Introduction to pH, pOH, and pKw

WEBVTT
Kind: captions
Language: en

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Sal: --the consistent theme
we're discovering in

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chemistry, that everything
is about random

00:00:04.540 --> 00:00:05.870
bumps between molecules.

00:00:05.870 --> 00:00:08.590
And when they bump randomly
into each other, a lot of

00:00:08.590 --> 00:00:10.690
different things can happen in
terms of knocking different

00:00:10.690 --> 00:00:12.890
parts of the molecules off of
each other and bonding to

00:00:12.890 --> 00:00:14.150
different things.

00:00:14.150 --> 00:00:18.180
This includes pure water, which
for the most part, one

00:00:18.180 --> 00:00:20.380
would think is a pretty
stable substance.

00:00:20.380 --> 00:00:21.390
And it really is.

00:00:21.390 --> 00:00:22.670
But it turns out-- let's
say if we start

00:00:22.670 --> 00:00:24.700
with 2 water molecules.

00:00:24.700 --> 00:00:27.000
And when I put a 2 here, and any
of these is going to turn

00:00:27.000 --> 00:00:29.300
into an equilibrium equation.

00:00:29.300 --> 00:00:32.229
It could represent 2 molecules,
it could represent

00:00:32.229 --> 00:00:33.400
2 moles of molecules.

00:00:33.400 --> 00:00:35.880
What matters is just the ratios
in these equations.

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And I think you know that.

00:00:37.210 --> 00:00:39.850
But let me just draw
2 for an example.

00:00:39.850 --> 00:00:43.350
So let's say I have
the oxygen end.

00:00:43.350 --> 00:00:45.370
Let's have another
oxygen here.

00:00:45.370 --> 00:00:47.680
They both are attached
to some hydrogens.

00:00:51.350 --> 00:00:54.700
They're sharing electrons with
some hydrogens here.

00:00:54.700 --> 00:00:57.900
And we know that oxygen is a lot
more electronegative than

00:00:57.900 --> 00:01:00.430
hydrogen, so it hogs all of the
electrons and what that

00:01:00.430 --> 00:01:04.620
creates is a partial positive
charge at the hydrogen side of

00:01:04.620 --> 00:01:05.570
the water molecule.

00:01:05.570 --> 00:01:07.670
They're almost naked protons
being attached

00:01:07.670 --> 00:01:08.720
to the water molecules.

00:01:08.720 --> 00:01:10.230
This is positive here.

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And then the oxygen sides have
a partial negative charge

00:01:13.390 --> 00:01:15.080
because they're hogging
all the electrons.

00:01:15.080 --> 00:01:19.130
And this bond between the
negative side of the oxygen

00:01:19.130 --> 00:01:21.650
molecules and the positive
side of another oxygen

00:01:21.650 --> 00:01:23.950
molecule-- this is called a
hydrogen bond-- this is what

00:01:23.950 --> 00:01:27.220
keeps water together and what
keeps it definitely in it's

00:01:27.220 --> 00:01:28.370
liquid or solid states.

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And in solid states, these bonds
become-- they only stay

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together in some lattice
structure.

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And in a liquid state, they
can kind of break

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and form new ones.

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But they keep them close to each
other, rubbing against

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each other.

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Now it turns out if you just
have exactly the right set of

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circumstances-- this isn't the
exact some circumstances, but

00:01:46.930 --> 00:01:47.700
I think you can imagine.

00:01:47.700 --> 00:01:50.190
If this moved just exactly close
enough, it had enough

00:01:50.190 --> 00:01:53.310
kinetic energy to move right
past this one, there's some

00:01:53.310 --> 00:01:58.800
probability-- and I'll draw an
equilibrium reaction here,

00:01:58.800 --> 00:02:01.702
because that is what we
are dealing with.

00:02:01.702 --> 00:02:03.430
The equilibrium reaction.

00:02:03.430 --> 00:02:06.910
There's some probability that
this hydrogen right here gets

00:02:06.910 --> 00:02:07.770
stuck to this guy.

00:02:07.770 --> 00:02:10.259
He gets so close, and maybe this
guy bounces in some other

00:02:10.259 --> 00:02:14.700
weird way that we get left
with-- let me draw the same 2

00:02:14.700 --> 00:02:18.500
characters, the 2 oxygens.

00:02:18.500 --> 00:02:20.020
Actually let, me draw
the second guy.

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Let's say that he just he just
somehow passed the other guy.

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So he had some movement.

00:02:25.440 --> 00:02:27.630
Let's say he moved in
that direction.

00:02:27.630 --> 00:02:30.010
This is obviously is not
scientific, but it gives you a

00:02:30.010 --> 00:02:32.950
narrative around what
could happen.

00:02:32.950 --> 00:02:35.070
This guy's hydrogen protons.

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And this was attracted
to this oxygen side.

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And if just under the right
circumstances-- Remember.

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Everything here is
probabalistic.

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There's always a probability of
almost anything happening.

00:02:44.440 --> 00:02:49.650
This guy can get attached to
the negative side of this

00:02:49.650 --> 00:02:51.210
water molecule, the
water side.

00:02:51.210 --> 00:02:54.930
And then this guy is only left
with one hydrogen right here.

00:02:54.930 --> 00:03:00.860
And then this molecule as a
whole, this is OH, hydroxide.

00:03:00.860 --> 00:03:03.080
And it has a minus
charge, right?

00:03:03.080 --> 00:03:06.320
Because water was a
neutral molecule.

00:03:06.320 --> 00:03:10.310
You had just as many protons
and electrons.

00:03:10.310 --> 00:03:13.610
Now we have 1 less proton
than electrons.

00:03:13.610 --> 00:03:18.520
So in this case, just the
hydrogen nucleus itself--

00:03:18.520 --> 00:03:20.680
which is just a proton because
we learned that hydrogen

00:03:20.680 --> 00:03:23.360
doesn't have any neutrons
in it's normal form.

00:03:23.360 --> 00:03:25.910
This is essentially a proton
when we draw a hydrogen.

00:03:25.910 --> 00:03:29.020
It gets bumped off or scraped
off and ends up on this water

00:03:29.020 --> 00:03:31.010
molecule, so this guy lost
a proton, has the

00:03:31.010 --> 00:03:32.760
same number of electrons.

00:03:32.760 --> 00:03:34.580
So he now has a negative
charge.

00:03:34.580 --> 00:03:37.480
This guy has the same number of
electrons, but now he's got

00:03:37.480 --> 00:03:38.310
an extra proton.

00:03:38.310 --> 00:03:39.550
So he has a positive charge.

00:03:39.550 --> 00:03:46.440
So this is H3O with
a positive charge.

00:03:46.440 --> 00:03:48.660
And this little version right
here where we gave a water

00:03:48.660 --> 00:03:53.930
molecule and it somehow gains
an extra hydrogen proton or

00:03:53.930 --> 00:03:54.980
hydrogen atom.

00:03:54.980 --> 00:03:56.230
This is called hydronium.

00:04:02.910 --> 00:04:06.000
And this process where water
can kind of spontaneously--

00:04:06.000 --> 00:04:07.730
and I'm not saying that water--
you know, a whole

00:04:07.730 --> 00:04:09.030
glass of water, this
is going to happen.

00:04:09.030 --> 00:04:10.630
It's actually happening
to a very

00:04:10.630 --> 00:04:12.210
small number of molecules.

00:04:12.210 --> 00:04:15.340
That if you view this is an
equilibrium reaction-- when

00:04:15.340 --> 00:04:19.360
you have 2 waters, the reaction
is much more weighted

00:04:19.360 --> 00:04:20.829
in the leftward direction.

00:04:20.829 --> 00:04:23.060
But this can happen, and that's
what the whole point of

00:04:23.060 --> 00:04:24.990
this is to think about.

00:04:24.990 --> 00:04:28.010
But this process is called
autoionization.

00:04:28.010 --> 00:04:30.410
Because you just have water by
itself, and just buy some

00:04:30.410 --> 00:04:33.110
random circumstances of some
molecules bumping into each

00:04:33.110 --> 00:04:35.970
other just right, some
subset of the water

00:04:35.970 --> 00:04:37.330
will ionize like this.

00:04:37.330 --> 00:04:40.810
Where one part will lose
a proton and the other

00:04:40.810 --> 00:04:42.290
part will gain one.

00:04:42.290 --> 00:04:43.540
So this is autoionization.

00:04:48.670 --> 00:04:50.600
And of course this is an
equilibrium reaction, so now

00:04:50.600 --> 00:04:52.430
these guys might go
bump into 2 other

00:04:52.430 --> 00:04:53.670
things and become water.

00:04:53.670 --> 00:04:55.630
Maybe they'll bump into
each other again and

00:04:55.630 --> 00:04:56.310
they'll become water.

00:04:56.310 --> 00:04:57.470
So goes back and forth.

00:04:57.470 --> 00:05:00.000
So there's some equilibrium
concentration of this.

00:05:00.000 --> 00:05:02.510
Let me write it as a proper
equilibrium reaction.

00:05:02.510 --> 00:05:05.930
So you could have 2, maybe
moles of water molecules.

00:05:05.930 --> 00:05:11.215
They are in equilibrium
with H3O positive.

00:05:14.030 --> 00:05:15.770
And all these are in an
aqueous solution.

00:05:15.770 --> 00:05:17.730
And that's a bit of a redundancy
because aqueous

00:05:17.730 --> 00:05:20.190
solution means that you're
dissolved in water.

00:05:20.190 --> 00:05:23.030
So of course water is
dissolved in water.

00:05:23.030 --> 00:05:25.730
But then H3O dissolved
in water.

00:05:25.730 --> 00:05:28.150
So all around here, you
have water molecules.

00:05:28.150 --> 00:05:28.955
This isn't isolation.

00:05:28.955 --> 00:05:32.190
You have gazillions of water
molecules all around.

00:05:32.190 --> 00:05:38.900
Plus OH minus aqueous.

00:05:38.900 --> 00:05:39.300
Now.

00:05:39.300 --> 00:05:44.130
Sometimes you'll see that exact

00:05:44.130 --> 00:05:45.250
reaction written as this.

00:05:45.250 --> 00:05:48.450
And it's important if you
realize it's the same thing.

00:05:48.450 --> 00:05:55.090
H2O in equilibrium with just
1 hydrogen ion popping off.

00:05:55.090 --> 00:05:56.670
Everything's in an
aqueous solution.

00:05:56.670 --> 00:05:58.460
Let me write that down.

00:05:58.460 --> 00:06:02.350
Everything is being dissolved
and water.

00:06:02.350 --> 00:06:08.270
Plus OH minus in an aqueous
solution And this equation

00:06:08.270 --> 00:06:12.030
right here-- and what
I wrote at the top.

00:06:12.030 --> 00:06:14.160
And let me cut and paste
this so you can see

00:06:14.160 --> 00:06:17.210
them at the same time.

00:06:17.210 --> 00:06:19.120
Copy.

00:06:19.120 --> 00:06:21.120
Go down here.

00:06:21.120 --> 00:06:23.070
And paste this.

00:06:23.070 --> 00:06:25.240
These are essentially
equivalent, although the

00:06:25.240 --> 00:06:27.530
second equation right
here describes

00:06:27.530 --> 00:06:30.050
what exactly happens.

00:06:30.050 --> 00:06:33.020
So the first equation is
creating a picture.

00:06:33.020 --> 00:06:36.080
You have one water molecule
and there's some small

00:06:36.080 --> 00:06:39.180
probability that one of its
hydrogens just pop off.

00:06:39.180 --> 00:06:41.800
And you're just left with a
hydrogen and a hydroxide,

00:06:41.800 --> 00:06:46.040
which is just an oxygen and
a hydrogen atom together.

