More on Oxidation States

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Kind: captions
Language: en

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Let's figure out the oxidation
states for some more

00:00:03.680 --> 00:00:05.710
constituent atoms
and molecules.

00:00:05.710 --> 00:00:13.130
So let's say I had
magnesium oxide.

00:00:13.130 --> 00:00:14.560
MgO.

00:00:14.560 --> 00:00:17.060
I'll do oxygen in a
different color.

00:00:17.060 --> 00:00:18.890
So what are going to be their
oxidation states?

00:00:18.890 --> 00:00:20.550
And you might know this already,
but let's look at the

00:00:20.550 --> 00:00:22.790
periodic table, because
it never hurts to get

00:00:22.790 --> 00:00:23.480
familiar with it.

00:00:23.480 --> 00:00:25.550
So we have magnesium.

00:00:25.550 --> 00:00:27.720
Magnesium has two valance
electrons.

00:00:27.720 --> 00:00:29.240
It's a Group 2 element.

00:00:29.240 --> 00:00:32.560
It would love to lose
those two electrons.

00:00:32.560 --> 00:00:34.620
Oxygen, we already know,
is one of the most

00:00:34.620 --> 00:00:38.990
electronegative atoms. It's so
electronegative that oxidized

00:00:38.990 --> 00:00:42.510
has essentially been
named after them.

00:00:42.510 --> 00:00:45.630
And we know that oxygen loves
to gain two electrons.

00:00:45.630 --> 00:00:47.490
So this is kind of a marriage
made in heaven.

00:00:47.490 --> 00:00:50.760
This guy wants to lose two
electrons and this guy wants

00:00:50.760 --> 00:00:52.800
to gain two electrons.

00:00:52.800 --> 00:00:54.350
So what's going to happen?

00:00:54.350 --> 00:00:57.630
The magnesium is going to
lose two electrons.

00:00:57.630 --> 00:00:58.790
It was neutral.

00:00:58.790 --> 00:01:03.510
So it's going to have a plus
2 charge, hypothetically.

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And then, the oxygen is going
to have a minus 2 charge,

00:01:06.870 --> 00:01:10.490
because it gained the
two electrons.

00:01:10.490 --> 00:01:14.710
So in this molecule of magnesium
oxide, the oxidation

00:01:14.710 --> 00:01:17.690
state of magnesium is plus 2.

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And the oxidation state of
the oxygen is minus 2.

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Now let's do a slightly
harder one.

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Let's say we had magnesium
hydroxide.

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So hydroxide is OH2.

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OH2 right there, where there's
two hydroxide groups in this.

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So, my temptation would
still be, look.

00:01:48.830 --> 00:01:52.840
Magnesium likes to lose its
electrons, its two electrons,

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which would make it's charge
positive-- it's hypothetical

00:01:55.420 --> 00:01:57.500
oxidation state positive.

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So my temptation is to say,
hey, magnesium here

00:01:59.980 --> 00:02:00.500
would be plus 2.

00:02:00.500 --> 00:02:03.330
So let me write that there.

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And remember, in order for
everything to work out, if

00:02:06.830 --> 00:02:10.840
it's a neutral compound, all of
the oxidation states in it

00:02:10.840 --> 00:02:11.940
have to add up to 1.

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So let's see if that's
going to work out.

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Now, oxygen.

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My impulse is that its
oxidation state

00:02:17.740 --> 00:02:19.910
tends to be minus 2.

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So let me write that down.

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And hydrogen, when it's bonded
with an oxygen-- remember.

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In this case, the hydrogen is
bonded with the oxygen first.

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And then that's bonded
to the magnesium.

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So the hydrogen is bonded
to an oxygen.

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Hydrogen, if it was bonded to a
magnesium, you might want to

00:02:38.790 --> 00:02:41.810
say, hey, maybe it'll take the
electrons and it'll have a

00:02:41.810 --> 00:02:43.180
negative oxidation state.

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But when hydrogen is bonded
with oxygen, it

00:02:46.290 --> 00:02:49.590
gives up the electrons.

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It only has one electron
to give up.

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So it has a plus 1
oxidation state.

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So let's see.

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At first, you might say,
hey, I'm adding up

00:02:57.120 --> 00:02:58.100
the oxidation states.

00:02:58.100 --> 00:03:00.840
Plus 2 minus 2 is 0 plus 1.

00:03:00.840 --> 00:03:03.840
I get a plus 1 oxidation
state here.

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That doesn't make sense, Sal.

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This is a neutral compound.

00:03:06.350 --> 00:03:08.700
And what you to remember is oh,
no, but you have two of

00:03:08.700 --> 00:03:10.960
these hydroxides right here.

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So what you do is you figure out
the sum of the oxidation

00:03:15.570 --> 00:03:17.390
states of the hydroxide.

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So that's minus 2 plus 1.

00:03:20.500 --> 00:03:23.150
So for the entire hydroxide
molecule, you

00:03:23.150 --> 00:03:25.820
have a minus 1 sum.

00:03:25.820 --> 00:03:27.460
And then you have two of them.

00:03:27.460 --> 00:03:27.750
Right?

00:03:27.750 --> 00:03:30.900
You have two hydroxide
molecules here.

00:03:30.900 --> 00:03:35.250
So the contribution to the
entire compound's oxidation

00:03:35.250 --> 00:03:37.570
state will be minus 1
for each hydroxide.

00:03:37.570 --> 00:03:39.500
But then you have two of them.

00:03:39.500 --> 00:03:42.960
So it's minus 2 and then plus
2 from the magnesium.

00:03:42.960 --> 00:03:45.600
And it all adds up to 0.

00:03:45.600 --> 00:03:47.440
So that worked out.

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Now, I want to do a little
bit of an aside.

00:03:49.350 --> 00:03:51.830
I want to go back to doing
some problems again.

00:03:51.830 --> 00:03:53.580
But I want to do a little bit
of an aside on some of my

00:03:53.580 --> 00:03:54.310
terminology.

00:03:54.310 --> 00:03:56.630
Because I've kind of used
oxidation state, and oxidized,

00:03:56.630 --> 00:04:00.360
and reduced interchangeably,
to a certain degree.

00:04:00.360 --> 00:04:05.030
But, we've done so many
problems with water

00:04:05.030 --> 00:04:10.560
autoionizing into-- actually,
let me do 2 moles of water.

00:04:10.560 --> 00:04:19.839
And it's in equilibrium with 1
mole of H30 plus OH minus.

00:04:19.839 --> 00:04:22.650
And obviously, everything is
in an aqueous solution.

00:04:22.650 --> 00:04:23.840
Now, let's look at the water.

00:04:23.840 --> 00:04:26.620
What are the oxidation states
in this water right here?

00:04:26.620 --> 00:04:28.620
Well, we've done this already
in the previous video.

00:04:28.620 --> 00:04:32.340
Oxidation state of oxygen is
minus 2, because it's hogging

00:04:32.340 --> 00:04:34.400
the two electrons from
the two hydrogens.

00:04:34.400 --> 00:04:36.705
Each hydrogen is giving
up an electron.

00:04:36.705 --> 00:04:39.210
So it has an oxidation
state of plus 1.

00:04:39.210 --> 00:04:40.400
And we see this molecule.

00:04:40.400 --> 00:04:41.620
Everything adds up.

00:04:41.620 --> 00:04:43.700
Because you have two hydrogens
with a plus 1.

00:04:43.700 --> 00:04:45.350
So that's plus 2.

00:04:45.350 --> 00:04:48.180
Plus 2 minus 2 for that one
oxygen, and you get to 0.

00:04:48.180 --> 00:04:49.990
And it's a neutral compound.

00:04:49.990 --> 00:04:52.650
Now here, what are the
oxidation states?

00:04:52.650 --> 00:04:58.660
So one of these hydrogens left
one of these water molecules

00:04:58.660 --> 00:05:01.220
and joined the other of the
water molecules without taking

00:05:01.220 --> 00:05:02.420
its electron with it.

00:05:02.420 --> 00:05:04.560
So it left the electron
over here.

00:05:04.560 --> 00:05:09.930
So that oxygen still has a
minus 2 oxidation state.

00:05:09.930 --> 00:05:13.290
And this hydrogen still
has a plus 1.

00:05:13.290 --> 00:05:15.190
And that's why you do
minus 2 plus 1.

00:05:15.190 --> 00:05:16.300
You get minus 1.

00:05:16.300 --> 00:05:18.660
And this time, it works out,
because that's the actual

00:05:18.660 --> 00:05:20.910
charge on this hydroxide ion.

00:05:20.910 --> 00:05:22.940
Now, here, what are the
oxidation states?

00:05:22.940 --> 00:05:28.230
Each of the hydrogens have
a plus 1 oxidation state.

00:05:28.230 --> 00:05:32.070
And then this oxygen
has a minus 2.

00:05:32.070 --> 00:05:33.860
And so if you look at the
charge for the entire

00:05:33.860 --> 00:05:38.730
molecule, plus 1 on three
hydrogens, so that's plus 3.

00:05:38.730 --> 00:05:39.610
I just added them up.

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Minus 2.

00:05:41.480 --> 00:05:45.080
So plus 3 minus 2, I should have
a plus 1 charge on this

00:05:45.080 --> 00:05:47.370
entire molecule, which
is the case.

00:05:47.370 --> 00:05:51.260
Now, my question to you is has
any of the oxidation states

00:05:51.260 --> 00:05:52.930
changed for any of the atoms?

00:05:52.930 --> 00:05:55.850
All of the hydrogens here--
and we could call

00:05:55.850 --> 00:05:58.120
this 2 moles of water.

00:05:58.120 --> 00:06:00.570
Or maybe I just have two
molecules of water.

00:06:00.570 --> 00:06:02.280
But I have four hydrogens
here.

00:06:02.280 --> 00:06:02.600
Right?

00:06:02.600 --> 00:06:04.890
And all of them had an
oxidation state of 1.

00:06:04.890 --> 00:06:07.050
On the right-hand side,
I have four hydrogens.

00:06:07.050 --> 00:06:10.050
All of them have an oxidation
state of 1.

00:06:10.050 --> 00:06:14.370
So although their oxidation
state is 1, in this reaction--

00:06:14.370 --> 00:06:16.860
and you can pick either
direction of the reaction--

00:06:16.860 --> 00:06:19.720
hydrogen has not
been oxidized.

00:06:19.720 --> 00:06:21.715
Its oxidation state
did not change.

00:06:24.270 --> 00:06:26.570
Maybe it was oxidized in a
previous reaction where the

00:06:26.570 --> 00:06:30.310
water was formed, but in this
reaction, it was not oxidized.

00:06:30.310 --> 00:06:34.160
Likewise, the oxygens-- we have
two oxygen molecules, or

00:06:34.160 --> 00:06:35.040
atoms, here.

00:06:35.040 --> 00:06:37.650
Each have a minus 2
oxidation state.

00:06:37.650 --> 00:06:39.380
Here, we have two oxygen
molecules.

00:06:39.380 --> 00:06:41.630
Each have a minus 2
oxidation state.

00:06:41.630 --> 00:06:46.000
Due to this reaction, at least,
no electrons changed

00:06:46.000 --> 00:06:48.140
hands in our oxidation
state world.

00:06:48.140 --> 00:06:52.250
So this is not an oxidation
or a reduction reaction.

00:06:52.250 --> 00:06:54.250
And I'm going to cover that in
detail in the next video.

00:06:54.250 --> 00:06:57.080
And I just want to be clear that
nothing here was oxidized

00:06:57.080 --> 00:07:01.160
or reduced, because
their oxidation

00:07:01.160 --> 00:07:03.180
states stayed the same.

00:07:03.180 --> 00:07:07.870
Because sometimes I'll
say, hey, look.

00:07:07.870 --> 00:07:10.580
Magnesium has an oxidation
state of plus 2.

00:07:10.580 --> 00:07:13.650
And oxygen has an oxidation
state of minus 2.

00:07:13.650 --> 00:07:15.310
Magnesium was oxidized.

00:07:15.310 --> 00:07:17.220
Two electrons were taken
away from it.

00:07:17.220 --> 00:07:18.670
And oxygen was reduced.

00:07:18.670 --> 00:07:20.180
Two electrons were
given to it.

00:07:20.180 --> 00:07:22.620
And I'll say that implying some
reaction that produced

00:07:22.620 --> 00:07:24.680
it, but that's not
always the case.

00:07:24.680 --> 00:07:26.120
You could have a reaction
where that

00:07:26.120 --> 00:07:27.120
necessarily didn't happen.

00:07:27.120 --> 00:07:29.760
But the oxidation state
for magnesium is

00:07:29.760 --> 00:07:31.050
definitely plus 2.

00:07:31.050 --> 00:07:35.520
And the oxidation state for the
oxygen, or the oxidation

00:07:35.520 --> 00:07:37.380
number, is minus 2.

