in this
molecular state, if it stayed
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together, right?
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But what happens is that the
sodium chloride actually
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disassociates, and we learned
all about it in
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that previous video.
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Each molecule or each sodium
chloride pair disassociates
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into two molecules,
into a sodium ion
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and a chlorine anion.
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And because of that, because
this disassociates into two,
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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
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times this.
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So my molality will
actually be 4.
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And this is an interesting
point.
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If I was dealing with--
and I wrote it here.
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So this right here is glucose,
and this is sodium chloride,
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or at least sodium chloride
in its crystal form.
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One molecule, I guess you can
view it, or one salt of it.
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I guess you could just view it
as one of these little pairs
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right here.
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But the interesting thing is
is you could have the same
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number of moles of sodium
chloride when you view it as a
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compound and glucose.
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But glucose, when it goes into
water, it just stays as one
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molecule of glucose.
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So a mole of glucose will
disassociate into a mole of
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glucose in water.
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Well, I guess it won't
disassociate.
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It'll just stay as one mole,
while a mole of sodium
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chloride will turn into
two moles because it
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disassociates.
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It turns into two separate
particles.
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So in my example, when I start
with a mole of this, I end
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up-- actually, once I dissolve
it in water, I ended up with 4
-
moles per kilogram of molality,
because this turns
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into two particles.
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So given that the molality
is 4 moles.
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2 moles of sodium, 2 moles
of chloride per kilogram.
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So I just use that constant that
I just got from Purdue.
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And I get the change in
temperature is equal to that
-
constant, 0.5, times 4, which
is equal to 2 degrees.
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So my boiling point will be
elevated by 2 degrees.
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Now, if I had the same number of
moles, if I had 2 moles of
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glucose dissolved into my water,
I'd only get half as
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much, half as much
of an increase.
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Because the molality would
be half as much.
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Because it doesn't turn
into two particles.
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In some textbooks, you'll
actually see it
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written like this.
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You'll actually see the same
formula written like change in
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boiling temperature, or vapor
temperature, or whatever you
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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.
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In this case, this number
would be 2, and i is the
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number of molecules or the
number of things that it
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disassociates into.
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So in this case, this
would have been 2.
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And that's where we would have
gotten 4 times k, which is
-
0.5, which is 2.
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In the case of water, this would
be-- oh, sorry, in the
-
case of the glucose, this
would still be 2.
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But it only turns into one
particle when it goes in the
-
water, so that would be 1.
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So you would only have a 1
degree increase in the boiling
-
point of water.
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Now, freezing point
is the same thing.
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Change in freezing
point is also
-
proportional to the molality.
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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.
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Of course, this k changes at
different pressures and for
-
different elements.
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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.
-