00:06:46.040 --> 00:06:50.250
But the reality is that these
hydrogen protons don't exist

00:06:50.250 --> 00:06:51.720
in water on their own.

00:06:51.720 --> 00:06:55.030
Whenever they are in water, in
an aqueous solution, they

00:06:55.030 --> 00:06:57.320
essentially get a ride with
another water molecule.

00:06:57.320 --> 00:07:00.460
And that's what happens here
with the hydronium ion.

00:07:00.460 --> 00:07:02.650
But the whole reason why I'm
going into this whole

00:07:02.650 --> 00:07:05.040
discussion about the
autoionization of water is

00:07:05.040 --> 00:07:08.850
because people really care about
the concentration of--

00:07:08.850 --> 00:07:12.880
depending on how I view it--
hydrogen atoms, hydrogen

00:07:12.880 --> 00:07:16.950
protons in a solution, or you
could say hydronium protons in

00:07:16.950 --> 00:07:17.320
a solution.

00:07:17.320 --> 00:07:18.840
The concentration of
these 2 things are

00:07:18.840 --> 00:07:20.160
going to be the same.

00:07:20.160 --> 00:07:20.420
Right?

00:07:20.420 --> 00:07:23.360
Because all of these really are
riding on another water

00:07:23.360 --> 00:07:24.920
molecule and becoming
hydronium.

00:07:27.710 --> 00:07:31.600
How do you measure this-- or a
better question is what is the

00:07:31.600 --> 00:07:33.190
equilibrium of this reaction?

00:07:33.190 --> 00:07:36.980
So it turns out that in just
regular water, the

00:07:36.980 --> 00:07:40.050
concentration-- and I'll maybe
I'll switch to this one.

00:07:40.050 --> 00:07:43.250
The concentration of-- this
is in regular water

00:07:43.250 --> 00:07:45.250
at 25 degrees Celsius.

00:07:45.250 --> 00:07:46.800
Let me write that down.

00:07:46.800 --> 00:07:52.910
25 degrees Celsius, which is
roughly room temperature.

00:07:52.910 --> 00:07:53.950
With pure water.

00:07:53.950 --> 00:07:56.940
The concentration
of pure water.

00:07:56.940 --> 00:07:59.460
In pure water, the concentration
of hydrogen

00:07:59.460 --> 00:08:02.260
ions, and you could also say
that that's the concentration

00:08:02.260 --> 00:08:06.650
of hydronium ions, or cations,
because they're positive.

00:08:06.650 --> 00:08:14.490
It equals 10 to the minus
seventh molar.

00:08:14.490 --> 00:08:14.810
Right?

00:08:14.810 --> 00:08:25.110
So for every 1 liter of water,
you have 10 to the minus 7

00:08:25.110 --> 00:08:27.920
moles of hydronium ions.

00:08:27.920 --> 00:08:29.840
Which is still a pretty good
number because it's 10 to the

00:08:29.840 --> 00:08:32.059
minus 7 moles, so it's
10 to the minus 7

00:08:32.059 --> 00:08:33.549
times Avogadro's number.

00:08:33.549 --> 00:08:34.940
So let me just do that.

00:08:34.940 --> 00:08:42.650
So it's 10 to the minus 7 moles
per liter of water,

00:08:42.650 --> 00:08:46.440
which is the same thing as 10 to
the minus 7 times-- a mole

00:08:46.440 --> 00:08:47.030
is just a number.

00:08:47.030 --> 00:08:50.810
6.022 times 10 to the 23.

00:08:50.810 --> 00:08:53.140
So you do the math here.

00:08:53.140 --> 00:08:54.370
You still end up with what?

00:08:54.370 --> 00:08:57.790
60 times 10 to the 16th
power molecule.

00:08:57.790 --> 00:08:59.910
So you still have a large
number of molecules.

00:08:59.910 --> 00:09:04.190
It's just a small fraction of
the total number molecules are

00:09:04.190 --> 00:09:07.430
actually these hydrogen or
these hydronium ions.

00:09:07.430 --> 00:09:11.610
And it turns out that your
concentration of hydroxides is

00:09:11.610 --> 00:09:17.100
also-- so your concentration
of hydroxides is also 10 to

00:09:17.100 --> 00:09:19.880
the minus seventh molars.

00:09:19.880 --> 00:09:22.790
So if we know that, we can now
actually figure out the

00:09:22.790 --> 00:09:27.790
equilibrium constant for this
autoionization of water.

00:09:27.790 --> 00:09:30.175
We can figure out the
equilibrium constant.

00:09:30.175 --> 00:09:31.270
So let's do that.

00:09:31.270 --> 00:09:32.070
And we'll do it with
the first one.

00:09:32.070 --> 00:09:35.390
So let's say you have--
I'll switch colors.

00:09:35.390 --> 00:09:39.210
H2O in an aqueous solution.

00:09:39.210 --> 00:09:43.060
It's in equilibrium
with hydrogen ions

00:09:43.060 --> 00:09:44.300
in an aqueous solution.

00:09:44.300 --> 00:09:46.950
But we know in reality
they've gotten rides

00:09:46.950 --> 00:09:49.580
with other water ions.

00:09:49.580 --> 00:09:51.710
With the other water molecules,
not ions.

00:09:51.710 --> 00:09:53.560
They turn them into
hydronium ions.

00:09:53.560 --> 00:09:59.930
Plus hydroxide molecule.

00:09:59.930 --> 00:10:02.620
The equilibrium for this
reaction, the equilibrium

00:10:02.620 --> 00:10:06.510
constant, we'll call it for
the K sub w for water.

00:10:06.510 --> 00:10:14.650
It equals the concentration of
the products of hydrogen ions

00:10:14.650 --> 00:10:18.640
times a concentration of
the hydroxide ions.

00:10:18.640 --> 00:10:21.060
Now, in a normal situation,
we would divide it by the

00:10:21.060 --> 00:10:24.820
concentration of
the reactants.

00:10:24.820 --> 00:10:27.440
But in this case, and we learned
this before, the

00:10:27.440 --> 00:10:29.540
reactants are what
the solvent is.

00:10:29.540 --> 00:10:30.540
We are in water.

00:10:30.540 --> 00:10:33.270
So you we won't include this.

00:10:33.270 --> 00:10:35.400
And if you want the intuition
behind that, remember this

00:10:35.400 --> 00:10:39.550
whole thing comes out from the
probability of-- this is the

00:10:39.550 --> 00:10:42.360
probably of the backwards
reaction happening as related

00:10:42.360 --> 00:10:44.140
to the concentrations
of these two things

00:10:44.140 --> 00:10:45.590
multiplied by each other.

00:10:45.590 --> 00:10:50.140
And the probably the forward
reaction happening is related

00:10:50.140 --> 00:10:53.580
to the probability of finding
two of the product-- of the

00:10:53.580 --> 00:10:54.920
reacting molecules
in one space.

00:10:54.920 --> 00:10:56.090
And you're always going
to find these

00:10:56.090 --> 00:10:57.360
molecules in some place.

00:10:57.360 --> 00:10:59.760
It's just going to be a constant
probability that some

00:10:59.760 --> 00:11:04.300
water spontaneously turns
into these two products.

00:11:04.300 --> 00:11:06.610
So whenever you have a reaction
like this equilibrium

00:11:06.610 --> 00:11:09.870
reaction, and the one side is
essentially just a solution,

00:11:09.870 --> 00:11:11.300
you just don't include it.

00:11:11.300 --> 00:11:13.190
So the equilibrium-- or you
could just think of

00:11:13.190 --> 00:11:14.980
it as a 1 down here.

00:11:14.980 --> 00:11:18.070
So the equilibrium constant of
this autoionization water is

00:11:18.070 --> 00:11:18.580
right there.

00:11:18.580 --> 00:11:22.170
And I just gave you what the
numbers were at room

00:11:22.170 --> 00:11:22.830
temperature.

00:11:22.830 --> 00:11:25.990
So the equilibrium constant is
equal to-- the concentration

00:11:25.990 --> 00:11:33.150
of hydrogen is 10 to the minus
7 times a concentration of

00:11:33.150 --> 00:11:37.080
hydroxide is also 10
to the minus 7.

00:11:37.080 --> 00:11:41.840
So it equals 10 to the
minus 14th power.

00:11:41.840 --> 00:11:42.335
Now.

00:11:42.335 --> 00:11:45.280
And that's a good thing
to memorize.

00:11:45.280 --> 00:11:49.100
The equilibrium constant of the
autoionization of water.

00:11:49.100 --> 00:11:51.380
And just so you know, this is
all going to lead into acids.

00:11:51.380 --> 00:11:55.770
Because essentially people care
a whole bunch about what

00:11:55.770 --> 00:11:57.330
this concentration
ends up being.

00:11:57.330 --> 00:11:58.180
And we're going to
touch on that a

00:11:58.180 --> 00:11:59.180
little bit in this video.

00:11:59.180 --> 00:12:02.650
Now it turns out that people
chemistry, for whatever

00:12:02.650 --> 00:12:05.190
reason, they don't like talking
in these terms. They

00:12:05.190 --> 00:12:07.720
don't like saying 10 to
the minus 14th power.

00:12:07.720 --> 00:12:10.630
So what they do is they take
the negative log of things.

00:12:10.630 --> 00:12:12.100
Negative log base 10.

00:12:12.100 --> 00:12:15.470
And if logs are intimidating to
you, review the videos on

00:12:15.470 --> 00:12:16.080
logarithms.

00:12:16.080 --> 00:12:21.830
So what they do is they take the
minus log of both sides.

00:12:21.830 --> 00:12:23.780
And when someone just writes log
without a number, you can

00:12:23.780 --> 00:12:24.780
assume it's base 10.

00:12:24.780 --> 00:12:29.630
But they take the minus log of
the equilibrium constant.

00:12:29.630 --> 00:12:34.510
The minus log base 10 of 10 to
the minus 14 is equal to--

00:12:34.510 --> 00:12:37.570
well 10 to what powers is
10 to the minus 14?

00:12:37.570 --> 00:12:39.690
Well, it's minus fourteen,
right?

00:12:39.690 --> 00:12:41.290
And then you have that
minus in front.

00:12:41.290 --> 00:12:49.650
You have this minus, and the
log part is just minus 14.

00:12:49.650 --> 00:12:54.200
And so the minus log of the
equilibrium constant of water

00:12:54.200 --> 00:12:56.560
is equal to 14.

00:12:56.560 --> 00:13:01.480
And this idea right here
is called a pKw.

00:13:01.480 --> 00:13:02.880
pK.

00:13:02.880 --> 00:13:06.910
Let me put the p in a different
color just so you

00:13:06.910 --> 00:13:08.760
see where it's-- pKw.

00:13:08.760 --> 00:13:12.100
But all p means, whenever you
see it in chemistry, it just

00:13:12.100 --> 00:13:15.130
means minus log base 10.

00:13:15.130 --> 00:13:16.690
Minus log base 10.

00:13:16.690 --> 00:13:19.630
And there's some debate as to
what p stands for, I try to

00:13:19.630 --> 00:13:22.430
think of it as power, although
it's a minus log, it's a

00:13:22.430 --> 00:13:23.500
exponent-- but whatever.

00:13:23.500 --> 00:13:26.640
It means you're taking the
minus log of something.

00:13:26.640 --> 00:13:30.310
And what it does is, it's just a
way kind of a way to get out

00:13:30.310 --> 00:13:32.840
of the exponent world-- although
I think it's nicer to

00:13:32.840 --> 00:13:34.150
stay in the exponent world
because you really know what

00:13:34.150 --> 00:13:35.980
you're talking about--
into some nice

00:13:35.980 --> 00:13:37.490
clean number like 14.