00:07:37.380 --> 00:07:38.960
But I think you know what I'm
talking about when I say it

00:07:38.960 --> 00:07:40.330
was oxidized.

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At some point, it went from
a neutral magnesium to a

00:07:44.130 --> 00:07:47.310
positively charged magnesium
by losing two electrons.

00:07:47.310 --> 00:07:49.510
So it got oxidized.

00:07:49.510 --> 00:07:53.280
Now, let's do some harder
problems. So hydrogen

00:07:53.280 --> 00:07:57.260
peroxide-- I've said multiple
times already that oxygen

00:07:57.260 --> 00:08:01.600
tends to have a minus
2 oxidation state.

00:08:01.600 --> 00:08:02.450
This is minus 1.

00:08:02.450 --> 00:08:04.910
I think you see the pattern.

00:08:04.910 --> 00:08:06.900
These guys are plus 1.

00:08:06.900 --> 00:08:09.100
Hydrogen is plus or minus 1.

00:08:09.100 --> 00:08:10.450
These guys are plus 2.

00:08:10.450 --> 00:08:11.520
I think you see the pattern.

00:08:11.520 --> 00:08:15.070
It's whether you want to
lose or gain electrons.

00:08:15.070 --> 00:08:17.450
You might say, well see,
water normally

00:08:17.450 --> 00:08:19.590
has a minus 2 oxidation.

00:08:19.590 --> 00:08:22.470
So you might be tempted
to do-- OK.

00:08:22.470 --> 00:08:25.470
Hydrogen has plus 1, because
it's bonding with water.

00:08:25.470 --> 00:08:28.690
And oxygen has a minus 2.

00:08:28.690 --> 00:08:31.440
But when you do that, you
immediately have a conundrum.

00:08:31.440 --> 00:08:34.240
This is a neutral molecule--
let's see.

00:08:34.240 --> 00:08:35.480
Two hydrogens plus 2.

00:08:35.480 --> 00:08:36.610
Two oxygens at minus 2.

00:08:36.610 --> 00:08:37.290
Minus 4.

00:08:37.290 --> 00:08:40.130
So this would end up with
a minus 4 total

00:08:40.130 --> 00:08:41.720
net oxidation state.

00:08:41.720 --> 00:08:42.780
And that's not the
case because this

00:08:42.780 --> 00:08:44.300
doesn't have any charge.

00:08:44.300 --> 00:08:45.980
So there's a conundrum here.

00:08:45.980 --> 00:08:48.390
And the conundrum is because,
if you actually look at the

00:08:48.390 --> 00:08:52.140
structure of hydrogen peroxide,
the oxygens are

00:08:52.140 --> 00:08:53.830
actually bonded to each other.

00:08:53.830 --> 00:08:56.120
That's where the peroxide
comes from.

00:08:56.120 --> 00:09:00.590
And then each of those are
bonded to a hydrogen.

00:09:00.590 --> 00:09:04.670
So in this case, especially in a
first-year chemistry course,

00:09:04.670 --> 00:09:07.190
the peroxide molecules,
especially hydrogen peroxide,

00:09:07.190 --> 00:09:09.110
tends to be that one
special case.

00:09:09.110 --> 00:09:11.660
There are others, but this is
the one special case where

00:09:11.660 --> 00:09:15.540
oxygen does not have a minus
2 oxidation state.

00:09:15.540 --> 00:09:17.550
Let's look at this and try to
figure out what oxygen's

00:09:17.550 --> 00:09:20.750
oxidation state would be
in hydrogen peroxide.

00:09:20.750 --> 00:09:23.735
So in this case, the
hydrogen-oxygen bond, oxygen

00:09:23.735 --> 00:09:26.460
is going to hog the electron
and hydrogen is

00:09:26.460 --> 00:09:27.320
going to lose it.

00:09:27.320 --> 00:09:28.780
So it's going to have
a plus 1 there.

00:09:28.780 --> 00:09:30.430
Same thing on the side.

00:09:30.430 --> 00:09:33.060
Oxygen, at least on this bond,
is going to have a plus 1.

00:09:33.060 --> 00:09:34.790
It's going to gain
an electron.

00:09:34.790 --> 00:09:37.380
What about from this other bond
with the other oxygen?

00:09:37.380 --> 00:09:39.760
Well, there's no reason why
one oxygen should hog the

00:09:39.760 --> 00:09:41.170
electron from the
other oxygen.

00:09:41.170 --> 00:09:43.560
So it's not going to have
any net impact on

00:09:43.560 --> 00:09:45.000
its oxidation state.

00:09:45.000 --> 00:09:49.470
So in this case, this oxygen's
oxidation state is plus 1.

00:09:49.470 --> 00:09:53.580
This oxygen's oxidation
state is also plus 1.

00:09:53.580 --> 00:09:59.620
So each of the hydrogens have an
oxidation number of plus 1.

00:09:59.620 --> 00:10:03.080
You said the oxygens have an
oxidation number of minus 1.

00:10:03.080 --> 00:10:05.930
And so you have a net of 0.

00:10:05.930 --> 00:10:09.260
2 times plus 1, plus 2
times minus 1, is 0.

00:10:09.260 --> 00:10:11.080
So that's just a special case.

00:10:11.080 --> 00:10:13.290
That's a good one to
be familiar with.

00:10:13.290 --> 00:10:14.170
Let's do another one.

00:10:14.170 --> 00:10:15.970
Iron 3 carbonate.

00:10:15.970 --> 00:10:18.410
And now, for the first time--
I remember when we first

00:10:18.410 --> 00:10:19.920
encountered iron 3 carbonate.

00:10:19.920 --> 00:10:22.990
You probably thought, hey, why
is it called iron 3 carbonate

00:10:22.990 --> 00:10:25.270
when there are only two
iron molecules, or

00:10:25.270 --> 00:10:26.350
two iron atoms, here?

00:10:26.350 --> 00:10:27.840
And you're about to learn why.

00:10:27.840 --> 00:10:30.310
Let's look at the oxidation
numbers.

00:10:30.310 --> 00:10:31.850
So oxygen.

00:10:31.850 --> 00:10:35.610
Oxygen's oxidation number
tends to be minus 2.

00:10:38.530 --> 00:10:41.760
Now, if carbon is bonding with
oxygen-- let's look at the

00:10:41.760 --> 00:10:43.330
periodic table.

00:10:43.330 --> 00:10:46.910
We have carbon bonding
with oxygen.

00:10:46.910 --> 00:10:48.720
Carbon can go either way.

00:10:48.720 --> 00:10:51.120
Carbon, sometimes it likes
to give away electrons.

00:10:51.120 --> 00:10:53.040
Sometimes it likes to
gain electrons.

00:10:53.040 --> 00:10:57.270
When carbon is bonding with
oxygen, this right here is the

00:10:57.270 --> 00:10:58.810
electron hog.

00:10:58.810 --> 00:11:01.160
If we had to say who's
taking the electrons,

00:11:01.160 --> 00:11:03.010
it's going to be oxygen.

00:11:03.010 --> 00:11:03.540
Right?

00:11:03.540 --> 00:11:07.040
So carbon is going to be giving
away its electrons.

00:11:07.040 --> 00:11:10.200
But how many electrons
can carbon give away?

00:11:10.200 --> 00:11:10.936
Well, let's see.

00:11:10.936 --> 00:11:14.720
It has 1, 2, 3, 4 valence
electrons.

00:11:14.720 --> 00:11:16.240
So the most it can really
do is give away

00:11:16.240 --> 00:11:19.390
four valence electrons.

00:11:19.390 --> 00:11:21.670
So let's go back to
the carbonate.

00:11:21.670 --> 00:11:27.040
So the carbon could at
most give away its

00:11:27.040 --> 00:11:29.930
four valence electrons.

00:11:29.930 --> 00:11:32.400
So what will be the net
oxidation number for the

00:11:32.400 --> 00:11:33.770
carbonate molecule?

00:11:33.770 --> 00:11:36.810
For the CO3?

00:11:36.810 --> 00:11:39.430
So this is a plus 4 oxidation,
because it only has

00:11:39.430 --> 00:11:40.270
four to give away.

00:11:40.270 --> 00:11:42.260
If it's bonding with oxygen,
it's going to give them away.

00:11:42.260 --> 00:11:44.630
Oxygen is more of a hog.

00:11:44.630 --> 00:11:47.640
Each oxygen has a minus 2.

00:11:47.640 --> 00:11:48.590
So let's think about it.

00:11:48.590 --> 00:11:54.210
I have plus 4 minus,
3 times minus 2.

00:11:54.210 --> 00:11:54.700
Right?

00:11:54.700 --> 00:11:56.870
I have 3 oxygen molecules.

00:11:56.870 --> 00:12:00.290
So I have 4 minus 6 is
equal to minus 2.

00:12:00.290 --> 00:12:02.730
So we can kind of view it as the
oxidation number for the

00:12:02.730 --> 00:12:06.990
entire carbonate molecule
is minus 2.

00:12:06.990 --> 00:12:11.210
Now, if this entire carbonate
molecule is minus 2, its

00:12:11.210 --> 00:12:16.920
contribution to the oxidation
state for this whole kind of--

00:12:16.920 --> 00:12:18.620
the carbonate part
of the molecule.

00:12:18.620 --> 00:12:20.980
We have 3 carbonate molecules.

00:12:20.980 --> 00:12:22.710
Each of them is contributing
minus 2.

00:12:22.710 --> 00:12:25.740
So I have a minus
6 contribution.

00:12:25.740 --> 00:12:29.380
If this is minus 6 and this is a
neutral molecule, then our 2

00:12:29.380 --> 00:12:32.880
irons are also going
to have to have a

00:12:32.880 --> 00:12:35.290
plus 6 oxidation state.

00:12:35.290 --> 00:12:38.030
Because it all has
to add up to 0.

00:12:38.030 --> 00:12:42.400
If both irons combined have
a plus 6 contribution to

00:12:42.400 --> 00:12:44.380
oxidation state, then each
of the irons must

00:12:44.380 --> 00:12:46.460
have a plus 3 oxidation.

00:12:46.460 --> 00:12:50.930
Or that, in our hypothetical
world, if this happens, at

00:12:50.930 --> 00:12:53.510
least three electrons are going
to favor the carbonate

00:12:53.510 --> 00:12:55.240
from each of the irons.

00:12:55.240 --> 00:12:57.980
So why is it called
iron 3 carbonate?

00:12:57.980 --> 00:13:01.340
I think you may have figured
this out by now.

00:13:01.340 --> 00:13:04.600
Because this is iron in its
third oxidation state.

00:13:04.600 --> 00:13:06.520
Iron-- a lot of the metals,
especially a lot of the

00:13:06.520 --> 00:13:09.790
transition metals-- can have
multiple oxidation states.

00:13:09.790 --> 00:13:11.970
When you have iron 3 carbonate,
you're literally

00:13:11.970 --> 00:13:14.130
saying, this is the third
oxidation state.

00:13:14.130 --> 00:13:18.120
Or iron's oxidation number
in this molecule

00:13:18.120 --> 00:13:20.780
will be positive 3.

00:13:20.780 --> 00:13:21.900
Now, let's do another one.

00:13:21.900 --> 00:13:23.000
This is interesting.

00:13:23.000 --> 00:13:24.010
Acetic acid.

00:13:24.010 --> 00:13:25.560
And I think is the first time
that I've actually shown you

00:13:25.560 --> 00:13:30.740
the formula for acetic acid.

00:13:30.740 --> 00:13:33.410
I won't go into the whole
organic chemistry of it.

00:13:33.410 --> 00:13:37.290
But let's try to figure out
what the different charges

00:13:37.290 --> 00:13:38.835
are, or the different
oxidation states.

00:13:41.510 --> 00:13:43.610
Sometimes you'll just see
it written like this.

00:13:43.610 --> 00:13:45.430
You'd say, OK.

00:13:45.430 --> 00:13:47.530
Oxygens, each of those are
going to have minus 2.

00:13:51.130 --> 00:13:52.570
Hydrogens are each going
to have plus 1.

00:13:56.270 --> 00:13:58.520
So how are we doing so far?

00:13:58.520 --> 00:14:02.100
So these oxygens are going
to contribute minus 4.

00:14:02.100 --> 00:14:06.200
And then the hydrogens--
here you have plus 3.

00:14:06.200 --> 00:14:09.260
And then here you have plus 1.

00:14:09.260 --> 00:14:10.925
You add these up and
you get to 0.

00:14:10.925 --> 00:14:11.740
And you're like, oh.

00:14:11.740 --> 00:14:14.430
So the carbons must have
no oxidation state.

00:14:14.430 --> 00:14:17.280
They must have an oxidation
number of 0.

00:14:17.280 --> 00:14:22.180
Because we're already at 0, if
we just consider the hydrogens

00:14:22.180 --> 00:14:23.370
and the oxygens.

00:14:23.370 --> 00:14:26.230
So let's look at that and see
if that's actually the case.

00:14:26.230 --> 00:14:29.360
So when carbon is bonding with
hydrogen, who's going to hog

00:14:29.360 --> 00:14:32.100
the electrons?

00:14:32.100 --> 00:14:35.330
When carbon is bonding
with hydrogen.

00:14:35.330 --> 00:14:37.320
Electronegativity-- as
you go to the right.

00:14:37.320 --> 00:14:39.330
Carbon is more electronegative.

00:14:39.330 --> 00:14:42.590
It likes to keep the electrons,
or hog them, more

00:14:42.590 --> 00:14:43.310
than hydrogen.

00:14:43.310 --> 00:14:46.080
So hydrogen is going to lose the
electrons in our oxidation

00:14:46.080 --> 00:14:46.860
state world.

00:14:46.860 --> 00:14:50.375
It's actually a covalent bond,
but of course, we know that

00:14:50.375 --> 00:14:52.430
when we're dealing with
oxidation states, we pretend

00:14:52.430 --> 00:14:53.630
that it's ionic.

00:14:53.630 --> 00:14:56.030
So in this case, your
hydrogens are

00:14:56.030 --> 00:14:58.100
going to lose electrons.

00:14:58.100 --> 00:15:00.920
So they're each going to have an
oxidation state of plus 1.

00:15:00.920 --> 00:15:03.400
That's consistent with
what we know so far.

00:15:03.400 --> 00:15:04.730
And actually, that's
another thing.

00:15:04.730 --> 00:15:06.770
When I did this exercise, right
here, I immediately

00:15:06.770 --> 00:15:10.420
assumed hydrogen has an
oxidation state of plus 1.

00:15:10.420 --> 00:15:12.190
I did that because, oh,
everything else in the

00:15:12.190 --> 00:15:15.000
molecule is carbon and oxygen,
which are more electronegative

00:15:15.000 --> 00:15:15.700
than the hydrogen.

00:15:15.700 --> 00:15:18.190
So the hydrogen is going to
go into its plus 1 state.

00:15:18.190 --> 00:15:20.960
If, over here, I had a bunch of
alkali and alkaline earth

00:15:20.960 --> 00:15:22.740
metals, I wouldn't be so sure.

00:15:22.740 --> 00:15:24.090
I'd say, oh, maybe hydrogen
would take

00:15:24.090 --> 00:15:25.910
electrons from them.

00:15:25.910 --> 00:15:27.280
But anyway.

00:15:27.280 --> 00:15:30.680
So these all gave an electron
to this carbon.

00:15:30.680 --> 00:15:36.785
So just from these hydrogens,
that carbon would have a minus

00:15:36.785 --> 00:15:39.740
3 oxidation state, right?

00:15:39.740 --> 00:15:41.260
These lost electrons.

00:15:41.260 --> 00:15:43.560
This guy gained three electrons,
so his charge

00:15:43.560 --> 00:15:45.430
goes down by 3.

00:15:45.430 --> 00:15:46.680
The carbon-carbon bond.

00:15:46.680 --> 00:15:48.720
Well, there's no reason one
carbon should take electrons

00:15:48.720 --> 00:15:49.570
from another carbon.

00:15:49.570 --> 00:15:51.620
All carbons are created equal.

00:15:51.620 --> 00:15:53.920
So there should be
no transfer here.

00:15:53.920 --> 00:15:56.160
So this carbon's oxidation
status is 3.

00:15:56.160 --> 00:15:57.230
Now what about on this side?

00:15:57.230 --> 00:16:01.560
So we know that this hydrogen
is going to have a plus 1

00:16:01.560 --> 00:16:02.920
oxidation state.

00:16:02.920 --> 00:16:05.470
It's going to give its electron
to this oxygen.

00:16:05.470 --> 00:16:08.870
This oxygen, like most oxygens,
are going to take up

00:16:08.870 --> 00:16:09.640
two electrons.

00:16:09.640 --> 00:16:12.980
One from this carbon, and
one from this hydrogen.

00:16:12.980 --> 00:16:16.460
So it's going to have a minus
2 oxidation state.

00:16:16.460 --> 00:16:19.140
This oxygen is also going
to take two electrons.

00:16:19.140 --> 00:16:20.600
In this case, both of
them are going to be

00:16:20.600 --> 00:16:22.370
from this orange carbon.

00:16:22.370 --> 00:16:24.870
So it's going to have a minus
2 oxidation state.

00:16:24.870 --> 00:16:26.840
So what's the oxidation
state of this carbon?

00:16:26.840 --> 00:16:33.190
It lost two electrons to this
guy up here, and it lost one

00:16:33.190 --> 00:16:35.450
electron to this oxygen
down here.

00:16:35.450 --> 00:16:37.640
Remember, this guy got one
electron from the carbon and

00:16:37.640 --> 00:16:39.050
one from the hydrogen.

00:16:39.050 --> 00:16:41.990
So it lost one electron
here, two there.

00:16:41.990 --> 00:16:43.770
It lost three electrons.

00:16:43.770 --> 00:16:49.050
So in that reality, it would
have a plus 3 charge.

00:16:49.050 --> 00:16:53.080
So it turns out that the average
oxidation state for

00:16:53.080 --> 00:16:54.930
the carbon in acetic
acid is 0.

00:16:54.930 --> 00:16:58.150
Because if you average minus
3 and plus 3, you get to 0.

00:16:58.150 --> 00:17:00.270
And that's why I said, oh,
maybe these are a 0.

00:17:00.270 --> 00:17:03.730
But if you actually write out
their oxidation numbers, this

00:17:03.730 --> 00:17:07.410
green C has a minus
3 oxidation state.

00:17:07.410 --> 00:17:10.660
And this orange C, this
orange carbon, has a

00:17:10.660 --> 00:17:12.880
plus 3 oxidation state.

00:17:12.880 --> 00:17:15.500
If you got this one, and I
don't think it's overly

00:17:15.500 --> 00:17:22.130
complex, you will be an
oxidation state jock.

00:17:22.130 --> 00:17:23.720
So I think you're all set now.

00:17:23.720 --> 00:17:26.515
In the next video, we're going
to start exploring oxidation

00:17:26.515 --> 00:17:28.150
reduction reactions.


Introduction to Oxidation States

WEBVTT
Kind: captions
Language: en

00:00:00.490 --> 00:00:07.600
Let's have the molecule
sodium chloride.

00:00:07.600 --> 00:00:09.840
Let's look at the
periodic table.

00:00:09.840 --> 00:00:12.870
Sodium, it's in alkali metal.

00:00:12.870 --> 00:00:14.770
It has one valence electron.

00:00:14.770 --> 00:00:16.190
It's in Group 1.

00:00:16.190 --> 00:00:17.800
And so when it only
has one, it really

00:00:17.800 --> 00:00:18.710
wants to give it away.

00:00:18.710 --> 00:00:24.350
And chlorine is a halogen,
and it's only one away.

00:00:24.350 --> 00:00:28.260
It just needs one electron to
have the full eight valence

00:00:28.260 --> 00:00:30.400
electrons in its outermost
shell.

00:00:30.400 --> 00:00:32.159
So it really wants to
take 1 electron.

00:00:32.159 --> 00:00:34.900
And we've talked about this
story multiple times,

00:00:34.900 --> 00:00:37.220
especially it early in the
chemistry playlist. So what we

00:00:37.220 --> 00:00:38.410
know what's going to happen.

00:00:38.410 --> 00:00:40.920
Sodium, since it wants to lose
an electron, is going to lose

00:00:40.920 --> 00:00:41.810
its electron.

00:00:41.810 --> 00:00:43.380
And it's going to give
it to chlorine.

00:00:43.380 --> 00:00:44.810
And chlorine is going
to gain it.

00:00:44.810 --> 00:00:47.020
And so you're going to have a
situation where sodium is

00:00:47.020 --> 00:00:51.950
going to have a plus 1 charge,
because it lost an electron.

00:00:51.950 --> 00:00:55.900
And then chlorine will have a
minus 1 charge because it

00:00:55.900 --> 00:00:57.480
gained an electron.

00:00:57.480 --> 00:01:00.910
And then they'll be attracted to
each other because this one

00:01:00.910 --> 00:01:02.570
is now a positive atom
and that one is

00:01:02.570 --> 00:01:03.540
now a negative atom.

00:01:03.540 --> 00:01:06.730
And just the Coulomb force will
make them want to be with

00:01:06.730 --> 00:01:07.140
each other.

00:01:07.140 --> 00:01:09.470
And we call this, of course,
an ionic bond.

00:01:09.470 --> 00:01:12.700
I haven't taught you anything
you don't know yet.

00:01:12.700 --> 00:01:15.510
And in an ionic bond, you
literally had a loss of

00:01:15.510 --> 00:01:19.190
electron from one compound
to another.

00:01:19.190 --> 00:01:19.870
Now.

00:01:19.870 --> 00:01:21.880
that's a very cut-and-dry
situation.

00:01:21.880 --> 00:01:24.280
But we saw other situations
where wasn't quite as

00:01:24.280 --> 00:01:25.300
cut-and-dry.

00:01:25.300 --> 00:01:27.350
For example, H2O.

00:01:27.350 --> 00:01:32.520
Where you have a oxygen-- but
you've seen this multiple

00:01:32.520 --> 00:01:34.300
times already, if
you're watching

00:01:34.300 --> 00:01:36.130
this playlist in order.

00:01:36.130 --> 00:01:38.790
And it's bonded to
2 hydrogens.

00:01:38.790 --> 00:01:41.710
But we know that oxygen is much,
much more-- in fact,

00:01:41.710 --> 00:01:46.110
nitrogen, oxygen, and fluorine
are the most electronegative

00:01:46.110 --> 00:01:49.880
atoms, which means that they
like to hog electrons the

00:01:49.880 --> 00:01:52.900
most. So when you have oxygen
bonded with pretty much

00:01:52.900 --> 00:01:56.130
anything-- but in particular in
this case, hydrogen-- it's

00:01:56.130 --> 00:01:59.910
going to to pull most of the
electrons in its direction.

00:01:59.910 --> 00:02:02.930
So even though these are
covalent bonds, the hydrogen

00:02:02.930 --> 00:02:06.250
is still sharing the
electrons that it

00:02:06.250 --> 00:02:08.660
has bonded with oxygen.

00:02:08.660 --> 00:02:11.940
The electrons spend most of
their time on the oxygen side

00:02:11.940 --> 00:02:13.220
of this whole affair.

00:02:13.220 --> 00:02:17.730
So you have a partial negative
charge on the oxygen end of

00:02:17.730 --> 00:02:18.430
the molecule.

00:02:18.430 --> 00:02:21.710
And you have partial positive
charges on the hydrogen.

00:02:21.710 --> 00:02:22.920
And we talked about this.

00:02:22.920 --> 00:02:24.100
And this is what leads
to hydrogen

00:02:24.100 --> 00:02:25.560
bonding, and all of that.

00:02:25.560 --> 00:02:29.990
What we're going to do here is
introduce-- it's almost an

00:02:29.990 --> 00:02:32.010
intellectual tool.

00:02:32.010 --> 00:02:34.160
We know that this isn't
an ionic bond.

00:02:34.160 --> 00:02:36.880
We know that this is actually
a covalent bond, that this

00:02:36.880 --> 00:02:37.630
atom is shared.

00:02:37.630 --> 00:02:41.060
But it spends most of
its time at the more

00:02:41.060 --> 00:02:42.170
electronegative atom.

00:02:42.170 --> 00:02:44.170
That's why you have this partial
negative charge.

00:02:44.170 --> 00:02:48.630
So we create this convention
called oxidation states.

00:02:48.630 --> 00:02:50.400
Let me write that down.

00:02:50.400 --> 00:02:52.925
And I'll clarify what the
word means in a second.

00:02:59.450 --> 00:03:02.830
And this is essentially
assigning-- With an ionic

00:03:02.830 --> 00:03:06.150
compound, you naturally assign
a charge, because each atom

00:03:06.150 --> 00:03:07.510
really does have a charge.

00:03:07.510 --> 00:03:10.000
But let's say we want to live in
a world where we don't like

00:03:10.000 --> 00:03:12.560
this partial charge,
partial negative.

00:03:12.560 --> 00:03:18.950
We want to say, look, if this
were, hypothetically, an ionic

00:03:18.950 --> 00:03:20.740
bond, what would it look like?

00:03:20.740 --> 00:03:22.430
Who would gain the
electron and who

00:03:22.430 --> 00:03:23.960
would lose the electron?

00:03:23.960 --> 00:03:28.070
So in the case of water, if you
were forced to say OK, who

00:03:28.070 --> 00:03:29.310
gained the electron?

00:03:29.310 --> 00:03:32.880
You'd say oxygen gained 2
electrons from the hydrogens,

00:03:32.880 --> 00:03:37.840
and the hydrogens lost each
1 electron to the oxygen.