00:13:37.490 --> 00:13:40.900
So when someone says that's
the pKw is 14, all they're

00:13:40.900 --> 00:13:44.420
saying is that the equilibrium
constant, which is just this

00:13:44.420 --> 00:13:47.540
part right here, is 10
to the minus 14.

00:13:47.540 --> 00:13:52.180
Now, the same convention
is used on the hydrogen

00:13:52.180 --> 00:13:54.660
concentration.

00:13:54.660 --> 00:13:58.130
The next few videos are all
going to revolve around the

00:13:58.130 --> 00:14:01.710
hydrogen concentration in
water, essentially.

00:14:01.710 --> 00:14:03.540
We'll talk a little bit about
what happens when you're

00:14:03.540 --> 00:14:04.810
outside of water.

00:14:04.810 --> 00:14:07.540
Now the hydrogen concentration
of water at room temperature,

00:14:07.540 --> 00:14:10.580
I've told you, has been
determined to be 10 to the

00:14:10.580 --> 00:14:13.540
minus 7 molars.

00:14:13.540 --> 00:14:15.940
People aren't happy just with
that, so what they want to do

00:14:15.940 --> 00:14:17.300
is do the same thing.

00:14:17.300 --> 00:14:20.670
They want to take the minus
log of both sides.

00:14:20.670 --> 00:14:23.440
So they say that pH--
and you've probably

00:14:23.440 --> 00:14:24.490
heard the word pH.

00:14:24.490 --> 00:14:25.510
It's a lot of cosmetics.

00:14:25.510 --> 00:14:26.390
pH balance.

00:14:26.390 --> 00:14:27.500
Now you know what it means.

00:14:27.500 --> 00:14:29.530
They're really just saying that
we have the right number

00:14:29.530 --> 00:14:32.560
of hydrogen ions in our lotion
or our perfume or whatever

00:14:32.560 --> 00:14:33.760
we're trying to sell you.

00:14:33.760 --> 00:14:42.930
So the pH is equal to the minus
log of the hydrogen ion

00:14:42.930 --> 00:14:47.505
concentration, which is equal
to the minus log, remember,

00:14:47.505 --> 00:14:51.740
it's just a base 10 if I don't
specify otherwise, of 10 to

00:14:51.740 --> 00:14:57.410
the minus 7, which is equal to--
well, 10 to what power is

00:14:57.410 --> 00:14:58.400
10 to the minus 7?

00:14:58.400 --> 00:15:00.580
This is equal to minus 7.

00:15:00.580 --> 00:15:03.450
Then we have the minus
outside, so then

00:15:03.450 --> 00:15:05.720
this becomes 7.

00:15:05.720 --> 00:15:06.100
Now.

00:15:06.100 --> 00:15:10.105
You can do that same exercise
for the pOH.

00:15:10.105 --> 00:15:11.910
pOH.

00:15:11.910 --> 00:15:15.900
The concentration of
hydroxide ions.

00:15:15.900 --> 00:15:20.990
What's the negative log of the
hydroxide concentration.

00:15:20.990 --> 00:15:23.260
The hydroxide concentration,
we saw, was the same

00:15:23.260 --> 00:15:25.570
concentration as the water
concentration.

00:15:25.570 --> 00:15:27.540
Oh, I didn't write it there.

00:15:27.540 --> 00:15:28.260
Where did I write it?

00:15:28.260 --> 00:15:28.470
Right there.

00:15:28.470 --> 00:15:29.770
10 of the minus seventh
molars.

00:15:29.770 --> 00:15:32.870
So water has the same
number of hydroxides

00:15:32.870 --> 00:15:33.930
and hydrogen ions.

00:15:33.930 --> 00:15:36.260
And that's because this water
is just associating into the

00:15:36.260 --> 00:15:37.400
two of them.

00:15:37.400 --> 00:15:42.000
But of course this is equal
to minus 7, right?

00:15:42.000 --> 00:15:42.980
Log-based.

00:15:42.980 --> 00:15:45.510
This number right here
is 10 to the minus 7.

00:15:45.510 --> 00:15:47.100
So it equals 7.

00:15:47.100 --> 00:15:52.640
So the pH and the pOH of water
is both equal to 7.

00:15:52.640 --> 00:15:53.390
Now.

00:15:53.390 --> 00:15:55.540
In the next few videos, we're
going to start covering the

00:15:55.540 --> 00:15:57.470
notion of acids and bases.

00:15:57.470 --> 00:16:00.280
And it turns out that an acid
is just something that

00:16:00.280 --> 00:16:03.220
increases the hydrogen
concentration.

00:16:03.220 --> 00:16:05.010
Well, that's one version
of acids.

00:16:05.010 --> 00:16:06.750
We'll talk about other
broader definitions.

00:16:06.750 --> 00:16:08.005
But it's called the
arrhenius acid.

00:16:08.005 --> 00:16:10.150
Which we'll talk about
in the next video.

00:16:10.150 --> 00:16:14.630
But if something is making the
hydrogen concentration go up,

00:16:14.630 --> 00:16:16.530
if we put something in water
that makes it go up, what's

00:16:16.530 --> 00:16:19.180
that going to do the pH?

00:16:19.180 --> 00:16:23.220
Well, if this number right here
goes up, then the log of

00:16:23.220 --> 00:16:26.500
that number is going to go up.

00:16:26.500 --> 00:16:28.680
But then the minus log
of that number--

00:16:28.680 --> 00:16:32.330
So let's say for example
10 to the minus 7.

00:16:32.330 --> 00:16:32.920
That's all good.

00:16:32.920 --> 00:16:36.690
But let's say we take it
from 10 to the minus 7.

00:16:36.690 --> 00:16:39.180
So 10 to minus 7, that's
kind of neutral

00:16:39.180 --> 00:16:40.750
water, doing nothing.

00:16:40.750 --> 00:16:42.590
We have a pH.

00:16:42.590 --> 00:16:43.450
The pH is 7.

00:16:43.450 --> 00:16:45.380
We just figured out
this calculation.

00:16:45.380 --> 00:16:48.510
That's where the hydrogen
concentration is equal to 10

00:16:48.510 --> 00:16:49.520
to minus 7.

00:16:49.520 --> 00:16:51.350
Now let's say we were to do
something to the water where

00:16:51.350 --> 00:16:54.710
the hydrogen concentration
were to increase.

00:16:54.710 --> 00:16:58.440
So we add hydrogen.

00:16:58.440 --> 00:17:00.570
We add concentration
of hydrogen.

00:17:00.570 --> 00:17:03.270
So let's say 10 to
the minus 3.

00:17:03.270 --> 00:17:05.770
This is obviously a much
higher concentration of

00:17:05.770 --> 00:17:07.349
hydrogen than that.

00:17:07.349 --> 00:17:10.440
So the hydrogen concentration
is now 10 to the minus 3.

00:17:10.440 --> 00:17:12.200
What's our new pH?

00:17:12.200 --> 00:17:17.369
So the new pH is going to be
equal to the minus log base 10

00:17:17.369 --> 00:17:19.690
of 10 to the minus 3.

00:17:19.690 --> 00:17:22.569
That's equal to that
minus there.

00:17:22.569 --> 00:17:25.660
10 to the minus 3 power is equal
to 10 to the minus 3.

00:17:25.660 --> 00:17:26.775
So it's minus 3.

00:17:26.775 --> 00:17:28.210
So this becomes plus.

00:17:28.210 --> 00:17:33.100
So now our pH is equal to 3.

00:17:33.100 --> 00:17:36.430
So the big take-away from this
is we care about the hydrogen

00:17:36.430 --> 00:17:37.240
concentration.

00:17:37.240 --> 00:17:39.980
When something increases the
hydrogen concentration, that

00:17:39.980 --> 00:17:42.630
is considered an acid.

00:17:42.630 --> 00:17:44.720
And when you increase the
hydrogen concentration, when

00:17:44.720 --> 00:17:47.420
you put an acid into an aqueous
solution, you are

00:17:47.420 --> 00:17:48.980
lowering the pH.

00:17:48.980 --> 00:17:52.890
So the lower the pH, the more
acidic you have, or the higher

00:17:52.890 --> 00:17:57.830
concentration of hydrogen
protons you have. And we'll

00:17:57.830 --> 00:18:00.630
talk in the next video about the
opposite of that, which is

00:18:00.630 --> 00:18:02.260
which is basic things.

00:18:02.260 --> 00:18:06.060
And bases increase the
concentration of hydroxide.

00:18:06.060 --> 00:18:08.680
Anyway, see you in
the next video.


Mini-video on ion size

WEBVTT
Kind: captions
Language: en

00:00:00.720 --> 00:00:05.070
In the video on solubility, I
draw little pictures of sodium

00:00:05.070 --> 00:00:08.910
and chloride ions when sodium
chloride dissolves or

00:00:08.910 --> 00:00:10.500
disassociates into water.

00:00:10.500 --> 00:00:13.860
This is sodium and
this is chloride.

00:00:13.860 --> 00:00:16.950
And my simple brain, when I
looked at it, I said, OK, how

00:00:16.950 --> 00:00:18.110
should I draw these things?

00:00:18.110 --> 00:00:21.270
I said, well, they're in the
same period, and sodium is a

00:00:21.270 --> 00:00:22.190
Group 1 element.

00:00:22.190 --> 00:00:27.890
It's an alkali metal, while
chlorine is a halogen, so

00:00:27.890 --> 00:00:29.690
chlorine's going to have a
smaller atomic radius.

00:00:29.690 --> 00:00:32.220
And the logic there, just to
review from the atomic table

00:00:32.220 --> 00:00:36.070
trends, is that both of their
valence electrons are in the

00:00:36.070 --> 00:00:37.050
third shell.

00:00:37.050 --> 00:00:40.140
Sodium only has 11 protons
pulling in the center.

00:00:40.140 --> 00:00:43.280
It has 11 in the center, and it
has only one electron out

00:00:43.280 --> 00:00:44.955
there in its valence shell.

00:00:44.955 --> 00:00:47.590
So the attraction isn't as
strong as the case of

00:00:47.590 --> 00:00:53.060
chlorine, which has 17 protons
in the center.

00:00:53.060 --> 00:00:59.090
Although it has more valence
electrons-- it has 7 of them--

00:00:59.090 --> 00:01:01.660
these protons are going
to have a stronger

00:01:01.660 --> 00:01:02.640
attraction on them.

00:01:02.640 --> 00:01:04.700
So if you just look at the trend
in the periodic table,

00:01:04.700 --> 00:01:08.920
you'd expect the sodium neutral
atom to be bigger than

00:01:08.920 --> 00:01:10.240
the chlorine neutral atom.

00:01:10.240 --> 00:01:13.010
Because this guy has more
protons pulling everything in.

00:01:13.010 --> 00:01:16.490
And that's how I drew the
ions in that video.

00:01:16.490 --> 00:01:18.420
I said, oh, when I disassociate
in water, I'll

00:01:18.420 --> 00:01:23.770
have a big sodium ion and
a smaller chlorine

00:01:23.770 --> 00:01:26.080
ion, which is incorrect.

00:01:26.080 --> 00:01:28.460
Because think about-- and this
was pointed out to me by one

00:01:28.460 --> 00:01:30.660
of the viewers, and they're
correct, and I should have

00:01:30.660 --> 00:01:31.630
realized it.

00:01:31.630 --> 00:01:34.450
What happens when you
ionize these things?

00:01:34.450 --> 00:01:36.800
This guy will lose an
electron, right?

00:01:36.800 --> 00:01:38.620
He gives the electron
to this guy.

00:01:38.620 --> 00:01:41.530
So his electron configuration
is actually going to look a

00:01:41.530 --> 00:01:44.400
lot more like neon.

00:01:44.400 --> 00:01:47.600
He now will have no electrons
in that third shell, in the

00:01:47.600 --> 00:01:49.160
third energy state.

00:01:49.160 --> 00:01:51.810
So now he's going to have an
atomic radius that's actually

00:01:51.810 --> 00:01:54.990
much more similar to
neon here, right?

00:01:54.990 --> 00:01:57.560
Because he's going to have
filled up the second shelf.

00:01:57.560 --> 00:02:01.200
So actually, the sodium ion,
this is completely incorrect.

00:02:01.200 --> 00:02:04.330
The sodium ion is going to have
an atomic radius not that

00:02:04.330 --> 00:02:05.440
different than neon.

00:02:05.440 --> 00:02:07.080
Actually, it will be even a
little smaller than neon

00:02:07.080 --> 00:02:10.789
because it has the same electron
configuration, but it

00:02:10.789 --> 00:02:12.360
has one more proton.

00:02:12.360 --> 00:02:16.740
So the sodium ion is actually
going to be smaller.

00:02:16.740 --> 00:02:18.930
Because it gets rid of the
electron in that third shell,

00:02:18.930 --> 00:02:24.600
and the chlorine cation, gained
an electron, so it has

00:02:24.600 --> 00:02:26.490
completely completed
its third shell.

00:02:26.490 --> 00:02:29.010
So here you have where
the chlorine ion

00:02:29.010 --> 00:02:30.710
is going to be bigger.

00:02:30.710 --> 00:02:33.220
So in that solubility video, I
should've actually switched

00:02:33.220 --> 00:02:34.830
the places between the
sodium and the

00:02:34.830 --> 00:02:36.600
chlorine, at least in size-wise.

00:02:36.600 --> 00:02:38.880
And, of course, I showed how
they disassociate in water,

00:02:38.880 --> 00:02:42.340
and this would be attracted to
the oxygen end of the water,

00:02:42.340 --> 00:02:43.580
and you have the hydrogen
end and all that.

00:02:43.580 --> 00:02:45.970
But you can watch the solubility
video for that.

00:02:45.970 --> 00:02:48.260
It doesn't change the real
takeaway from the video.

00:02:48.260 --> 00:02:50.970
But I think this is a really
interesting point that it

00:02:50.970 --> 00:02:55.380
brings up, that when you ionize
these neutral atoms, it

00:02:55.380 --> 00:02:57.340
can significantly change,
especially significantly

00:02:57.340 --> 00:03:00.440
change their relative
atomic sizes.

00:03:00.440 --> 00:03:02.930
Anyway, hopefully, you found
that interesting.


Introduction to kinetics

WEBVTT
Kind: captions
Language: en

00:00:00.874 --> 00:00:02.385
- [Voiceover] When
you're studying chemistry

00:00:02.385 --> 00:00:03.894
you'll often see reactions,

00:00:03.894 --> 00:00:05.903
in fact you always see reactions.

00:00:05.903 --> 00:00:08.902
For example if you have hydrogen gas

00:00:09.665 --> 00:00:11.086
it's a diatomic molecule,

00:00:11.086 --> 00:00:13.558
'cause hydrogen bonds with
itself in the gassy state,

00:00:13.558 --> 00:00:15.459
plus iodine gas,

00:00:17.403 --> 00:00:18.944
I2, that's also in the gassy state,

00:00:18.944 --> 00:00:20.385
it's very easy to just sort of,

00:00:20.385 --> 00:00:22.018
oh you know, if you put 'em together

00:00:22.018 --> 00:00:24.202
they're going to react
and form the product,

00:00:24.725 --> 00:00:26.209
if you have two moles of,

00:00:26.209 --> 00:00:28.528
hydrogen, two moles of iodine,

00:00:28.528 --> 00:00:31.383
so it's gonna form two moles of

00:00:31.383 --> 00:00:34.314
hydrogen iodide.

00:00:35.053 --> 00:00:36.731
That's all nice and neat

00:00:36.731 --> 00:00:38.873
and it makes it seem like
it's a very clean thing

00:00:38.873 --> 00:00:41.069
that happens without much fuss.

00:00:41.319 --> 00:00:43.069
But we know that that isn't the reality

00:00:43.069 --> 00:00:45.271
and we also know that this
doesn't happen just instantly,

00:00:45.271 --> 00:00:47.001
it's not like you can
just take some hydrogen,

00:00:47.001 --> 00:00:49.777
put it with some iodine,

00:00:49.777 --> 00:00:52.179
and it just magically
turns into hydrogen iodide.

00:00:52.179 --> 00:00:53.827
That there's some process going on,

00:00:53.827 --> 00:00:57.530
that these gaseous state
particles are bouncing around,

00:00:57.530 --> 00:00:59.541
and somehow they must
bounce into each other

00:00:59.541 --> 00:01:02.341
and break bonds that they were in before,

00:01:02.341 --> 00:01:03.867
and form new ones,

00:01:03.867 --> 00:01:05.201
and that's what we're going to study now.

00:01:05.201 --> 00:01:08.428
This whole study of how
the reaction progresses,

00:01:08.428 --> 00:01:11.104
and the rates of the
reactions is called kinetics.

00:01:11.576 --> 00:01:13.356
Which is a very fancy word,

00:01:13.356 --> 00:01:14.571
but you're probably familiar with it

00:01:14.571 --> 00:01:16.370
because we've talked a
lot about kinetic energy.

00:01:16.945 --> 00:01:18.180
Kinetics.

00:01:20.897 --> 00:01:24.022
Which is just the study
of the rate of reactions.

00:01:24.202 --> 00:01:26.564
How fast do they happen,
and how do they happen?

00:01:26.857 --> 00:01:28.281
So let's just in our minds,

00:01:28.281 --> 00:01:30.332
come up with a intuitive way

00:01:30.332 --> 00:01:33.065
that hydrogen and iodine can combine.

00:01:33.140 --> 00:01:34.860
So let's think about
what hydrogen looks like.

00:01:34.860 --> 00:01:36.620
So if we get our periodic table out,

00:01:36.620 --> 00:01:38.764
hydrogen's got one valence electron

00:01:38.764 --> 00:01:40.708
so if they have two hydrogen atoms

00:01:40.708 --> 00:01:42.245
they can share them with each other.

00:01:42.245 --> 00:01:43.836
And then iodine,

00:01:44.805 --> 00:01:47.408
iodine has seven valence electrons,

00:01:47.408 --> 00:01:49.800
so if they each share one
they get complete as well.

00:01:49.800 --> 00:01:51.880
So let's just review that right now.

00:01:51.880 --> 00:01:53.336
So hydrogen

00:01:53.336 --> 00:01:54.976
this hydrogen might have one,

00:01:55.622 --> 00:01:58.197
well, will have one electron out there.

00:01:58.330 --> 00:01:59.961
And then you can have another hydrogen

00:01:59.961 --> 00:02:02.163
that has another electron out there,

00:02:02.163 --> 00:02:04.663
and then if they form
a bond they share this,

00:02:04.663 --> 00:02:07.240
this hydrogen can pretend
like he has this electron,

00:02:07.240 --> 00:02:09.431
this hydrogen can pretend
like she has that electron,

00:02:09.431 --> 00:02:10.261
and then they're happy.

00:02:10.261 --> 00:02:13.447
They both feel like they've
completed their 1S shell.

00:02:13.447 --> 00:02:15.827
Same thing on the iodine side.

00:02:16.613 --> 00:02:17.950
Where you have two iodines,

00:02:17.950 --> 00:02:19.905
they both have seven valence electrons.

00:02:20.514 --> 00:02:22.260
They're halogens,

00:02:22.260 --> 00:02:23.527
you know that already.

00:02:24.332 --> 00:02:26.195
Halogens are the group seven elements,

00:02:26.795 --> 00:02:28.824
so they have seven electrons

00:02:29.004 --> 00:02:31.348
this guy's got one here,
this guy's got one here,

00:02:31.348 --> 00:02:33.243
if this guy can pretend
like he's got that electron,

00:02:33.243 --> 00:02:35.220
he's happy, he has
eight valence electrons.

00:02:35.220 --> 00:02:37.178
If this guy can pretend
like he's got that one,

00:02:37.178 --> 00:02:38.084
same thing.

00:02:38.084 --> 00:02:39.801
So there's a bond right here,

00:02:39.801 --> 00:02:43.853
and this is why hydrogen is
a diatomic molecular gas,

00:02:44.041 --> 00:02:45.741
and this is why iodine is the same.

00:02:45.833 --> 00:02:48.553
Now, when they're in the gaseous state,

00:02:48.553 --> 00:02:50.929
you have a bunch of these things that are

00:02:50.929 --> 00:02:53.239
moving around bumping into each other,

00:02:53.545 --> 00:02:54.420
I'll do it like this.

00:02:54.420 --> 00:02:56.456
So the hydrogen might
look something like this,

00:02:57.668 --> 00:03:00.768
the hydrogen is these two atomic spheres

00:03:00.772 --> 00:03:02.051
that are bonded together,

00:03:02.051 --> 00:03:05.180
these electrons in between
that are keeping them bonded.

00:03:05.483 --> 00:03:07.235
The iodine might look something like this,

00:03:07.235 --> 00:03:08.828
it's a much bigger molecule.

00:03:10.785 --> 00:03:12.740
Where it's bonded together like this,

00:03:13.192 --> 00:03:16.044
it's also sharing some
electrons in a covalent bond,

00:03:16.531 --> 00:03:17.984
everything's probabilistic.

00:03:17.984 --> 00:03:20.961
So in order for these two
molecules to turn to this,

00:03:20.961 --> 00:03:22.854
somehow these bonds have to be broken,

00:03:22.854 --> 00:03:24.498
and new bonds have to be formed.

00:03:24.734 --> 00:03:26.278
And what has to happen is that these guys,

00:03:26.278 --> 00:03:27.525
there's a ton of these guys.

00:03:27.525 --> 00:03:29.071
I could draw a bunch of them.

00:03:29.071 --> 00:03:30.790
Or I could copy and paste.

00:03:31.636 --> 00:03:33.234
So there's a bunch of...

00:03:34.177 --> 00:03:37.094
There's a bunch of
hydrogen molecules around,

00:03:37.094 --> 00:03:40.658
and some of these iodine

00:03:40.918 --> 00:03:42.730
gas molecules around.

00:03:45.246 --> 00:03:46.631
So what has to happen in order for us

00:03:46.631 --> 00:03:47.918
to get the hydrogen iodide is,

00:03:47.918 --> 00:03:48.783
they have to collide.

00:03:48.783 --> 00:03:50.654
And they have to collide
in exactly the right way.

00:03:50.654 --> 00:03:52.635
So let's say this guy,
wish I could show it.

00:03:52.635 --> 00:03:54.315
Let's say he's moving, this is neat,

00:03:54.315 --> 00:03:55.587
I'm just dragging and dropping.

00:03:55.587 --> 00:03:56.355
But he's moving.

00:03:56.355 --> 00:03:59.935
He has to hit this hydrogen
molecule just right,

00:04:00.274 --> 00:04:02.808
And maybe just right, if
he just happens to hit it

00:04:02.808 --> 00:04:04.623
and bounce it with enough energy,

00:04:04.623 --> 00:04:05.959
then all of the sudden,

00:04:05.959 --> 00:04:07.711
let's say we get to this point right here,

00:04:08.741 --> 00:04:10.624
these electrons are
gonna say "Hey, you know,

00:04:10.624 --> 00:04:12.971
"it's nice to be shared this way,

00:04:12.971 --> 00:04:14.465
"we're in a stable configuration,

00:04:14.465 --> 00:04:16.156
"we're filling the 1S
shell, but look at this,

00:04:16.156 --> 00:04:18.685
"there's this iodine that's close by

00:04:18.685 --> 00:04:20.114
"and they really want me,

00:04:20.114 --> 00:04:21.828
"they're much more electro-negative

00:04:21.828 --> 00:04:23.423
"than me, the hydrogen".