00:03:37.840 --> 00:03:42.260
So the hydrogen would have a
plus 1 charge, each of them.

00:03:42.260 --> 00:03:47.300
And then the oxygen would
have a minus 2 charge.

00:03:47.300 --> 00:03:49.210
Now, I want to be very
clear with this.

00:03:49.210 --> 00:03:50.840
This isn't what really
happened.

00:03:50.840 --> 00:03:53.790
This is just our little
intellectual game that we're

00:03:53.790 --> 00:03:55.550
playing called an
oxidation state.

00:03:55.550 --> 00:03:58.590
It's going to be really useful
later to understand why some

00:03:58.590 --> 00:03:59.700
reactions occur.

00:03:59.700 --> 00:04:01.480
But I just want to
be very clear.

00:04:01.480 --> 00:04:05.320
This is a hypothetical charge
if these were ionic bonds.

00:04:05.320 --> 00:04:08.070
So you're just saying, whoever
is the more electronegative--

00:04:08.070 --> 00:04:10.070
and remember, electronegativity,
it goes

00:04:10.070 --> 00:04:12.340
from the bottom left
to the top right.

00:04:12.340 --> 00:04:14.890
So these are the most
electronegative atoms, which

00:04:14.890 --> 00:04:19.269
means they love to hog electrons
the most. These are

00:04:19.269 --> 00:04:21.110
the least. Or you could also
call them the most

00:04:21.110 --> 00:04:24.080
electropositive, which means
they like to give away

00:04:24.080 --> 00:04:26.430
electrons the most.

00:04:26.430 --> 00:04:27.800
So what you would do
is you say, OK.

00:04:27.800 --> 00:04:29.810
The more electronegative,
let's just say that they

00:04:29.810 --> 00:04:32.710
actually get the electrons.

00:04:32.710 --> 00:04:35.070
And that the more
electropositive atoms, they

00:04:35.070 --> 00:04:36.140
give the electrons.

00:04:36.140 --> 00:04:37.700
Even though we know that it's
something in between.

00:04:37.700 --> 00:04:39.050
It's actually partial.

00:04:39.050 --> 00:04:40.030
Now these numbers.

00:04:40.030 --> 00:04:47.760
These hypothetical ionization
of these hydrogen molecules.

00:04:47.760 --> 00:04:49.580
This is called their
oxidation number.

00:04:53.900 --> 00:05:04.720
The oxidation number of hydrogen
in H2O is plus 1.

00:05:04.720 --> 00:05:09.420
So we could write a plus 1 for
each of the hydrogens there.

00:05:09.420 --> 00:05:14.040
And then oxidation number for
the oxygen is minus 2.

00:05:17.230 --> 00:05:20.150
And the way we talk about it,
we say that the hydrogen

00:05:20.150 --> 00:05:23.330
molecules here have
been oxidized.

00:05:23.330 --> 00:05:27.700
And it's kind of like saying
that you've been-- In this

00:05:27.700 --> 00:05:31.010
case, they were oxidized
by oxygen.

00:05:31.010 --> 00:05:33.120
And this is actually a very
confusing point, or at least

00:05:33.120 --> 00:05:34.210
it was to me initially.

00:05:34.210 --> 00:05:36.500
Because when I first learned
about oxidation, I was like,

00:05:36.500 --> 00:05:38.890
oh, that's what oxygen
does to things.

00:05:38.890 --> 00:05:42.180
Because the word has the word,
oxygen, or at least the

00:05:42.180 --> 00:05:43.910
beginning part of oxygen.

00:05:43.910 --> 00:05:47.255
So I thought, oh, oxidation must
mean what oxygen does to

00:05:47.255 --> 00:05:49.100
other atoms, which
means it takes

00:05:49.100 --> 00:05:50.460
electrons away from them.

00:05:50.460 --> 00:05:52.880
Usually it takes 2 electrons
for itself.

00:05:52.880 --> 00:05:55.390
Maybe it took 1 from each of the
other atoms. So if you've

00:05:55.390 --> 00:06:00.000
been oxidized, you've had
electrons taken away from you.

00:06:00.000 --> 00:06:02.530
And so you'll have a
positive charge.

00:06:02.530 --> 00:06:05.190
Now, that interpretation
is only partially true.

00:06:05.190 --> 00:06:09.550
The reality is it does not have
to be oxygen that's doing

00:06:09.550 --> 00:06:10.840
the oxidation.

00:06:10.840 --> 00:06:15.440
So for example, let me
do hydrogen fluoride.

00:06:19.650 --> 00:06:22.060
H F.

00:06:22.060 --> 00:06:26.250
I've shed my habit of writing
fluorine as Fl.

00:06:26.250 --> 00:06:30.060
I've now remembered that its
elemental symbol is just F.

00:06:30.060 --> 00:06:33.585
So hydrogen fluoride, if it was
in an aqueous solution,

00:06:33.585 --> 00:06:35.860
it'd be hydrofluoric acid.

00:06:35.860 --> 00:06:40.420
That's hydrogen here bonding
with fluorine, one of the most

00:06:40.420 --> 00:06:43.980
electronegative atoms. So what's
going to happen here?

00:06:43.980 --> 00:06:47.960
Once again, the reality is that
hydrogen is sharing--

00:06:47.960 --> 00:06:49.550
it's a covalent bond
with fluorine.

00:06:49.550 --> 00:06:52.190
But the electron spends most
of its time here, on the

00:06:52.190 --> 00:06:53.630
fluorine atom.

00:06:53.630 --> 00:06:56.130
So you're going to have a
partial negative charge here,

00:06:56.130 --> 00:06:57.900
and a partial positive.

00:06:57.900 --> 00:07:00.240
But we don't like this partial,
halfway game.

00:07:00.240 --> 00:07:00.980
We want to say, look.

00:07:00.980 --> 00:07:05.020
If this was, hypothetically, an
ionic bond, if one of these

00:07:05.020 --> 00:07:07.300
people have to gain or
lose an electron, how

00:07:07.300 --> 00:07:08.290
would it play out?

00:07:08.290 --> 00:07:11.350
In that situation, this guy
likes to hog electrons more

00:07:11.350 --> 00:07:12.610
than hydrogen does.

00:07:12.610 --> 00:07:18.270
So the fluorine would gain an
electron and have an oxidation

00:07:18.270 --> 00:07:20.000
number of minus 1.

00:07:20.000 --> 00:07:22.520
And the hydrogen would lose
an electron, and have an

00:07:22.520 --> 00:07:24.950
oxidation number of plus 1.

00:07:24.950 --> 00:07:27.480
In this case, hydrogen
has been oxidized.

00:07:31.590 --> 00:07:35.140
And notice, there's no
oxygen to be seen.

00:07:35.140 --> 00:07:38.620
So the way I think about it,
fluorine did to hydrogen what

00:07:38.620 --> 00:07:39.890
oxygen would have done.

00:07:39.890 --> 00:07:40.640
For example.

00:07:40.640 --> 00:07:43.540
If I say that you've been
Bernie Madoff'ed.

00:07:43.540 --> 00:07:47.250
Bernie Madoff might not be the
actual individual who's taking

00:07:47.250 --> 00:07:49.870
your money and putting it into
a Ponzi scheme, but it would

00:07:49.870 --> 00:07:51.870
mean that someone else is doing
the same thing that

00:07:51.870 --> 00:07:54.240
Bernie Madoff would
have done to you.

00:07:54.240 --> 00:07:57.010
So in the same way, even though
there's no oxygen here,

00:07:57.010 --> 00:08:00.232
fluorine has oxidized
the hydrogen.

00:08:00.232 --> 00:08:04.160
Now, I'll introduce another
word, and that's reduction.

00:08:04.160 --> 00:08:07.310
Reduction is the opposite
of oxidation.

00:08:07.310 --> 00:08:08.560
I'll write it in blue.

00:08:12.530 --> 00:08:17.573
And this just means a
reduction of charge.

00:08:25.990 --> 00:08:29.970
So in this hydrogen fluorine
molecule, or hydrogen fluoride

00:08:29.970 --> 00:08:33.350
molecule, hydrogen has
been oxidized.

00:08:33.350 --> 00:08:36.030
It's been oxidized because
electrons have been

00:08:36.030 --> 00:08:37.000
taken away from it.

00:08:37.000 --> 00:08:39.809
That's what oxygen would
have done to it.

00:08:39.809 --> 00:08:41.950
And so it has, now,
a positive charge.

00:08:41.950 --> 00:08:44.280
And its oxidation number
is plus 1.

00:08:44.280 --> 00:08:47.900
Because exactly one electron
would have been taken away

00:08:47.900 --> 00:08:50.930
from this in a hypothetical
ionic bond.

00:08:50.930 --> 00:08:52.750
Now, fluorine has
been reduced.

00:08:55.370 --> 00:09:03.040
Its oxidation state
was reduced by 1.

00:09:03.040 --> 00:09:04.690
If there was no hydrogen
around, it

00:09:04.690 --> 00:09:05.890
would've been neutral.

00:09:05.890 --> 00:09:08.050
But now it has a
minus 1 charge.

00:09:08.050 --> 00:09:11.110
Because in our hypothetical
world of there's no partial

00:09:11.110 --> 00:09:12.830
charge, it's more
electronegative.

00:09:12.830 --> 00:09:15.400
It took the atom from
the hydrogen.

00:09:15.400 --> 00:09:16.900
So it has a minus 1 charge.

00:09:16.900 --> 00:09:19.280
Its charge has been reduced.

00:09:19.280 --> 00:09:21.880
Its oxidation state
has been reduced.

00:09:21.880 --> 00:09:25.380
Your oxidation state is just
that hypothetical charge.

00:09:25.380 --> 00:09:27.210
And that's how I think of it.

00:09:27.210 --> 00:09:30.990
Reduction means reduction in
your hypothetical charge.

00:09:30.990 --> 00:09:34.270
Oxidation means, what would
oxygen have done do you?

00:09:34.270 --> 00:09:36.360
Which means it would've taken
away electrons, which would

00:09:36.360 --> 00:09:38.410
mean you have a positive
charge.

00:09:38.410 --> 00:09:40.640
But if you look in some
textbooks, or some teachers,

00:09:40.640 --> 00:09:41.890
they'll give you a mnemonic.

00:09:45.790 --> 00:09:48.250
Leo the lion says, GER.

00:09:48.250 --> 00:09:51.310
I'll say, says, in small,
because it's really irrelevant

00:09:51.310 --> 00:09:52.560
to the mnemonic.

00:09:58.250 --> 00:10:03.370
And this is just a way of
remembering that Leo means

00:10:03.370 --> 00:10:11.600
Losing Electrons is equal
to Oxidation.

00:10:11.600 --> 00:10:22.590
And that Gaining Electrons
is equal to Reduction.

00:10:22.590 --> 00:10:24.770
Now, to me, when you first said,
oh wait, I'm gaining

00:10:24.770 --> 00:10:26.360
electrons, but I'm
getting reduced.

00:10:26.360 --> 00:10:27.200
What's getting reduced?

00:10:27.200 --> 00:10:29.000
What's getting reduced is
your charge, because

00:10:29.000 --> 00:10:30.070
electrons are negative.

00:10:30.070 --> 00:10:32.030
You're gaining something
with a negative charge.

00:10:32.030 --> 00:10:34.380
So that's where the reduction
comes from.

00:10:34.380 --> 00:10:38.550
Oxygen comes from the fact that
oxygen will normally make

00:10:38.550 --> 00:10:39.820
you lose electrons.

00:10:39.820 --> 00:10:42.230
It will normally take electrons
away from you, even

00:10:42.230 --> 00:10:44.260
though oxygen might
not have anything

00:10:44.260 --> 00:10:45.830
to do with the reaction.

00:10:45.830 --> 00:10:49.560
Now, we can look at the periodic
table, and we can

00:10:49.560 --> 00:10:55.290
guess, in most molecules, what
an oxidation state of a given

00:10:55.290 --> 00:10:56.600
atom will be.

00:10:56.600 --> 00:10:58.790
All of these guys are
alkali earth metals.

00:10:58.790 --> 00:11:00.450
And this is really a review.

00:11:00.450 --> 00:11:01.960
It's kind of taught
as something new.

00:11:01.960 --> 00:11:03.710
But we know all of these guys.

00:11:03.710 --> 00:11:08.920
They love to give up electrons,
because that makes

00:11:08.920 --> 00:11:09.610
them stable.

00:11:09.610 --> 00:11:11.560
Because they have this one
electron in their outer shell.

00:11:11.560 --> 00:11:15.310
So they tend to have an
oxidation state of plus 1,

00:11:15.310 --> 00:11:18.010
which means that they tend
to give away an electron.

00:11:18.010 --> 00:11:21.670
For example, if I write
sodium chloride.

00:11:21.670 --> 00:11:24.850
In this case, the oxidation
state is truly reflective of

00:11:24.850 --> 00:11:26.480
their charge.