00:04:23.450 --> 00:04:25.148
So maybe they're kind of attracted here,

00:04:25.148 --> 00:04:26.402
they don't know whether they wanna be

00:04:26.402 --> 00:04:28.459
here between that hydrogen

00:04:28.459 --> 00:04:30.059
and this right here between that.

00:04:30.059 --> 00:04:32.780
And so they kind of enter
this higher energy state.

00:04:32.780 --> 00:04:35.613
And similarly, you know
these guys they say,

00:04:35.613 --> 00:04:36.992
"Hey, wouldn't it be nicer,

00:04:36.992 --> 00:04:39.968
"I don't have to be here, I
could kind of go back home

00:04:39.968 --> 00:04:43.995
"to my home atom if
this guy comes in here".

00:04:44.017 --> 00:04:45.523
Because then we're gonna have,

00:04:45.523 --> 00:04:47.435
then we're gonna have
eight valence electrons

00:04:47.435 --> 00:04:49.070
and the same thing's happening here.

00:04:49.516 --> 00:04:51.268
And this complex right here,

00:04:51.268 --> 00:04:53.456
this kind of,

00:04:53.634 --> 00:04:55.421
right when the collision happens,

00:04:55.421 --> 00:04:57.132
this is actually a state,

00:04:57.132 --> 00:04:58.445
this is the high energy state

00:04:58.445 --> 00:05:00.140
of the transition state of the reaction,

00:05:00.140 --> 00:05:02.489
and this is called an activated complex.

00:05:03.739 --> 00:05:05.176
Activated complex.

00:05:10.715 --> 00:05:14.108
Sometimes you know I just
drew it kind of visually,

00:05:14.108 --> 00:05:16.011
but you could draw it like this.

00:05:16.011 --> 00:05:19.175
So hydrogen has a covalent
bond with another hydrogen,

00:05:20.356 --> 00:05:23.019
and then here comes along some iodine

00:05:23.526 --> 00:05:26.480
that has a covalent bond
with some other iodine,,

00:05:26.692 --> 00:05:29.209
but all the sudden these
guys like to bond as well.

00:05:29.243 --> 00:05:30.556
So they start forming,

00:05:30.556 --> 00:05:32.875
so there is kind of a you know,

00:05:32.875 --> 00:05:35.939
there's a little bit of an
attraction on that side too.

00:05:35.939 --> 00:05:38.380
So this is another way of
drawing the activation complex.

00:05:38.380 --> 00:05:39.893
But this is a high energy state,

00:05:39.893 --> 00:05:41.506
'cause in order for the electrons,

00:05:41.506 --> 00:05:42.628
the way you can think of it,

00:05:42.628 --> 00:05:44.717
to kind of go from that bond to this bond,

00:05:44.717 --> 00:05:46.590
or this bond to that bond,

00:05:46.590 --> 00:05:47.559
or to go back,

00:05:47.559 --> 00:05:50.077
they have to enter into
a higher energy state.

00:05:50.077 --> 00:05:51.916
A less stable energy state,

00:05:51.916 --> 00:05:53.263
than they were before.

00:05:53.263 --> 00:05:55.124
But they do that if there's enough energy,

00:05:55.985 --> 00:05:57.118
'cause you can go from,

00:05:57.118 --> 00:06:00.440
so you're going from both
of these things separate,

00:06:00.892 --> 00:06:02.351
let me just draw them separate,

00:06:02.644 --> 00:06:04.740
so you have both of them separate,

00:06:05.878 --> 00:06:07.279
you have the hydrogen separate,

00:06:07.279 --> 00:06:09.435
plus the iodine separate.

00:06:10.286 --> 00:06:12.519
They go to this, which
is a higher energy state.

00:06:12.519 --> 00:06:14.968
But if they can get to
that higher energy state,

00:06:14.968 --> 00:06:16.536
if there's enough energy for the collision

00:06:16.536 --> 00:06:19.127
and they have enough kinetic energy

00:06:19.127 --> 00:06:20.703
when they hit in the right orientation,

00:06:20.703 --> 00:06:23.757
then, from this activated complex

00:06:23.757 --> 00:06:25.053
or this higher energy state,

00:06:25.053 --> 00:06:26.885
it will then go to the
lowest energy state,

00:06:26.885 --> 00:06:28.606
and the lowest energy state is

00:06:28.606 --> 00:06:30.687
the hydrogen iodide.

00:06:33.021 --> 00:06:35.011
I wanna draw the iodide,

00:06:35.665 --> 00:06:37.043
and then the hydrogen.

00:06:39.385 --> 00:06:40.550
This is actually,

00:06:40.550 --> 00:06:42.947
this is actually a lower
energy state than this.

00:06:43.119 --> 00:06:45.159
But in order to get here

00:06:45.159 --> 00:06:46.863
you have to go through
a higher energy state.

00:06:46.863 --> 00:06:48.011
And I could do that

00:06:48.011 --> 00:06:49.600
with an energy diagram.

00:06:49.600 --> 00:06:50.727
So if we say that,

00:06:50.727 --> 00:06:53.795
let's say the X-axis is the
progression of the reaction,

00:07:01.245 --> 00:07:02.189
and actually you know

00:07:02.189 --> 00:07:03.672
we don't know how fast it's progressing,

00:07:03.672 --> 00:07:05.293
but this you can kind of view it as time

00:07:05.293 --> 00:07:06.822
on some dimension,

00:07:06.822 --> 00:07:09.183
and let's say this is
the potential energy.

00:07:10.138 --> 00:07:11.847
I wanna draw thicker lines.

00:07:12.771 --> 00:07:14.478
See this is the potential energy.

00:07:15.626 --> 00:07:17.758
Right there, let me make
this line thicker as well.

00:07:21.870 --> 00:07:25.510
So this is the potential energy.

00:07:26.322 --> 00:07:29.161
So initially, you are at this reality,

00:07:29.161 --> 00:07:32.074
and we can kind of view it as
the combined potential energy,

00:07:32.074 --> 00:07:33.426
so this is where eventually,

00:07:33.426 --> 00:07:34.627
we start off here,

00:07:35.027 --> 00:07:39.424
and this is the H2 plus I2

00:07:40.025 --> 00:07:41.866
And a lower potential energy is

00:07:41.866 --> 00:07:43.413
when we were in the hydrogen iodide.

00:07:43.413 --> 00:07:45.793
So this is the lower
potential energy down here.

00:07:47.414 --> 00:07:49.225
Lower potential energy down here.

00:07:49.621 --> 00:07:53.781
This is the 2HI, right?

00:07:53.781 --> 00:07:54.730
But to get here,

00:07:54.730 --> 00:07:56.293
we have to enter this
higher activation energy,

00:07:56.293 --> 00:07:57.557
where the electrons have to get,

00:07:57.557 --> 00:07:59.524
they have to have some
energy to kind of be able to

00:07:59.524 --> 00:08:01.692
at least figure out what they
wanna do with their lives.

00:08:02.052 --> 00:08:03.916
And so you have to add
energy to the system,

00:08:03.916 --> 00:08:05.220
you don't always have to add it,

00:08:05.220 --> 00:08:07.261
but if it doesn't happen spontaneously

00:08:07.261 --> 00:08:09.116
you're gonna have to add
some energy to the system

00:08:10.438 --> 00:08:12.542
to get to this activated state.

00:08:16.024 --> 00:08:19.022
RIght, so this is when we
were at this thing right here.

00:08:19.976 --> 00:08:20.725
We're there.

00:08:20.725 --> 00:08:22.114
So some energy has to be in the system,

00:08:22.114 --> 00:08:23.258
and this energy,

00:08:23.258 --> 00:08:25.378
the difference between the energy we're at

00:08:25.378 --> 00:08:28.276
when we were just hydrogen
molecules and iodine molecules,

00:08:28.276 --> 00:08:29.760
and the energy we have to get to

00:08:29.760 --> 00:08:31.369
to get this activated state,

00:08:31.369 --> 00:08:33.089
this distance right here

00:08:34.304 --> 00:08:36.417
this is the activation energy.

00:08:38.988 --> 00:08:42.069
If we're able to get to somehow put enough

00:08:42.069 --> 00:08:43.197
energy in the system,

00:08:43.197 --> 00:08:44.566
then this thing will happen,

00:08:44.566 --> 00:08:45.918
they'll collide with enough energy

00:08:45.918 --> 00:08:47.893
and bonds will be broken and reformed.

00:08:47.893 --> 00:08:49.546
Activation energy.

00:08:49.645 --> 00:08:53.345
Sometimes it's written as
Ea energy of activation,

00:08:53.420 --> 00:08:55.334
and in the future we'll maybe do reactions

00:08:55.334 --> 00:08:57.101
where we actually measure
the activation energy.

00:08:57.101 --> 00:08:58.853
But the important thing is
to conceptually understand

00:08:58.853 --> 00:09:00.175
that it's there,

00:09:00.175 --> 00:09:03.496
that things just don't
spontaneously go from here to here.

00:09:03.950 --> 00:09:06.044
And I won't go in deeply
into catalysts right now,

00:09:06.044 --> 00:09:07.916
but you've probably heard
of the word catalyst

00:09:07.916 --> 00:09:09.652
or something being catalyzed.

00:09:09.652 --> 00:09:10.916
And that's something,

00:09:10.916 --> 00:09:12.580
some other agent,

00:09:12.580 --> 00:09:15.256
some other thing in the reactions.

00:09:15.462 --> 00:09:16.620
So right now,

00:09:17.296 --> 00:09:18.813
so right now we're doing,

00:09:19.843 --> 00:09:22.808
we have H2 plus I2,

00:09:23.518 --> 00:09:26.606
yielding 2H hydrogen iodides.

00:09:27.236 --> 00:09:28.708
Now you could have a catalyst,

00:09:28.708 --> 00:09:29.874
and I'll just say plus C.

00:09:30.925 --> 00:09:32.439
And I actually don't
know what a good catalyst

00:09:32.439 --> 00:09:33.811
would be for this reaction,

00:09:33.821 --> 00:09:36.882
and how a catalyst operates is,

00:09:36.882 --> 00:09:38.972
it can actually operate in
many many different ways,

00:09:38.972 --> 00:09:40.572
so that's why I don't
wanna do it in this video.

00:09:40.572 --> 00:09:43.535
But what a catalyst is, is
something that doesn't change.

00:09:43.535 --> 00:09:45.588
It doesn't get consumed in the reaction.

00:09:45.588 --> 00:09:47.348
The catalyst was there
before the reaction,

00:09:47.348 --> 00:09:49.380
the catalyst is there after the reaction.

00:09:49.380 --> 00:09:51.679
But what it does is it makes the reaction

00:09:51.679 --> 00:09:53.831
happen either faster,

00:09:53.831 --> 00:09:55.758
or it lowers the amount of energy

00:09:55.758 --> 00:09:57.039
for the reaction to happen.

00:09:57.039 --> 00:09:58.190
Which is kind of the same thing.

00:09:58.190 --> 00:09:59.303
So if you have a catalyst,

00:09:59.303 --> 00:10:01.086
then this activation energy

00:10:01.086 --> 00:10:03.075
will be lower.

00:10:03.414 --> 00:10:04.679
And what it does is, it makes it,

00:10:04.679 --> 00:10:07.238
it might easily, it might be some molecule

00:10:07.238 --> 00:10:09.137
that allows some other transition state

00:10:09.137 --> 00:10:11.097
that has less of a potential energy

00:10:11.097 --> 00:10:13.633
so that you require less heat or less

00:10:13.633 --> 00:10:15.608
concentration of the molecules

00:10:15.608 --> 00:10:17.664
for them to bump into each
other in the right direction,

00:10:17.664 --> 00:10:19.536
to get to that other state.

00:10:19.536 --> 00:10:21.253
So you require less energy.

00:10:21.697 --> 00:10:23.481
So given how we understand

00:10:24.281 --> 00:10:25.968
how these kinetics occur,

00:10:25.968 --> 00:10:28.171
these molecules interact with each other,

00:10:28.171 --> 00:10:30.000
what do you think are the things that

00:10:30.000 --> 00:10:33.339
will drive whether a
reaction happens or not?

00:10:33.558 --> 00:10:35.544
I mean we already know that if
we have a positive catalyst,

00:10:35.544 --> 00:10:37.474
there's something called
a negative catalyst that

00:10:37.474 --> 00:10:39.030
will actually slow down a reaction.

00:10:39.030 --> 00:10:40.886
But if we have a positive catalyst,

00:10:41.670 --> 00:10:42.636
it lowers...

00:10:45.002 --> 00:10:46.843
Obviously it lowers the activation energy,

00:10:46.843 --> 00:10:50.176
so this makes reaction faster.

00:10:54.631 --> 00:10:57.572
More molecules are gonna bump
into each other just right

00:10:57.760 --> 00:10:59.056
to be able to get over this hump

00:10:59.056 --> 00:11:00.464
'cause the hump will be lower,

00:11:00.464 --> 00:11:01.485
when you have a catalyst.

00:11:01.485 --> 00:11:03.493
Also if you increase the concentration.

00:11:04.016 --> 00:11:06.895
Right? If you increase your concentration,

00:11:07.635 --> 00:11:08.893
of molecules,

00:11:09.449 --> 00:11:10.550
if the concentration goes up,

00:11:10.550 --> 00:11:12.755
then you just have more stuff
to bump into each other.

00:11:13.551 --> 00:11:14.383
Right?

00:11:14.383 --> 00:11:15.319
There's just the likelihood.

00:11:15.319 --> 00:11:16.503
Everything is probabilistic.

00:11:16.503 --> 00:11:19.328
You know when people write
these reaction equations,

00:11:19.328 --> 00:11:22.455
it all seems nice and simple
and very clear and it happens.

00:11:22.455 --> 00:11:24.357
But no, in the real world,

00:11:24.357 --> 00:11:26.527
you just have things
bumping into each other.

00:11:26.527 --> 00:11:28.454
And when we do biology videos

00:11:28.454 --> 00:11:29.742
it'll be fascinating to talk about.

00:11:29.742 --> 00:11:32.144
Because all of, every
biological process is

00:11:32.144 --> 00:11:33.664
really just a chemical process.

00:11:34.110 --> 00:11:35.734
And it's really just a byproduct of

00:11:35.734 --> 00:11:38.474
all of these things
bumping into each other.

00:11:38.742 --> 00:11:40.869
And you can imagine the
more concentration you have

00:11:40.869 --> 00:11:42.805
of the things that need
to bump into each other,

00:11:42.805 --> 00:11:44.141
the more likely you're gonna get

00:11:44.141 --> 00:11:45.714
just that perfect bump,

00:11:45.714 --> 00:11:47.317
and that perfect amount of kinetic energy

00:11:47.317 --> 00:11:48.782
for the reaction to happen.

00:11:49.447 --> 00:11:51.552
And actually I'll make a
little other note here.

00:11:51.552 --> 00:11:53.795
This reaction you might
say, "Okay I have some,

00:11:53.795 --> 00:11:55.302
"let's say I'm at this energy,

00:11:55.302 --> 00:11:56.588
"how do I ever get over this?

00:11:56.588 --> 00:11:57.885
"How does this ever react?".

00:11:57.885 --> 00:11:59.965
Well remember, in a gas,

00:11:59.965 --> 00:12:01.862
the kinetic energies of
all of the molecules,

00:12:01.862 --> 00:12:02.988
they're not uniform.

00:12:02.988 --> 00:12:04.568
Some gases have,

00:12:04.568 --> 00:12:06.431
some molecules will have
higher kinetic energy,

00:12:06.431 --> 00:12:07.128
some will have lower,

00:12:07.128 --> 00:12:08.627
temperature just gives you the average.

00:12:08.627 --> 00:12:11.224
So there's always some
probability that two

00:12:11.224 --> 00:12:13.166
maybe high kinetic energy molecules

00:12:13.166 --> 00:12:15.702
will bump into each other just perfectly,

00:12:16.132 --> 00:12:17.281
surpass the kinetic--

00:12:17.281 --> 00:12:18.609
So they have enough kinetic energy

00:12:18.609 --> 00:12:20.284
to get into the activation state,

00:12:20.284 --> 00:12:22.240
and then they can go to the lower state,

00:12:22.240 --> 00:12:23.717
which is the hydrogen iodide.

00:12:23.841 --> 00:12:24.817
So even at a,

00:12:24.817 --> 00:12:26.724
at all temperatures this will occur,

00:12:26.724 --> 00:12:28.577
but obviously if you
increase the temperature,

00:12:28.577 --> 00:12:29.959
if you increase the temperature

00:12:29.959 --> 00:12:31.685
that reaction is more likely.

00:12:32.129 --> 00:12:33.016
So that's the other one.

00:12:33.016 --> 00:12:34.121
So temperature.

00:12:34.121 --> 00:12:35.600
Temperature is actually
probably the biggest.

00:12:36.753 --> 00:12:39.292
Temperature is probably
the single biggest thing

00:12:39.292 --> 00:12:42.159
that will make the reaction happen faster.

00:12:43.707 --> 00:12:44.644
So all of these things,

00:12:44.644 --> 00:12:45.876
you want higher temperature,

00:12:45.876 --> 00:12:47.146
higher reaction.

00:12:47.650 --> 00:12:48.640
And then if you just wanna think about

00:12:48.640 --> 00:12:50.114
the molecules itself,

00:12:50.114 --> 00:12:52.497
if you have molecules where

00:12:52.497 --> 00:12:54.728
their original bonds are weak,

00:12:55.497 --> 00:12:57.098
they're more likely to
be able to interact.

00:12:57.098 --> 00:12:58.497
And there's other things
you could talk about,

00:12:58.497 --> 00:12:59.618
the molecular shape,

00:12:59.618 --> 00:13:01.417
how available certain atoms are

00:13:01.417 --> 00:13:02.740
to interact with other atoms,

00:13:02.740 --> 00:13:04.050
and that really becomes significant

00:13:04.050 --> 00:13:05.857
when we start going into biology.

00:13:05.857 --> 00:13:08.169
And then the last one, and
you probably realize this,

00:13:08.169 --> 00:13:09.853
is just the surface area.

00:13:10.852 --> 00:13:12.552
If you increase the surface area,

00:13:12.552 --> 00:13:15.186
so we were just doing
gas-gas interactions,

00:13:15.186 --> 00:13:16.849
which almost by definition have

00:13:16.849 --> 00:13:18.383
pretty good surface area interactions.

00:13:18.383 --> 00:13:19.839
But if the surface area goes up,

00:13:19.839 --> 00:13:21.887
then the reaction also goes up.

00:13:21.887 --> 00:13:23.175
The reaction rate.

00:13:24.328 --> 00:13:25.364
And how do you think about that?

00:13:25.364 --> 00:13:26.852
Well, think about the reaction of,

00:13:27.431 --> 00:13:29.406
think about the reaction
of, you know we've done this

00:13:29.406 --> 00:13:30.455
multiple times.

00:13:30.455 --> 00:13:32.347
Sodium chloride,

00:13:33.408 --> 00:13:35.475
solid, so solid salt,

00:13:35.895 --> 00:13:38.226
plus liquid water,

00:13:40.293 --> 00:13:41.443
leads to

00:13:41.894 --> 00:13:44.773
sodium, well we could think
of it a lot of different ways,

00:13:44.998 --> 00:13:48.970
but we could think of it
as sodium ion aqueous,

00:13:50.343 --> 00:13:53.610
plus chloride anions,
there's a cation anion,

00:13:54.007 --> 00:13:55.298
aqueous.

00:13:55.480 --> 00:13:56.611
So it gets dissolved.

00:13:56.611 --> 00:13:57.677
And how does that happen?

00:13:57.677 --> 00:13:59.312
If you have a big block of ice,

00:13:59.312 --> 00:14:00.481
not of ice, of salt.

00:14:01.696 --> 00:14:03.443
You have a big, I'll do salt in grey.

00:14:03.443 --> 00:14:05.943
If you have a big block of salt in there,

00:14:06.573 --> 00:14:08.541
and you have, so there's a bunch of

00:14:08.541 --> 00:14:10.194
sodium and chloride

00:14:10.978 --> 00:14:13.126
atoms in it.

00:14:13.623 --> 00:14:15.341
And you have water all around it,

00:14:17.202 --> 00:14:18.957
the water is only gonna
be able to interact

00:14:18.957 --> 00:14:20.325
with the surface molecules,

00:14:20.325 --> 00:14:22.169
and slowly dissolve away the salt.

00:14:22.297 --> 00:14:24.718
Slowly make polar bonds.

00:14:24.718 --> 00:14:26.101
These are actually,

00:14:26.101 --> 00:14:28.981
well, polar dipole bonds with a different,

00:14:29.488 --> 00:14:32.457
the different sodium or chloride ions.

00:14:32.771 --> 00:14:35.643
But if you were to break
this up into smaller cubes,

00:14:37.021 --> 00:14:38.225
if you were to break it up or really

00:14:38.225 --> 00:14:40.234
crush it into really small pieces,

00:14:41.446 --> 00:14:43.197
then all of the sudden the surface area

00:14:43.197 --> 00:14:45.429
that the water molecules
can interact with,

00:14:45.429 --> 00:14:48.185
it can actually interact with
more of the sodium chloride.

00:14:48.185 --> 00:14:49.984
So the reaction will happen faster.

00:14:49.984 --> 00:14:50.864
So surface area,

00:14:50.864 --> 00:14:52.375
you increase the surface
area of interaction,

00:14:52.375 --> 00:14:54.119
and you'll also increase
the reaction rate.

00:14:54.119 --> 00:14:56.417
If you're trying to do it with two fluids,

00:14:56.417 --> 00:14:58.242
what you could do is you could kind of

00:14:58.242 --> 00:14:59.688
spray one fluid into the other,

00:14:59.688 --> 00:15:00.703
so you have little droplets

00:15:00.703 --> 00:15:02.403
so you also increase the surface area.

00:15:02.646 --> 00:15:03.366
So anyway,

00:15:03.366 --> 00:15:05.819
this is kind of an introduction
to the idea of kinetics.

00:15:05.958 --> 00:15:08.326
But hopefully gives you a
sense that these reactions,

00:15:08.326 --> 00:15:10.383
and I want you to really think
about chemistry this way,

00:15:10.383 --> 00:15:11.528
not think about it as,

00:15:11.528 --> 00:15:13.045
"Oh it's just some formula
I have to remember".

00:15:13.045 --> 00:15:16.252
That that these really are
bumps and bruises between atoms,

00:15:16.252 --> 00:15:18.306
that it's probabilistic and it's messy

00:15:18.306 --> 00:15:19.994
and we really have to think about

00:15:19.994 --> 00:15:22.939
what will make it more likely
that these things collide

00:15:22.939 --> 00:15:25.734
in just the perfect way for
the reactions to happen.


Boiling point elevation and freezing point depression

WEBVTT
Kind: captions
Language: en

00:00:00.790 --> 00:00:04.310
Let's think about what might
happen to the boiling point or

00:00:04.310 --> 00:00:08.550
the freezing point of any
solution if we start adding

00:00:08.550 --> 00:00:10.860
particles, or we start
adding solute to it.