00:11:26.480 --> 00:11:29.230
Its charge is equal to
its oxidation state.

00:11:29.230 --> 00:11:31.460
It gave away an electron.

00:11:31.460 --> 00:11:35.640
And that's true for all of the
alkali metals right here.

00:11:35.640 --> 00:11:37.160
Remember, I don't include
hydrogen there.

00:11:37.160 --> 00:11:38.810
Because hydrogen is a little
bit of a special case.

00:11:38.810 --> 00:11:41.690
It could have been thrown here
on the periodic table.

00:11:41.690 --> 00:11:44.000
Because it has one electron in
its outermost shell, which is

00:11:44.000 --> 00:11:45.160
just its only shell.

00:11:45.160 --> 00:11:48.870
But it's also happy to get two,
and have a configuration

00:11:48.870 --> 00:11:49.500
like helium.

00:11:49.500 --> 00:11:51.460
So you can almost view it
as it's very close to

00:11:51.460 --> 00:11:52.610
completing its shell.

00:11:52.610 --> 00:11:59.040
So it sometimes has alkali
metal-type properties.

00:11:59.040 --> 00:12:00.930
And sometimes it has halogen
properties, where it wants to

00:12:00.930 --> 00:12:02.400
gain electrons.

00:12:02.400 --> 00:12:06.510
So hydrogen-- Let's say hydrogen
is bonding with one

00:12:06.510 --> 00:12:07.740
of these guys, right?

00:12:07.740 --> 00:12:09.450
So let's say you had
lithium hydride.

00:12:16.040 --> 00:12:19.630
So in this case, lithium,
is right here.

00:12:19.630 --> 00:12:21.720
It loves to lose
its electrons.

00:12:21.720 --> 00:12:23.770
So it'll lose its electron.

00:12:23.770 --> 00:12:27.780
So it'll have an oxidation
state of plus 1.

00:12:27.780 --> 00:12:30.070
And it will lose its electron
to hydrogen.

00:12:30.070 --> 00:12:34.060
Because hydrogen is more
electronegative than lithium.

00:12:34.060 --> 00:12:36.110
Because hydrogen, you give
it 1 electron then it has

00:12:36.110 --> 00:12:38.510
electron configuration
like helium.

00:12:38.510 --> 00:12:41.230
So hydrogen will be minus 1.

00:12:41.230 --> 00:12:44.610
In this situation, hydrogen
bonds with people roughly on

00:12:44.610 --> 00:12:46.680
the left-hand side of
the periodic table.

00:12:46.680 --> 00:12:49.920
So lithium-- just to review
our terminology.

00:12:49.920 --> 00:12:53.270
Lithium was oxidized
by the hydrogen.

00:12:53.270 --> 00:12:56.050
Hydrogen was reduced
by the lithium.

00:12:58.570 --> 00:12:59.790
Its charge went down.

00:12:59.790 --> 00:13:01.300
Their oxidation number?

00:13:01.300 --> 00:13:04.310
Plus 1 for the lithium, minus
1 for the hydrogen.

00:13:04.310 --> 00:13:07.860
What is the total sum of
an oxidation state

00:13:07.860 --> 00:13:08.730
for a neutral molecule?

00:13:08.730 --> 00:13:12.600
In this case, you add up the
charges and you get, oh, it's

00:13:12.600 --> 00:13:14.210
equal to 0.

00:13:14.210 --> 00:13:16.860
And that's a big takeaway.

00:13:16.860 --> 00:13:20.680
Then, in general, if you have
a neutral molecule-- let me

00:13:20.680 --> 00:13:21.930
write that down.

00:13:28.120 --> 00:13:29.370
I'll just say neutral
compound.

00:13:33.410 --> 00:13:42.730
Then the oxidation
states add to 0.

00:13:42.730 --> 00:13:45.150
And if you have a non-neutral
compound, if you have a plus 1

00:13:45.150 --> 00:13:48.830
charge, the oxidation states
of all of the molecules in

00:13:48.830 --> 00:13:50.830
your compound are going to
add up to your charge.

00:13:54.680 --> 00:13:56.050
Well, let me give
another example.

00:13:56.050 --> 00:13:57.960
So in this case, I had hydrogen

00:13:57.960 --> 00:13:59.330
bonding with these guys.

00:13:59.330 --> 00:14:02.250
In that case, hydrogen is the
one that took the electron.

00:14:02.250 --> 00:14:03.530
Let's say I had OH minus.

00:14:16.450 --> 00:14:19.430
So hydrogen, in this case,
when it's dealing with

00:14:19.430 --> 00:14:21.230
something that's not on the
left-hand side, when it's

00:14:21.230 --> 00:14:23.640
dealing with a super
electronegative atom,

00:14:23.640 --> 00:14:26.780
hydrogen's oxidation
state is plus 1.

00:14:26.780 --> 00:14:28.030
Right?

00:14:32.390 --> 00:14:35.660
Now, oxygen's typical oxidation
state, when it bonds

00:14:35.660 --> 00:14:38.530
with almost everything else--
and I'll give a special case

00:14:38.530 --> 00:14:40.770
in a few moments-- is minus 2.

00:14:40.770 --> 00:14:43.190
Because it normally takes two
electrons from other things.

00:14:43.190 --> 00:14:44.440
We saw that with the water.

00:14:44.440 --> 00:14:49.010
Oxygen's oxidation state, in
most cases, is minus 2.

00:14:49.010 --> 00:14:51.600
So even though it hasn't gained
two electrons here, we

00:14:51.600 --> 00:14:53.340
would write its oxidation
number as

00:14:53.340 --> 00:14:56.180
minus 2 for the oxygen.

00:14:56.180 --> 00:14:59.420
And then you add up the
two oxidation states,

00:14:59.420 --> 00:15:02.930
minus 2 plus 1.

00:15:02.930 --> 00:15:04.400
Well, actually I'll
take that back.

00:15:04.400 --> 00:15:06.640
Oxygen has gained an extra
electron because this is

00:15:06.640 --> 00:15:07.840
already an ion.

00:15:07.840 --> 00:15:11.500
So in this case, this OH, oxygen
has stolen an electron

00:15:11.500 --> 00:15:12.280
from someone else.

00:15:12.280 --> 00:15:15.150
Let me draw that.

00:15:15.150 --> 00:15:23.260
And then oxygen normally only
has 1, 2, 3, 4-- so oxygen has

00:15:23.260 --> 00:15:25.410
completed its valence shell.

00:15:25.410 --> 00:15:28.970
So essentially, it took
an extra electron

00:15:28.970 --> 00:15:29.900
from some place else.

00:15:29.900 --> 00:15:33.430
So it definitely does have a
minus 2 charge in this case.

00:15:33.430 --> 00:15:35.480
Oxygen has a minus 2 charge.

00:15:35.480 --> 00:15:38.110
Hydrogen has a plus 1 charge,
because this electron was

00:15:38.110 --> 00:15:40.000
taken from it by the oxygen.

00:15:40.000 --> 00:15:43.605
And so the total charge of this
molecule is minus 2 plus

00:15:43.605 --> 00:15:45.480
1, which is minus 1.

00:15:45.480 --> 00:15:48.700
And that's why you have a
minus 1 charge on OH.

00:15:48.700 --> 00:15:51.000
So you add up the oxidation
states, and you're going to

00:15:51.000 --> 00:15:53.690
get the charge of the
atom under question.

00:15:53.690 --> 00:15:56.280
Now, we already saw that most
of these guys have an

00:15:56.280 --> 00:15:58.310
oxidation state of plus 1.

00:15:58.310 --> 00:16:02.350
These guys here, the alkaline
earth metals, have an oxygen

00:16:02.350 --> 00:16:05.370
state of plus 2, because they
like to give away two atoms.

00:16:05.370 --> 00:16:07.950
We already saw hydrogen.

00:16:07.950 --> 00:16:11.680
If they bond with people here,
they take the electrons.

00:16:11.680 --> 00:16:13.100
So they get reduced.

00:16:13.100 --> 00:16:15.160
So it has an oxidation
state of minus 1.

00:16:15.160 --> 00:16:17.570
But if hydrogen bonds with these
guys on the right-hand

00:16:17.570 --> 00:16:19.310
side, it gives away
the electrons.

00:16:19.310 --> 00:16:21.350
So it would have an oxidation
state of plus 1.

00:16:21.350 --> 00:16:23.280
We saw that with hydrogen
fluoride.

00:16:23.280 --> 00:16:24.110
It would be the case
with hydrogen

00:16:24.110 --> 00:16:25.940
chloride, hydrogen bromide.

00:16:25.940 --> 00:16:27.280
You saw the case with water.

00:16:27.280 --> 00:16:30.020
All of these cases, hydrogen is
the one that's giving away

00:16:30.020 --> 00:16:30.740
the electrons.

00:16:30.740 --> 00:16:32.500
So it gets oxidized.

00:16:32.500 --> 00:16:37.390
And these guys get reduced, or
their the oxidizing agent.

00:16:37.390 --> 00:16:40.990
Now, what about these
guys over here?

00:16:40.990 --> 00:16:43.680
What about our halogens?

00:16:43.680 --> 00:16:48.320
Well, they all like
to take electrons.

00:16:48.320 --> 00:16:53.010
So their typical oxidation
state is minus 1.

00:16:53.010 --> 00:16:55.170
Oxygen's typical oxidation
state?

00:16:55.170 --> 00:16:57.410
We just saw it's minus 2.

00:16:57.410 --> 00:16:59.580
And I'll show you an example,
which is really one of the few

00:16:59.580 --> 00:17:01.100
cases where oxygen
doesn't have a

00:17:01.100 --> 00:17:03.300
minus 2 oxidation state.

00:17:03.300 --> 00:17:05.430
And then there's a lot of
these other elements, in

00:17:05.430 --> 00:17:06.910
between, that can have multiple

00:17:06.910 --> 00:17:08.270
different oxidation states.

00:17:08.270 --> 00:17:09.599
And we'll see that in
a lot of examples.

00:17:09.599 --> 00:17:12.060
But if you know that hydrogen
is plus or minus 1, if

00:17:12.060 --> 00:17:15.020
hydrogen is bonding with these
guys, it's a plus 1. it gives

00:17:15.020 --> 00:17:16.180
away electrons.

00:17:16.180 --> 00:17:18.480
If it bonds with these
guys, it's a minus 1.

00:17:18.480 --> 00:17:19.990
it takes the electrons.

00:17:19.990 --> 00:17:22.960
If you know oxygen tends to take
two electrons, so it has

00:17:22.960 --> 00:17:24.780
a minus 2 oxidation state.

00:17:24.780 --> 00:17:26.780
And the halogens take one.

00:17:26.780 --> 00:17:29.270
If you know that, you can
normally figure out all of the

00:17:29.270 --> 00:17:30.920
other people in the compound.

00:17:30.920 --> 00:17:32.390
So what I'll do is I'll leave
you there for now.

00:17:32.390 --> 00:17:33.670
I realize I'm running
over time.

00:17:33.670 --> 00:17:36.260
In the next video, we're going
to do some maybe more

00:17:36.260 --> 00:17:38.160
difficult examples.


Weak Acid Titration

WEBVTT
Kind: captions
Language: en

00:00:00.950 --> 00:00:03.510
Let's say I have a bucket of
aqueous solution, and it

00:00:03.510 --> 00:00:05.440
contains some weak acid in it.

00:00:05.440 --> 00:00:06.550
And so weak acid.

00:00:06.550 --> 00:00:09.080
We've done this multiple
times.

00:00:09.080 --> 00:00:12.620
It's in equilibrium with
its dissociated state.

00:00:12.620 --> 00:00:13.585
So it would be in equilibrium.

00:00:13.585 --> 00:00:15.980
Let me write the aqueous.

00:00:15.980 --> 00:00:18.540
So it's a weak acid, where the
A could be any other kind of

00:00:18.540 --> 00:00:20.000
molecular group.

00:00:20.000 --> 00:00:22.480
That could be the conjugate
base of the weak acid.

00:00:22.480 --> 00:00:33.810
So that's in equilibrium with
some hydrogen ions, so it'll

00:00:33.810 --> 00:00:35.670
produce some of them, but
some of this is going to

00:00:35.670 --> 00:00:37.190
be around as well.

00:00:37.190 --> 00:00:39.900
Plus its conjugate base.

00:00:39.900 --> 00:00:41.150
A minus.

00:00:41.150 --> 00:00:42.740
Aqueous.

00:00:42.740 --> 00:00:46.680
And just like we did with the
strong acid titration, let's

00:00:46.680 --> 00:00:48.900
measure it's pH.

00:00:48.900 --> 00:00:50.226
So I'll do that right here.

00:00:52.930 --> 00:00:55.430
Actually, I want to do
it a little higher.

00:00:55.430 --> 00:00:59.340
So let's do it right there.

00:00:59.340 --> 00:01:00.670
Let me make a little
scale here.