00:00:10.860 --> 00:00:14.470
For our visualization, let's
just think about water again.

00:00:14.470 --> 00:00:15.550
It doesn't have to be water.

00:00:15.550 --> 00:00:17.880
It can be any solvent, but let's
just think about water

00:00:17.880 --> 00:00:19.640
in its liquid state.

00:00:19.640 --> 00:00:24.100
The particles are reasonably
disorganized because of their

00:00:24.100 --> 00:00:27.150
kinetic energy, but they still
have that hydrogen bonds that

00:00:27.150 --> 00:00:30.340
wants to make them be
near each other.

00:00:30.340 --> 00:00:32.240
So this is in the liquid
state, and they have a

00:00:32.240 --> 00:00:34.740
reasonable amount of
kinetic energy.

00:00:34.740 --> 00:00:37.120
You know, each of these
particles is moving in some

00:00:37.120 --> 00:00:39.460
direction, rubbing against each
other, bouncing off of

00:00:39.460 --> 00:00:40.490
each other.

00:00:40.490 --> 00:00:44.910
Now, to move it into the solid
state, or to freeze it, what

00:00:44.910 --> 00:00:46.250
has to happen?

00:00:46.250 --> 00:00:48.620
The ice has to enter kind of
a crystalline structure.

00:00:48.620 --> 00:00:51.710
It has to get pretty organized,
so let's say it has

00:00:51.710 --> 00:00:54.530
to look something like this.

00:00:54.530 --> 00:00:56.610
The water molecules are going
to have a regular structure

00:00:56.610 --> 00:01:01.460
where the hydrogen bonds
dominate any kind of kinetic

00:01:01.460 --> 00:01:03.340
movement they want to do, and
all the kinetic movement,

00:01:03.340 --> 00:01:04.980
they're just vibrating
in place.

00:01:04.980 --> 00:01:07.840
So you have to get a
little bit orderly

00:01:07.840 --> 00:01:10.060
right there, right?

00:01:10.060 --> 00:01:12.310
And then, obviously, this
lattice structure goes on and

00:01:12.310 --> 00:01:17.150
on with a gazillion
water molecules.

00:01:17.150 --> 00:01:19.470
But the interesting
thing is that this

00:01:19.470 --> 00:01:20.880
somehow has to get organized.

00:01:20.880 --> 00:01:25.540
And what happens if we start
introducing molecules into

00:01:25.540 --> 00:01:26.250
this water?

00:01:26.250 --> 00:01:30.130
Let's say the example of
sodium-- actually, I won't do

00:01:30.130 --> 00:01:30.750
any example.

00:01:30.750 --> 00:01:32.770
Let's just say some arbitrary
molecule, if I were to

00:01:32.770 --> 00:01:35.070
introduce it there, if I
were to put something--

00:01:35.070 --> 00:01:35.930
let me draw it again.

00:01:35.930 --> 00:01:40.350
So now I'll just use that same--
I'll introduce some

00:01:40.350 --> 00:01:43.300
molecules, and let's say they're
pretty large, so they

00:01:43.300 --> 00:01:46.060
push all of these water
molecules out of the way.

00:01:46.060 --> 00:01:48.900
So the water molecules are now
on the outside of that, and

00:01:48.900 --> 00:01:52.890
let's have another one that's
over here, some relatively

00:01:52.890 --> 00:01:55.860
large molecules of solute
relative to water, and this is

00:01:55.860 --> 00:01:58.890
because a water molecule
really isn't that big.

00:01:58.890 --> 00:02:02.310
Now, do you think it's going
to be easier or harder to

00:02:02.310 --> 00:02:03.220
freeze this?

00:02:03.220 --> 00:02:06.040
Are you going to have to remove
more or less energy to

00:02:06.040 --> 00:02:08.199
get to a frozen state?

00:02:08.199 --> 00:02:10.979
Well, because these molecules,
they're not going to be part

00:02:10.979 --> 00:02:13.690
of this lattice structure
because frankly, they wouldn't

00:02:13.690 --> 00:02:14.780
even fit into it.

00:02:14.780 --> 00:02:16.990
They're actually going to make
it harder for these water

00:02:16.990 --> 00:02:20.495
molecules to get organized
because to get organized, they

00:02:20.495 --> 00:02:22.500
have to get at the right
distance for the hydrogen

00:02:22.500 --> 00:02:23.330
bonds to form.

00:02:23.330 --> 00:02:26.600
But in this case, even as you
start removing heat from the

00:02:26.600 --> 00:02:33.000
system, maybe the ones that
aren't near the solute

00:02:33.000 --> 00:02:35.440
particles, they'll start to
organize with each other.

00:02:39.020 --> 00:02:41.500
But then when you introduce a
solute particle, let's say a

00:02:41.500 --> 00:02:44.740
solute particle is sitting
right here.

00:02:44.740 --> 00:02:47.340
It's going to be very hard for
someone to organize with this

00:02:47.340 --> 00:02:50.240
guy, to get near enough for
the hydrogen bond to start

00:02:50.240 --> 00:02:52.470
taking hold.

00:02:52.470 --> 00:02:54.280
This distance would make
it very difficult.

00:02:54.280 --> 00:02:57.220
And so the way I think about
it is that these solute

00:02:57.220 --> 00:03:00.170
particles make the structure
irregular, or they add more

00:03:00.170 --> 00:03:01.710
disorder, and we'll eventually
talk about

00:03:01.710 --> 00:03:03.000
entropy and all of that.

00:03:03.000 --> 00:03:05.500
But they make it more irregular,
and it's making it

00:03:05.500 --> 00:03:08.130
harder to get into
a regular form.

00:03:08.130 --> 00:03:12.100
And so the intuition is is that
this should lower the

00:03:12.100 --> 00:03:13.960
boiling point or make
it-- oh, sorry,

00:03:13.960 --> 00:03:14.780
lower the melting point.

00:03:14.780 --> 00:03:27.440
So solute particles make you
have a lower boiling point.

00:03:27.440 --> 00:03:30.300
Let's say if we're talking
about water at standard

00:03:30.300 --> 00:03:33.870
temperature and pressure or at
one atmosphere then instead of

00:03:33.870 --> 00:03:36.390
going to 0 degrees, you might
have to go to negative 1 or

00:03:36.390 --> 00:03:38.330
negative 2 degrees, and we're
going to talk a little bit

00:03:38.330 --> 00:03:39.800
about what that is.

00:03:39.800 --> 00:03:43.400
Now, what's the intuition of
what this will do when you

00:03:43.400 --> 00:03:45.080
want to go into a gaseous
state, when you

00:03:45.080 --> 00:03:45.850
want to boil it?

00:03:45.850 --> 00:03:49.320
So my initial gut was, hey, I'm
already in a disordered

00:03:49.320 --> 00:03:53.390
state, which is closer to what
a gas is, so wouldn't that

00:03:53.390 --> 00:03:54.620
make it easier to boil?

00:03:54.620 --> 00:03:57.040
But it turns out it also makes
it harder to boil, and this is

00:03:57.040 --> 00:03:58.170
how I think about it.

00:03:58.170 --> 00:04:00.780
Remember, everything with
boiling deals with what's

00:04:00.780 --> 00:04:04.020
happening at the surface, and
we talked about that in our

00:04:04.020 --> 00:04:05.160
vapor pressure.

00:04:05.160 --> 00:04:08.980
So at the surface, we said if
I have a bunch of water

00:04:08.980 --> 00:04:15.390
molecules in the liquid state,
we knew that although the

00:04:15.390 --> 00:04:19.000
average temperature might not
be high enough for the water

00:04:19.000 --> 00:04:21.680
molecules to evaporate, that
there's a distribution of

00:04:21.680 --> 00:04:22.480
kinetic energies.

00:04:22.480 --> 00:04:24.580
And some of these water
molecules on the surface

00:04:24.580 --> 00:04:27.070
because the surface ones
might be going

00:04:27.070 --> 00:04:29.030
fast enough to escape.

00:04:29.030 --> 00:04:33.340
And when they escape into vapor,
then they create a

00:04:33.340 --> 00:04:34.510
vapor pressure above here.

00:04:34.510 --> 00:04:37.550
And if that vapor pressure is
high enough, you can almost

00:04:37.550 --> 00:04:40.910
view them as linemen blocking
the way for more molecules to

00:04:40.910 --> 00:04:43.530
kind of run behind them as they
block all of the other

00:04:43.530 --> 00:04:46.930
ambient air pressure
above them.

00:04:46.930 --> 00:04:49.980
So if there's enough of them and
they have enough energy,

00:04:49.980 --> 00:04:54.180
they can start to push back or
to push outward is the way I

00:04:54.180 --> 00:04:57.410
think about it, so that more
guys can come in behind them.

00:04:57.410 --> 00:05:00.910
So I hope that lineman analogy
doesn't completely lose you.

00:05:00.910 --> 00:05:04.210
Now, what happens if you were
to introduce solute into it?

00:05:04.210 --> 00:05:06.210
Some of the solute particle
might be down here.

00:05:06.210 --> 00:05:09.170
It probably doesn't have much
of an effect down here, but

00:05:09.170 --> 00:05:12.290
some of it's going to be
bouncing on the surface, so

00:05:12.290 --> 00:05:15.020
they're going to be taking up
some of the surface area.

00:05:15.020 --> 00:05:17.310
And because, and this is at
least how I think of it, since

00:05:17.310 --> 00:05:19.590
they're going to be taking up
some of the surface area,

00:05:19.590 --> 00:05:22.300
you're going to have less
surface area exposed to the

00:05:22.300 --> 00:05:25.200
solvent particle or to the
solution or the stuff that'll

00:05:25.200 --> 00:05:26.080
actually vaporize.

00:05:26.080 --> 00:05:27.820
You're going to have a
lower vapor pressure.

00:05:33.610 --> 00:05:36.250
And remember, your boiling
point is when the vapor

00:05:36.250 --> 00:05:39.020
pressure, when you have enough
particles with enough kinetic

00:05:39.020 --> 00:05:41.820
energy out here to start
pushing against the

00:05:41.820 --> 00:05:44.340
atmospheric pressure, when the
vapor pressure is equal to the

00:05:44.340 --> 00:05:45.800
atmospheric pressure,
you start boiling.

00:05:45.800 --> 00:05:49.100
But because of these guys, I
have a lower vapor pressure.

00:05:49.100 --> 00:05:51.170
So I'm going to have to add even
more kinetic energy, more

00:05:51.170 --> 00:05:54.030
heat to the system in order to
get enough vapor pressure up

00:05:54.030 --> 00:05:57.130
here to start pushing back
the atmospheric pressure.

00:05:57.130 --> 00:06:02.440
So solute also raises
the boiling point.

00:06:05.450 --> 00:06:08.230
So the way that you can think
about it is solute, when you

00:06:08.230 --> 00:06:11.780
add something to a solution,
it's going to make it want to

00:06:11.780 --> 00:06:13.830
be in the liquid state more.

00:06:13.830 --> 00:06:15.520
Whether you lower the
temperature, it's going to

00:06:15.520 --> 00:06:17.580
want to stay in liquid as
opposed to ice, and if you

00:06:17.580 --> 00:06:19.000
raise the temperature, it's
going to want to stay in

00:06:19.000 --> 00:06:20.530
liquid as opposed to gas.

00:06:20.530 --> 00:06:22.460
I found this neat-- hopefully,
it shows up

00:06:22.460 --> 00:06:23.850
well on this video.

00:06:23.850 --> 00:06:26.205
I have to give due credit, this
is from chem.purdue.edu/

00:06:26.205 --> 00:06:34.820
gchelp/solutions/eboil.html, but
I thought it was a pretty

00:06:34.820 --> 00:06:37.690
neat graphic, or at least
a visualization.