00:01:00.670 --> 00:01:02.520
So this is 0.

00:01:02.520 --> 00:01:06.920
Let's say that right
there is 7.

00:01:06.920 --> 00:01:09.210
That this right here is 14.

00:01:09.210 --> 00:01:12.620
So our first measurement of the
pH of our solution-- we

00:01:12.620 --> 00:01:13.920
have a maybe a liter of it.

00:01:13.920 --> 00:01:16.370
Let's say we have 1 liter
of our solution.

00:01:16.370 --> 00:01:18.680
We don't know the concentration
of this thing.

00:01:18.680 --> 00:01:21.000
But let's say our first
measurement, we go and we say

00:01:21.000 --> 00:01:23.470
OK, it's got a pH of--
I don't know.

00:01:23.470 --> 00:01:23.650
Something here.

00:01:23.650 --> 00:01:25.470
Let's say it's got a pH of 3.

00:01:25.470 --> 00:01:28.680
So it's acidic, not ultra
acidic, but it's acidic.

00:01:28.680 --> 00:01:30.590
So it's got a pH of
3 right there.

00:01:30.590 --> 00:01:32.180
This whole scale on the
left-hand side.

00:01:32.180 --> 00:01:35.000
This is all pH, just like
in the last video.

00:01:35.000 --> 00:01:36.695
This is pH.

00:01:36.695 --> 00:01:39.390
The pH is going up as
you go up the scale.

00:01:39.390 --> 00:01:40.740
That's our first measurement.

00:01:40.740 --> 00:01:43.790
Now we're going to titrate this
weak acid, just like we

00:01:43.790 --> 00:01:46.110
titrated the strong base.

00:01:46.110 --> 00:01:48.870
And normally when you do this--
I mean, you can, in

00:01:48.870 --> 00:01:50.320
theory, titrate with anything.

00:01:50.320 --> 00:01:54.580
But you normally titrate-- if
you're titrating an acid,

00:01:54.580 --> 00:01:57.040
whether it's weak or strong--
you always titrate with a

00:01:57.040 --> 00:01:58.530
strong base.

00:01:58.530 --> 00:02:02.260
And if you are titrating a
base, whether it's weaker

00:02:02.260 --> 00:02:04.980
strong, the reagent, or the
thing that you're going to

00:02:04.980 --> 00:02:07.640
add, is going to be
a strong acid.

00:02:07.640 --> 00:02:12.460
So in this case, even though
we're going to drop some drops

00:02:12.460 --> 00:02:14.480
into a weak acid, we're
going to drop

00:02:14.480 --> 00:02:16.720
drops of a strong base.

00:02:16.720 --> 00:02:21.250
So let's use sodium
hydroxide again.

00:02:21.250 --> 00:02:22.520
NaOH.

00:02:22.520 --> 00:02:24.420
And just to make things
interesting, to make the math

00:02:24.420 --> 00:02:29.052
a little interesting, let's say
it's a 0.5 molar solution.

00:02:29.052 --> 00:02:32.620
And we're going to add it
in increments of little

00:02:32.620 --> 00:02:34.460
100-milliliter increments.

00:02:34.460 --> 00:02:36.050
A tenth of a liter.

00:02:36.050 --> 00:02:39.600
But the interesting thing is
really the shape of the curve

00:02:39.600 --> 00:02:43.800
that forms, or the titration
curve, or the pH curve.

00:02:43.800 --> 00:02:48.140
Let me write what the
reaction of NaOH.

00:02:48.140 --> 00:02:49.340
Just so you have that
in your head.

00:02:49.340 --> 00:02:53.010
NaOH, sodium hydroxide,
and I did this in

00:02:53.010 --> 00:02:53.700
the previous video.

00:02:53.700 --> 00:02:57.900
It disassociates completely
in water.

00:02:57.900 --> 00:03:01.520
And so it turns into Na plus--
all this is aqueous, of

00:03:01.520 --> 00:03:04.410
course-- plus OH minus.

00:03:04.410 --> 00:03:05.870
So when you throw even
a little bit.

00:03:05.870 --> 00:03:11.175
Let's say you throw in-- this
is 0.5 molar solution-- 100

00:03:11.175 --> 00:03:12.425
milliliters.

00:03:16.050 --> 00:03:20.180
That's equal to one
tenth of a liter.

00:03:20.180 --> 00:03:22.850
This 0.5 molar, that's
its molarity.

00:03:22.850 --> 00:03:24.680
So it says hey, I'm going
to have half a

00:03:24.680 --> 00:03:26.580
mole of this stuff.

00:03:26.580 --> 00:03:28.120
Or really, of this stuff.

00:03:28.120 --> 00:03:30.100
I'm going to have half
a mole of hydroxide

00:03:30.100 --> 00:03:32.060
ions for every liter.

00:03:32.060 --> 00:03:35.690
So in one tenth of a liter,
that means that I'm

00:03:35.690 --> 00:03:37.330
going to have what?

00:03:37.330 --> 00:03:38.980
I'm going to have
half of that.

00:03:38.980 --> 00:03:46.480
So I'm going have 1/20,
or 0.05 moles.

00:03:46.480 --> 00:03:50.030
So let's say this point right
here's is 100 milliliters.

00:03:50.030 --> 00:03:52.510
I've added 100 milliliters
of our solution.

00:03:52.510 --> 00:03:55.770
That's going to have
0.5 moles of OH.

00:03:55.770 --> 00:03:57.950
Because remember, we could say
it's 0.5 moles of this, but

00:03:57.950 --> 00:03:59.870
whenever this is in solution,
it disassociates

00:03:59.870 --> 00:04:00.720
completely into this.

00:04:00.720 --> 00:04:03.010
So if you have 0.5 moles of
this, you really have 0.5

00:04:03.010 --> 00:04:04.220
moles of this.

00:04:04.220 --> 00:04:06.860
This will be in this form
completely, because it's a

00:04:06.860 --> 00:04:08.280
strong base.

00:04:08.280 --> 00:04:11.280
So if I have 0.5 moles of this
and I throw it into the

00:04:11.280 --> 00:04:13.040
solution, what's
it going to do?

00:04:13.040 --> 00:04:18.769
It's going to start sopping up
some of our hydrogen ions.

00:04:18.769 --> 00:04:24.690
So it's going to increase the
pH, and it's going to keep

00:04:24.690 --> 00:04:25.440
increasing.

00:04:25.440 --> 00:04:26.910
Right?

00:04:26.910 --> 00:04:28.310
And this is a buffer.

00:04:28.310 --> 00:04:30.470
And this is a key thing
to remember.

00:04:30.470 --> 00:04:32.730
This is going to sop some
of this thing up.

00:04:32.730 --> 00:04:33.690
But this is a buffer.

00:04:33.690 --> 00:04:35.740
This is a weak acid.

00:04:35.740 --> 00:04:36.730
So it's in equilibrium.

00:04:36.730 --> 00:04:40.930
So it has this reserve over
here of the acid form.

00:04:40.930 --> 00:04:46.830
So when you start lowering this
by adding a strong base

00:04:46.830 --> 00:04:49.640
to it, to sop this up, what's
going to happen?

00:04:49.640 --> 00:04:52.320
The reaction is going to move
in this direction to kind of

00:04:52.320 --> 00:04:55.520
backfill, to fill or
to almost replace.

00:04:55.520 --> 00:05:00.170
It won't get quite to where it
was before, but it's going to

00:05:00.170 --> 00:05:03.560
dampen the stress on
the equilibrium.

00:05:03.560 --> 00:05:07.170
So what's going to happen is the
concentration of your weak

00:05:07.170 --> 00:05:09.440
acid is going to go down.

00:05:09.440 --> 00:05:11.770
It's going to try to backfill
this stuff.

00:05:11.770 --> 00:05:14.980
But every time the reaction goes
in this direction, this

00:05:14.980 --> 00:05:16.630
is going to go up.

00:05:16.630 --> 00:05:16.900
Right?

00:05:16.900 --> 00:05:20.140
Remember, nothing is sopping
up the conjugate

00:05:20.140 --> 00:05:21.240
base up here, right?

00:05:21.240 --> 00:05:23.410
We're just sopping up
the hydrogen ions.

00:05:23.410 --> 00:05:26.840
As we sop up more and more
hydrogen ions, this reaction

00:05:26.840 --> 00:05:29.560
goes more and more in the
rightward direction.

00:05:29.560 --> 00:05:31.380
But as it goes more and more
in the rightward direction,

00:05:31.380 --> 00:05:34.550
we're just adding more
and more to this.

00:05:34.550 --> 00:05:36.770
And the base just takes more
and more of this out.

00:05:36.770 --> 00:05:38.940
So essentially when the base
is taking this out, it's

00:05:38.940 --> 00:05:42.160
really taking this out-- or it's
taking half of this out.

00:05:42.160 --> 00:05:44.330
It's taking the H part
of this out.

00:05:44.330 --> 00:05:47.200
Because that converts to that,
and it gets sopped up by the

00:05:47.200 --> 00:05:48.430
by the reagent.

00:05:48.430 --> 00:05:50.000
The strong base.

00:05:50.000 --> 00:05:55.450
And all of this ends
up on this side, as

00:05:55.450 --> 00:05:58.210
the conjugate base.

00:05:58.210 --> 00:06:02.640
So at some point, when we have
added the exact same amount of

00:06:02.640 --> 00:06:08.370
OH as there is of this stuff, as
there is of this stuff plus

00:06:08.370 --> 00:06:09.230
the hydrogen.

00:06:09.230 --> 00:06:12.900
When we've sopped up all
of the hydrogen,

00:06:12.900 --> 00:06:14.590
what's going to happen?

00:06:14.590 --> 00:06:18.720
Well, right when you get above
that point, when you start

00:06:18.720 --> 00:06:22.650
adding even more, you're going
to start getting very basic.

00:06:22.650 --> 00:06:24.750
It's just going to skyrocket
up the pH.

00:06:24.750 --> 00:06:26.880
Because then you're adding OHs,
and they're not being

00:06:26.880 --> 00:06:29.250
canceled out by any hydrogen
over there.

00:06:29.250 --> 00:06:31.460
And there's nothing to kind
of backfill the hydrogen.

00:06:31.460 --> 00:06:33.140
So you're going to get to
a very high reaction.

00:06:38.610 --> 00:06:44.560
When we add enough of the sodium
hydroxide that this

00:06:44.560 --> 00:06:47.970
completely neutralizes this
and this, and all we

00:06:47.970 --> 00:06:50.070
have left is that.

00:06:50.070 --> 00:06:54.055
Well, you might say, oh, we have
a pH of 7 because all of

00:06:54.055 --> 00:06:54.905
the acid is neutralized.

00:06:54.905 --> 00:06:57.560
But we don't have a pH of 7 like
we did with the strong

00:06:57.560 --> 00:07:00.990
base, because here, as we
sopped up stuff, we kept

00:07:00.990 --> 00:07:06.180
adding conjugate
base over here.

00:07:06.180 --> 00:07:07.830
Remember, and it's
a real base.

00:07:07.830 --> 00:07:09.180
It actually has basic
properties.

00:07:09.180 --> 00:07:13.690
If this is a strong acid, if
this was hydrogen chloride,

00:07:13.690 --> 00:07:15.070
I'd still have the chloride
over here.

00:07:15.070 --> 00:07:17.900
But chloride, even though it's
the conjugate base, it has no

00:07:17.900 --> 00:07:18.820
basic properties.

00:07:18.820 --> 00:07:21.290
It does not increase the
pH of a solution.

00:07:21.290 --> 00:07:25.360
But this stuff right here does
increase the pH of a solution.

00:07:25.360 --> 00:07:27.460
And we've done that in
previous videos.

00:07:27.460 --> 00:07:30.750
So once all of this is sopped
up, your equivalence point, or

00:07:30.750 --> 00:07:33.900
the point at which the number
of moles of this is equal to

00:07:33.900 --> 00:07:37.190
the number of moles of this,
is going to have a high pH.

00:07:37.190 --> 00:07:42.050
So it's going to be like
right over there.

00:07:42.050 --> 00:07:45.380
And then as you add more and
more of your reagent, or your

00:07:45.380 --> 00:07:48.420
strong base, your pH is going
to get higher and higher and

00:07:48.420 --> 00:07:50.440
approach 14 or even
go above it.

00:07:50.440 --> 00:07:52.060
So your titration curve
is going look

00:07:52.060 --> 00:07:53.310
something like this.

00:07:56.410 --> 00:07:57.690
And once again, you look
at the steepest

00:07:57.690 --> 00:07:59.940
point on the curve.

00:07:59.940 --> 00:08:01.626
It's right there.

00:08:01.626 --> 00:08:03.480
And you say, OK, that's
the equivalence point.

00:08:03.480 --> 00:08:06.580
That's the point at which
I had an equal amount of

00:08:06.580 --> 00:08:09.920
hydroxide having sopped up all
of the hydrogen, and I've run

00:08:09.920 --> 00:08:12.800
out of all of this stuff to
backfill the hydrogen.

00:08:12.800 --> 00:08:14.210
So you look at your
equivalence point.

00:08:14.210 --> 00:08:16.910
And you say, OK, if that
equivalence point was when I

00:08:16.910 --> 00:08:18.170
added, I don't know.

00:08:18.170 --> 00:08:23.500
Maybe that's when you added
2 liters of your reagent.

00:08:23.500 --> 00:08:25.900
So you added 2 liters
of your reagent.

00:08:25.900 --> 00:08:28.800
Your basic reagent,
right here.

00:08:28.800 --> 00:08:30.360
Or your titrant.

00:08:30.360 --> 00:08:31.490
Or titrator.

00:08:31.490 --> 00:08:33.270
I think that's another
word for it.

00:08:33.270 --> 00:08:34.720
How many moles have you added?

00:08:34.720 --> 00:08:39.600
Well it's 2 liters, it's 0.5
molar, so you've added 1 mole.

00:08:39.600 --> 00:08:44.530
So here you've added
1 mole of OH.

00:08:48.830 --> 00:08:52.250
And when you added 1 mole of OH,
you sopped up all of the

00:08:52.250 --> 00:08:54.060
whatever mole you had of this.

00:08:54.060 --> 00:08:55.980
So you clearly had
1 mole of that.

00:08:55.980 --> 00:09:02.660
So over here, your original
concentration of your-- I

00:09:02.660 --> 00:09:04.850
mean, you really could say your
concentration of your HA

00:09:04.850 --> 00:09:10.550
plus your initial equilibrium
hydrogen is equal to 1 molar.

00:09:10.550 --> 00:09:15.580
Now in most chemistry classes,
this number is way bigger than

00:09:15.580 --> 00:09:17.610
this number for most
weak acids.

00:09:17.610 --> 00:09:18.460
And you could look at that.

00:09:18.460 --> 00:09:22.230
Because if you look at some of
the pKa's for some weak acids,

00:09:22.230 --> 00:09:25.590
you'll see that this is some--
well, I won't go into the

00:09:25.590 --> 00:09:26.900
math, but this number's
a lot lower.

00:09:26.900 --> 00:09:30.010
So this is an indication,
essentially, of your initial

00:09:30.010 --> 00:09:31.960
concentration of
your weak acid.

00:09:31.960 --> 00:09:34.590
So you essentially just keep
titrating it, figure out the

00:09:34.590 --> 00:09:37.070
inflection point, you say, the
inflection point happened when

00:09:37.070 --> 00:09:40.330
I added 2 liters of the
titrator to it, which

00:09:40.330 --> 00:09:42.170
corresponded to 1 mole.

00:09:42.170 --> 00:09:46.000
So therefore I must have had 1
mole of my original acid in

00:09:46.000 --> 00:09:48.060
the equation.

00:09:48.060 --> 00:09:49.060
Another interesting thing.

00:09:49.060 --> 00:09:52.800
So if you said OK, I had 1 mole
of my original acid in

00:09:52.800 --> 00:09:53.430
the equation.

00:09:53.430 --> 00:09:58.250
So you say, this was 1 molar
originally, before I started

00:09:58.250 --> 00:09:59.520
titrating it all.

00:09:59.520 --> 00:10:03.695
And if I said that this is a 1
liter solution-- this is 1

00:10:03.695 --> 00:10:04.990
mole, not 1 molar.

00:10:04.990 --> 00:10:07.410
But if I know it's a 1-liter
solution, now I know that

00:10:07.410 --> 00:10:10.060
originally I had 1 mole
in 1 liter, so I had

00:10:10.060 --> 00:10:12.550
a molarity of 1.

00:10:12.550 --> 00:10:18.020
Now, let me go over something
else that's interesting about

00:10:18.020 --> 00:10:20.350
a basic reaction.

00:10:20.350 --> 00:10:21.810
Actually, let me do that in
the next video, because I

00:10:21.810 --> 00:10:23.960
think this one's getting
a little long.


Strong Acid Titration

WEBVTT
Kind: captions
Language: en

00:00:00.730 --> 00:00:06.070
Let's have 1 molar of-- let
me pick a strong acid.

00:00:06.070 --> 00:00:07.730
Let's say it's hydrochloric
acid.

00:00:10.880 --> 00:00:11.580
HCl.

00:00:11.580 --> 00:00:14.810
And we know, of course, that
since it's a strong acid it

00:00:14.810 --> 00:00:16.309
disassociates completely.

00:00:16.309 --> 00:00:20.370
So HCl in an aqueous solution--
or a solution of

00:00:20.370 --> 00:00:25.700
water-- disassociates completely
into hydrogen ions

00:00:25.700 --> 00:00:28.660
in an aqueous solution, which
is really hydronium.

00:00:28.660 --> 00:00:30.390
I've said that multiple times.

00:00:30.390 --> 00:00:34.670
Plus your chlorine anion
in an aqueous solution.

00:00:34.670 --> 00:00:37.300
So if you actually have a molar
of this, as soon as you

00:00:37.300 --> 00:00:40.580
put it in the water, you really
have 1 molar of this.

00:00:40.580 --> 00:00:44.120
So your hydrogen concentration
is really 1 molar.

00:00:44.120 --> 00:00:47.960
And of course we know, what's
the pH of that?

00:00:47.960 --> 00:00:53.170
Well, the pH is just the minus
log base 10 of your hydrogen

00:00:53.170 --> 00:00:55.270
concentration, which is 1.

00:00:55.270 --> 00:00:58.760
Which is the minus log of 0.

00:00:58.760 --> 00:01:00.610
So minus 0, which
is equal to 0.

00:01:00.610 --> 00:01:04.209
So your pH is going
to be equal to 0.

00:01:04.209 --> 00:01:07.560
And let's say for use in the
later part of this, let's say

00:01:07.560 --> 00:01:10.940
I have a liter of this.

00:01:10.940 --> 00:01:11.110
Right.

00:01:11.110 --> 00:01:13.290
This just tells me how many
moles per liter, and now I

00:01:13.290 --> 00:01:14.680
just told you that
I have 1 liter.

00:01:14.680 --> 00:01:17.210
So you know that you
have 1 mole.

00:01:17.210 --> 00:01:19.810
Let me make a little
chart here.

00:01:19.810 --> 00:01:23.650
I'll make a little box here.

00:01:23.650 --> 00:01:29.100
And let's make this
access right here.

00:01:29.100 --> 00:01:32.050
This is going to be my pH.

00:01:32.050 --> 00:01:34.800
And right when I take that first
measurement-- let's say

00:01:34.800 --> 00:01:41.700
this is 0 here, this is 7,
and this is 14 up here.

00:01:41.700 --> 00:01:45.660
When I take that first
measurement, my solution of

00:01:45.660 --> 00:01:52.270
hydrogen chloride, or
hydrochloric acid, has a pH 0.

00:01:52.270 --> 00:01:56.170
Now, what I'm going to do is
I'm going to titrate this

00:01:56.170 --> 00:01:57.530
aqueous solution of acid.

00:01:57.530 --> 00:02:01.240
And titrate just means drop
something else into it in

00:02:01.240 --> 00:02:03.650
controlled increments
to see what happens.

00:02:03.650 --> 00:02:06.121
And what I'm doing here
is going to be

00:02:06.121 --> 00:02:07.800
an acid-base titration.

00:02:07.800 --> 00:02:10.199
I have an acid here, and what
I'm actually going to titrate

00:02:10.199 --> 00:02:16.110
it with, or the reagent that
I'm going to add to it--

00:02:16.110 --> 00:02:19.540
The reagent is essentially a
reactant that, when you view

00:02:19.540 --> 00:02:21.400
it this way, that you're
adding to the solution.

00:02:21.400 --> 00:02:22.320
So it's the titrant.

00:02:22.320 --> 00:02:24.600
So in this case, my solution
already has

00:02:24.600 --> 00:02:25.825
hydrochloric acid in it.

00:02:25.825 --> 00:02:27.360
The pH is already 0.

00:02:27.360 --> 00:02:28.480
But what I'm going to
do is I'm going to

00:02:28.480 --> 00:02:29.880
add a strong base.

00:02:32.440 --> 00:02:34.410
Let's say sodium hydroxide.

00:02:34.410 --> 00:02:36.740
That's my favorite
strong base.

00:02:36.740 --> 00:02:40.500
And let's say I have a solution
that I have prepared

00:02:40.500 --> 00:02:42.230
ahead of time.

00:02:42.230 --> 00:02:45.595
Let's say it's a 1
molar solution.

00:02:48.670 --> 00:02:51.330
But I'm not going to add all
of it at the same time.

00:02:51.330 --> 00:02:52.780
I'm going to add it
in increments of

00:02:52.780 --> 00:02:54.300
a tenth of a liter.

00:02:54.300 --> 00:03:02.020
So this is 1 liter
that I've added.

00:03:02.020 --> 00:03:06.150
And then this would
be 2 liters.

00:03:06.150 --> 00:03:09.440
And so this would be
500 milliliters.

00:03:09.440 --> 00:03:11.240
500 mL.

00:03:11.240 --> 00:03:12.720
This is 1 liter.

00:03:12.720 --> 00:03:16.000
This is 1.5 liters.

00:03:16.000 --> 00:03:19.130
This 2 liters.

00:03:19.130 --> 00:03:22.740
And so on this axis, I'm going
to slowly add more and more of

00:03:22.740 --> 00:03:24.420
this hydroxide.

00:03:24.420 --> 00:03:25.670
So what's going to happen?

00:03:29.560 --> 00:03:32.610
As I slowly add hydroxide, it's
going to increase the pH.

00:03:32.610 --> 00:03:34.750
Why is it going to
increase the pH?

00:03:34.750 --> 00:03:35.950
Because this OH.

00:03:35.950 --> 00:03:40.130
Remember, hydroxide
is a strong base.

00:03:40.130 --> 00:03:44.750
So the NaOH disassociates
completely

00:03:44.750 --> 00:03:46.440
in an aqueous solution.

00:03:46.440 --> 00:03:51.820
To OH minus plus
sodium cation.

00:03:51.820 --> 00:03:52.370
Right?

00:03:52.370 --> 00:03:54.900
Now if I'm adding this to this
stuff here, what do you

00:03:54.900 --> 00:03:56.650
think's going to happen?

00:03:56.650 --> 00:03:59.840
Well this stuff right here,
these OHs are just going to

00:03:59.840 --> 00:04:01.960
sop up your hydrogens.

00:04:01.960 --> 00:04:04.430
So for the most part--
obviously, everything is

00:04:04.430 --> 00:04:04.950
probabalistic.

00:04:04.950 --> 00:04:07.380
And whenever we're dealing with
chemistry, there's always

00:04:07.380 --> 00:04:09.715
some molecules that won't
completely react, and there

00:04:09.715 --> 00:04:12.230
will always be some
concentration of anything.

00:04:12.230 --> 00:04:15.000
And that's why you actually
can never get to a 0

00:04:15.000 --> 00:04:17.990
concentration of hydrogen, no
matter how much base you add.

00:04:17.990 --> 00:04:20.570
Because there will always be
some hydrogen molecules that

00:04:20.570 --> 00:04:23.365
are just hiding just perfectly,
or they're in

00:04:23.365 --> 00:04:24.870
perfect equilibrium so
they don't react.

00:04:24.870 --> 00:04:26.720
But most of them will react.

00:04:26.720 --> 00:04:30.110
But as I add more H, these
are going to disappear.

00:04:30.110 --> 00:04:33.450
So my hydrogen concentration
is going to go down.

00:04:33.450 --> 00:04:37.360
The pH is a negative log of
that, so your pH will go up.

00:04:37.360 --> 00:04:41.950
Your pH will go up slowly--
well, it will look slowly,

00:04:41.950 --> 00:04:44.050
because it's a log chart,
so maybe it's doing

00:04:44.050 --> 00:04:45.610
something like this.

00:04:45.610 --> 00:04:48.230
Your pH concentration is going
to look something like-- as

00:04:48.230 --> 00:04:49.060
you add more and more.

00:04:49.060 --> 00:04:50.930
But you might say, oh that's
such a slow movement.

00:04:50.930 --> 00:04:52.250
Why isn't it occurring faster.

00:04:52.250 --> 00:04:54.560
But you have to think
about something.

00:04:54.560 --> 00:05:02.940
This right here, if this is a pH
of 1, this means-- so, at a

00:05:02.940 --> 00:05:04.810
pH of 0, this means
your hydrogen

00:05:04.810 --> 00:05:07.410
concentration is 1 molar.

00:05:07.410 --> 00:05:11.130
At a pH of 1, this means your
hydrogen concentration is at

00:05:11.130 --> 00:05:12.640
10 to the minus 1.

00:05:12.640 --> 00:05:15.480
Which is 0.1 molar.