00:06:37.690 --> 00:06:39.970
This is just the surface of
water molecules, and it gives

00:06:39.970 --> 00:06:43.060
you a sense of just how things
vaporize as well.

00:06:43.060 --> 00:06:46.090
There's some things on the
surface that just bounce off.

00:06:46.090 --> 00:06:48.970
And here's an example where
they visualized sodium

00:06:48.970 --> 00:06:50.390
chloride at the surface.

00:06:50.390 --> 00:06:52.910
And because the sodium chloride
is kind of bouncing

00:06:52.910 --> 00:06:56.120
around on the surface with the
water molecules, fewer of

00:06:56.120 --> 00:06:59.880
those water molecules kind of
have the room to escape, so

00:06:59.880 --> 00:07:03.050
the boiling point
gets elevated.

00:07:03.050 --> 00:07:06.080
Now, the question is by how
much does it get elevated?

00:07:06.080 --> 00:07:08.900
And this is one of the neat
things in life is that the

00:07:08.900 --> 00:07:11.730
answer is actually
quite simple.

00:07:11.730 --> 00:07:17.860
The change in boiling or
freezing point, so the change

00:07:17.860 --> 00:07:24.970
in temperature of vaporization,
is equal to some

00:07:24.970 --> 00:07:31.120
constant times the number of
moles, or at least the mole

00:07:31.120 --> 00:07:37.590
concentration, the molality,
times the molality of the

00:07:37.590 --> 00:07:41.200
solute that you're putting
into your solution.

00:07:41.200 --> 00:07:51.230
So, for example, let's say I
have 1 kilogram of-- so let's

00:07:51.230 --> 00:07:55.790
say my solvent is water.

00:07:55.790 --> 00:07:57.840
I'll switch colors.

00:07:57.840 --> 00:08:00.110
And I have 1 kilogram of water,
and let's say we're

00:08:00.110 --> 00:08:02.020
just at atmospheric pressure.

00:08:02.020 --> 00:08:08.040
And let's say I have some
sodium chloride, NaCl.

00:08:08.040 --> 00:08:14.170
And let's say I have
2 moles of NaCl.

00:08:14.170 --> 00:08:15.420
I'll have 2 moles.

00:08:17.670 --> 00:08:23.480
The question is how much will
this raise the boiling point

00:08:23.480 --> 00:08:24.610
of this water?

00:08:24.610 --> 00:08:31.070
So first of all, you just have
to figure out the molality,

00:08:31.070 --> 00:08:36.299
which is just equal to the
number of moles of solute,

00:08:36.299 --> 00:08:42.159
this 2 moles, divided
by the number of

00:08:42.159 --> 00:08:43.570
kilograms of solvent.

00:08:43.570 --> 00:08:51.610
So let's say we have 1
kilogram of solvent.

00:08:51.610 --> 00:08:54.090
This was, of course, moles.

00:08:54.090 --> 00:08:56.820
So our molality is 2
moles per kilogram.

00:08:56.820 --> 00:08:59.840
So we just have to figure out
what this constant is, and

00:08:59.840 --> 00:09:01.225
then we'll know the temperature
elevation.

00:09:01.225 --> 00:09:03.430
And actually, that same
Purdue site, they

00:09:03.430 --> 00:09:04.510
gave a list of tables.

00:09:04.510 --> 00:09:07.146
I haven't run the experiments
myself.

00:09:07.146 --> 00:09:08.760
They have some neat
charts here.

00:09:08.760 --> 00:09:11.640
But they say, OK water, normal
boiling point is 100 degrees

00:09:11.640 --> 00:09:15.200
Celsius at standard atmospheric
pressure.

00:09:15.200 --> 00:09:19.740
And then they say that the
constant is 0.512 Celsius

00:09:19.740 --> 00:09:21.360
degrees per mole.

00:09:21.360 --> 00:09:24.070
So let's just say 0.5.

00:09:24.070 --> 00:09:25.320
So it equals 0.5.

00:09:27.700 --> 00:09:28.950
So k is equal to 0.5.

00:09:32.430 --> 00:09:35.670
And I want to be very clear here
because this is a very--

00:09:35.670 --> 00:09:38.040
I won't say a subtle point, but
it's an interesting point.

00:09:38.040 --> 00:09:41.350
So I said that there's 2-- the
molality of-- I just realized

00:09:41.350 --> 00:09:42.030
I made a mistake.

00:09:42.030 --> 00:09:44.710
I said the molality of
sodium chloride is 2.

00:09:44.710 --> 00:09:47.620
2 moles per kilograms. But
that would be if sodium

00:09:47.620 --> 00:09:51.940
chloride stayed in this
molecular state, if it stayed

00:09:51.940 --> 00:09:53.840
together, right?

00:09:53.840 --> 00:09:56.150
But what happens is that the
sodium chloride actually

00:09:56.150 --> 00:09:57.790
disassociates, and we learned
all about it in

00:09:57.790 --> 00:09:59.430
that previous video.

00:09:59.430 --> 00:10:04.750
Each molecule or each sodium
chloride pair disassociates

00:10:04.750 --> 00:10:08.810
into two molecules,
into a sodium ion

00:10:08.810 --> 00:10:12.430
and a chlorine anion.

00:10:12.430 --> 00:10:15.820
And because of that, because
this disassociates into two,

00:10:15.820 --> 00:10:18.950
the molality is actually going
to be two times the number of

00:10:18.950 --> 00:10:22.670
moles of sodium chloride I have.
So it's going to be two

00:10:22.670 --> 00:10:23.730
times this.

00:10:23.730 --> 00:10:26.040
So my molality will
actually be 4.

00:10:26.040 --> 00:10:26.890
And this is an interesting
point.

00:10:26.890 --> 00:10:30.700
If I was dealing with--
and I wrote it here.

00:10:30.700 --> 00:10:36.400
So this right here is glucose,
and this is sodium chloride,

00:10:36.400 --> 00:10:38.370
or at least sodium chloride
in its crystal form.

00:10:38.370 --> 00:10:43.080
One molecule, I guess you can
view it, or one salt of it.

00:10:43.080 --> 00:10:45.310
I guess you could just view it
as one of these little pairs

00:10:45.310 --> 00:10:46.320
right here.

00:10:46.320 --> 00:10:48.780
But the interesting thing is
is you could have the same

00:10:48.780 --> 00:10:51.320
number of moles of sodium
chloride when you view it as a

00:10:51.320 --> 00:10:53.230
compound and glucose.

00:10:53.230 --> 00:10:56.730
But glucose, when it goes into
water, it just stays as one

00:10:56.730 --> 00:10:58.460
molecule of glucose.

00:10:58.460 --> 00:11:01.990
So a mole of glucose will
disassociate into a mole of

00:11:01.990 --> 00:11:03.170
glucose in water.

00:11:03.170 --> 00:11:04.950
Well, I guess it won't
disassociate.

00:11:04.950 --> 00:11:07.510
It'll just stay as one mole,
while a mole of sodium

00:11:07.510 --> 00:11:10.490
chloride will turn into
two moles because it

00:11:10.490 --> 00:11:11.480
disassociates.

00:11:11.480 --> 00:11:13.890
It turns into two separate
particles.

00:11:13.890 --> 00:11:18.780
So in my example, when I start
with a mole of this, I end

00:11:18.780 --> 00:11:26.910
up-- actually, once I dissolve
it in water, I ended up with 4

00:11:26.910 --> 00:11:28.890
moles per kilogram of molality,
because this turns

00:11:28.890 --> 00:11:30.280
into two particles.

00:11:30.280 --> 00:11:34.600
So given that the molality
is 4 moles.

00:11:34.600 --> 00:11:38.240
2 moles of sodium, 2 moles
of chloride per kilogram.

00:11:38.240 --> 00:11:42.270
So I just use that constant that
I just got from Purdue.

00:11:42.270 --> 00:11:46.100
And I get the change in
temperature is equal to that

00:11:46.100 --> 00:11:54.910
constant, 0.5, times 4, which
is equal to 2 degrees.

00:11:54.910 --> 00:11:59.440
So my boiling point will be
elevated by 2 degrees.

00:11:59.440 --> 00:12:03.970
Now, if I had the same number of
moles, if I had 2 moles of

00:12:03.970 --> 00:12:08.460
glucose dissolved into my water,
I'd only get half as

00:12:08.460 --> 00:12:10.610
much, half as much
of an increase.

00:12:10.610 --> 00:12:13.150
Because the molality would
be half as much.

00:12:13.150 --> 00:12:15.210
Because it doesn't turn
into two particles.

00:12:15.210 --> 00:12:16.720
In some textbooks, you'll
actually see it

00:12:16.720 --> 00:12:18.090
written like this.

00:12:18.090 --> 00:12:22.990
You'll actually see the same
formula written like change in

00:12:22.990 --> 00:12:25.860
boiling temperature, or vapor
temperature, or whatever you

00:12:25.860 --> 00:12:30.250
want to think, is equal to k
times m times i, where they'll

00:12:30.250 --> 00:12:34.270
say this is the molality
of the compound

00:12:34.270 --> 00:12:35.110
you're talking about.

00:12:35.110 --> 00:12:39.880
In this case, this number
would be 2, and i is the

00:12:39.880 --> 00:12:43.330
number of molecules or the
number of things that it

00:12:43.330 --> 00:12:44.640
disassociates into.

00:12:44.640 --> 00:12:46.910
So in this case, this
would have been 2.

00:12:46.910 --> 00:12:49.520
And that's where we would have
gotten 4 times k, which is

00:12:49.520 --> 00:12:51.020
0.5, which is 2.

00:12:51.020 --> 00:12:53.180
In the case of water, this would
be-- oh, sorry, in the

00:12:53.180 --> 00:12:55.390
case of the glucose, this
would still be 2.

00:12:55.390 --> 00:12:57.400
But it only turns into one
particle when it goes in the

00:12:57.400 --> 00:12:58.640
water, so that would be 1.

00:12:58.640 --> 00:13:02.370
So you would only have a 1
degree increase in the boiling

00:13:02.370 --> 00:13:03.370
point of water.

00:13:03.370 --> 00:13:06.460
Now, freezing point
is the same thing.

00:13:06.460 --> 00:13:10.500
Change in freezing
point is also

00:13:10.500 --> 00:13:13.180
proportional to the molality.

00:13:13.180 --> 00:13:15.800
And you can either say the
molality of the original

00:13:15.800 --> 00:13:18.690
non-in-water compound times
the number of compounds it

00:13:18.690 --> 00:13:22.290
disassociates into, although
this k is going to be

00:13:22.290 --> 00:13:26.440
different for freezing than
it is for boiling.

00:13:26.440 --> 00:13:29.660
Of course, this k changes at
different pressures and for

00:13:29.660 --> 00:13:30.370
different elements.

00:13:30.370 --> 00:13:33.950
But the really big takeaway is
just to realize that even if

00:13:33.950 --> 00:13:37.160
you have a mole of this and a
mole of that, and they're

00:13:37.160 --> 00:13:39.810
going to be dissolved into the
same amount of water, because

00:13:39.810 --> 00:13:42.160
this dissociates into two
particles and this

00:13:42.160 --> 00:13:45.170
disassociates into only one
for every-- or this

00:13:45.170 --> 00:13:47.620
disassociates into two moles for
every mole of the crystal

00:13:47.620 --> 00:13:50.610
you have-- this doesn't
disassociate; it just stays as

00:13:50.610 --> 00:13:53.810
one-- this'll have twice as
large of an effect on the

00:13:53.810 --> 00:13:57.300
freezing point change or on the
boiling point elevation

00:13:57.300 --> 00:13:59.320
than the glucose will.