00:05:15.480 --> 00:05:20.410
So even though on a log chart
it looks like you've made a

00:05:20.410 --> 00:05:22.190
very small movement, you've
actually gone

00:05:22.190 --> 00:05:23.640
by a factor of 10.

00:05:23.640 --> 00:05:26.205
In terms of your reduction of
your hydrogen concentration.

00:05:29.870 --> 00:05:34.360
You started off with 1 molar
of your hydrogen.

00:05:34.360 --> 00:05:38.910
By the time you've added roughly
900 milliliters of

00:05:38.910 --> 00:05:41.530
your solution, you probably
would have gotten

00:05:41.530 --> 00:05:42.220
rid of most of it.

00:05:42.220 --> 00:05:43.580
And obviously the denomenator's
changing,

00:05:43.580 --> 00:05:46.270
because I'm actually
adding volume here.

00:05:46.270 --> 00:05:46.600
Right?

00:05:46.600 --> 00:05:49.220
Because I'm actually adding
some volume here.

00:05:49.220 --> 00:05:50.120
So you would have to add them.

00:05:50.120 --> 00:05:51.850
But you get the I
general idea.

00:05:51.850 --> 00:05:55.380
That these actual molecules of
hydroxide are going to react

00:05:55.380 --> 00:05:56.420
with these things.

00:05:56.420 --> 00:05:59.370
They're going to essentially
turn into water and disappear

00:05:59.370 --> 00:06:02.340
into the solution.

00:06:02.340 --> 00:06:04.430
So these are going to
disappear with those

00:06:04.430 --> 00:06:06.010
disappearing, for
the most part.

00:06:06.010 --> 00:06:09.120
Which leads in an overall
reduction concentration of

00:06:09.120 --> 00:06:11.080
your hydrogen ions, which
will lead to an

00:06:11.080 --> 00:06:12.500
increase in your pH.

00:06:12.500 --> 00:06:15.590
Until some point where you have
pretty much sopped up as

00:06:15.590 --> 00:06:17.540
many of these things
as you can.

00:06:17.540 --> 00:06:19.520
There will always be a few of
them, but you've sopped up as

00:06:19.520 --> 00:06:20.780
many of them as you can.

00:06:20.780 --> 00:06:27.310
And so any incremental hydroxide
you add will

00:06:27.310 --> 00:06:30.950
actually go to build the
hydroxide concentration.

00:06:30.950 --> 00:06:33.140
Where the hydroxide
concentration is going to be

00:06:33.140 --> 00:06:35.690
greater than your hydrogen
concentration.

00:06:35.690 --> 00:06:37.480
But even better than that,
there's going to be some point

00:06:37.480 --> 00:06:40.780
where you've added just enough
of this that you have an equal

00:06:40.780 --> 00:06:43.030
amount of this and this.

00:06:43.030 --> 00:06:46.410
You have an equal amount of
hydroxide and hydrogen.

00:06:46.410 --> 00:06:48.200
Right?

00:06:48.200 --> 00:06:50.370
What happens, first of all, when
there's an equal amount

00:06:50.370 --> 00:06:52.380
of hydroxide and hydrogen?

00:06:52.380 --> 00:06:54.240
Well, then you're neutral.

00:06:54.240 --> 00:06:55.320
Right?

00:06:55.320 --> 00:06:58.390
If your hydroxide concentration
is equal to your

00:06:58.390 --> 00:07:02.280
hydrogen concentration,
you're neutral.

00:07:02.280 --> 00:07:03.320
This is just like water.

00:07:03.320 --> 00:07:07.810
You're at a pH of 7.

00:07:07.810 --> 00:07:11.010
So there's some point where your
hydroxide concentration

00:07:11.010 --> 00:07:14.270
is equal to your hydrogen
concentration, and your pH is

00:07:14.270 --> 00:07:15.730
going to be 7.

00:07:15.730 --> 00:07:17.680
And then above that,
you're just adding

00:07:17.680 --> 00:07:18.990
more and more hydroxide.

00:07:18.990 --> 00:07:21.690
The hydroxide concentration is
overwhelming your hydrogen

00:07:21.690 --> 00:07:22.630
concentration.

00:07:22.630 --> 00:07:24.580
And so you're going to get
really, really basic.

00:07:24.580 --> 00:07:27.040
And since this is a strong base,
you're eventually going

00:07:27.040 --> 00:07:29.340
to get to a pH of 14.

00:07:29.340 --> 00:07:31.490
Because you're essentially going
to have-- well, you're

00:07:31.490 --> 00:07:32.400
not going to have 1 molar.

00:07:32.400 --> 00:07:33.760
Let me erase that point.

00:07:33.760 --> 00:07:38.320
You're going to have
half a molar of OHs

00:07:38.320 --> 00:07:39.240
at the end of this.

00:07:39.240 --> 00:07:41.830
So your graph is going to look
something like this.

00:07:48.230 --> 00:07:49.850
Now, I mean, you're still
going to have hydrogen

00:07:49.850 --> 00:07:52.070
concentration, but it's going
to go really small.

00:07:52.070 --> 00:07:55.450
It's going to get really,
really small.

00:07:55.450 --> 00:07:56.030
Now.

00:07:56.030 --> 00:08:00.330
What's interesting about what
just happened here?

00:08:00.330 --> 00:08:07.570
At this point right here, your
OH is equal to your hydrogen.

00:08:07.570 --> 00:08:13.460
And the way I think about it is
the great majority of your

00:08:13.460 --> 00:08:17.210
moles of hydrogen were sopped
up at that point.

00:08:17.210 --> 00:08:20.980
So in order to stop up 1 mole of
hydrogen, you essentially,

00:08:20.980 --> 00:08:23.880
for the most part-- and there's
always things that

00:08:23.880 --> 00:08:25.740
don't get completely sopped up--
but you'll essentially

00:08:25.740 --> 00:08:33.919
have to add 1 mole
of OH to sop up

00:08:33.919 --> 00:08:35.280
your 1 mole of hydrogen.

00:08:35.280 --> 00:08:37.289
Remember, this is 1 molar and
you had 1 liter of it.

00:08:37.289 --> 00:08:41.650
So you had 1 mole of
hydrogen to sop up.

00:08:41.650 --> 00:08:44.700
So you need 1 mole of OH to sop
up, and how do you get 1

00:08:44.700 --> 00:08:46.040
mole of OH?

00:08:46.040 --> 00:08:49.130
Well, we said the concentration
of the solution

00:08:49.130 --> 00:08:51.110
was 1 molar.

00:08:51.110 --> 00:08:54.170
So once you put a liter in
there-- because that's 1 mole

00:08:54.170 --> 00:08:56.240
per liter-- once you put a liter
in there, you would have

00:08:56.240 --> 00:09:00.430
put 1 mole of hydroxide
cations in there

00:09:00.430 --> 00:09:01.390
to sop up the hydrogen.

00:09:01.390 --> 00:09:04.240
So this is the equivalence
point.

00:09:04.240 --> 00:09:07.180
Where have essentially, you have
an equal amount and very

00:09:07.180 --> 00:09:10.430
low concentrations of both
hydroxide and hydrogen.

00:09:10.430 --> 00:09:13.370
You've essentially sopped up all
of the hydrogen, but you

00:09:13.370 --> 00:09:15.880
also have sopped up all
of the hydroxide that

00:09:15.880 --> 00:09:17.170
you've added so far.

00:09:17.170 --> 00:09:19.660
After this point, any hydroxide
you add will

00:09:19.660 --> 00:09:23.520
essentially contribute to
increasing the pH even more.

00:09:23.520 --> 00:09:27.560
Or it'll kind of overwhelm your
hydrogen concentration.

00:09:27.560 --> 00:09:28.760
Now what's interesting
about this?

00:09:28.760 --> 00:09:31.510
In this example, I told you, oh,
I have hydrochloric acid

00:09:31.510 --> 00:09:33.870
and I gave you its concentration
ahead of time.

00:09:33.870 --> 00:09:35.070
But let's say I didn't
tell you that.

00:09:35.070 --> 00:09:37.730
Let's say it was some mystery
substance, and you just

00:09:37.730 --> 00:09:40.000
measured the mystery
substance's pH.

00:09:40.000 --> 00:09:43.970
At this point, you said
OK, it's pH is 0.

00:09:43.970 --> 00:09:47.560
And you titrated it with
this sodium hydroxide.

00:09:47.560 --> 00:09:50.250
And you said, well, gee, when
I added 1 mole-- because you

00:09:50.250 --> 00:09:56.810
figured out at a liter of the
solution-- of hydroxide ions.

00:09:56.810 --> 00:10:00.010
At that point, I've essentially
eliminated all of

00:10:00.010 --> 00:10:03.990
the hydrogen ions that were
in my original solution.

00:10:03.990 --> 00:10:07.300
So I must have had 1 mole
of whatever acid

00:10:07.300 --> 00:10:08.890
this was over here.

00:10:08.890 --> 00:10:11.620
So this would have told you that
my initial concentration

00:10:11.620 --> 00:10:18.680
was 1 mole of H plus.

00:10:18.680 --> 00:10:21.450
And since we know it's easy to
see how much volume you have,

00:10:21.450 --> 00:10:22.180
you see I have a liter.

00:10:22.180 --> 00:10:23.850
So you say my original
concentration of

00:10:23.850 --> 00:10:25.730
this acid is 1 molar.

00:10:25.730 --> 00:10:28.490
And you could also know that
what you're dealing with is a

00:10:28.490 --> 00:10:29.740
strong acid.

00:10:33.060 --> 00:10:35.460
And I encourage you to watch the
next to do I do the same

00:10:35.460 --> 00:10:37.390
thing with a weak acid.

00:10:37.390 --> 00:10:40.980
But because it's a strong acid
and a strong base reacting

00:10:40.980 --> 00:10:44.810
with each other, when you have
the same number moles of both,

00:10:44.810 --> 00:10:47.690
you get exactly to a pH of 7.

00:10:47.690 --> 00:10:50.620
They completely neutralize each
other, and you have a

00:10:50.620 --> 00:10:52.350
completely neutral solution.

00:10:52.350 --> 00:10:56.510
If you had a strong base with a
weak acid, the strong base--

00:10:56.510 --> 00:10:58.840
once you have the same amount as
the weak acid, it would've

00:10:58.840 --> 00:10:59.770
neutralized it.

00:10:59.770 --> 00:11:01.090
But you would have also
had some of the

00:11:01.090 --> 00:11:03.370
conjugate weak base left.

00:11:03.370 --> 00:11:05.950
So you would have actually
ended up with a basic pH.

00:11:05.950 --> 00:11:07.060
And I'm going to do that
in the next video.

00:11:07.060 --> 00:11:12.440
But if you see that the
equivalence point occurs at a

00:11:12.440 --> 00:11:15.850
pH of 7, you'd know that you
were titrating-- what you were

00:11:15.850 --> 00:11:19.150
dealing with was
a strong acid.

00:11:19.150 --> 00:11:21.780
Because it was completely
neutralized, not more than

00:11:21.780 --> 00:11:25.270
neutralized, by the
strong base.

00:11:25.270 --> 00:11:27.560
And if you want to eye-ball
where is equivalence point,

00:11:27.560 --> 00:11:29.150
you look for the steepest
point in the graph.

00:11:29.150 --> 00:11:31.840
Because that's the point where
you're going from having a

00:11:31.840 --> 00:11:35.090
majority of hydrogen versus
hydroxide, to having a

00:11:35.090 --> 00:11:37.120
majority of hydroxide
versus hydrogen.

00:11:37.120 --> 00:11:39.660
So it creates this inflection
in the graph, and you just

00:11:39.660 --> 00:11:41.240
would have to look
at this chart.

00:11:41.240 --> 00:11:43.210
And say, I added a liter
of this solution.

00:11:43.210 --> 00:11:46.280
The concentration of this
solution is 1 mole per liter.

00:11:46.280 --> 00:11:47.220
It's 1 molar.

00:11:47.220 --> 00:11:48.840
It has a molarity of 1.

00:11:48.840 --> 00:11:51.870
So I must have added
1 mole of hydroxide

00:11:51.870 --> 00:11:52.820
to get to this point.

00:11:52.820 --> 00:11:55.480
So I must have neutralized
1 mole hydrogen.

00:11:55.480 --> 00:11:58.140
So there must have been 1 mole
of hydrogen to begin with.

00:12:01.130 --> 00:12:04.330
And since I had 1 liter of
solution, I had 1 mole

00:12:04.330 --> 00:12:06.100
hydrogen and 1 liter
of solution.

00:12:06.100 --> 00:12:10.160
So my original molarity was 1.

00:12:10.160 --> 00:12:12.160
Anyway, I don't want to
confuse you too much.

00:12:12.160 --> 00:12:14.340
In the next video, I'm going to
try to do the same thing,

00:12:14.340 --> 00:12:16.880
but I'm going to do it
with a weak acid.