WEBVTT

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Let's figure out the oxidation
states for some more

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constituent atoms
and molecules.

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So let's say I had
magnesium oxide.

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MgO.

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I'll do oxygen in a
different color.

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So what are going to be their
oxidation states?

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And you might know this already,
but let's look at the

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periodic table, because
it never hurts to get

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familiar with it.

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So we have magnesium.

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Magnesium has two valance
electrons.

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It's a Group 2 element.

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It would love to lose
those two electrons.

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Oxygen, we already know,
is one of the most

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electronegative atoms. It's so
electronegative that oxidized

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has essentially been
named after them.

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And we know that oxygen loves
to gain two electrons.

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So this is kind of a marriage
made in heaven.

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This guy wants to lose two
electrons and this guy wants

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to gain two electrons.

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So what's going to happen?

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The magnesium is going to
lose two electrons.

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It was neutral.

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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,

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because it gained the
two electrons.

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So in this molecule of magnesium
oxide, the oxidation

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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.

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Magnesium likes to lose its
electrons, its two electrons,

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

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oxidation state positive.

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So my temptation is to say,
hey, magnesium here

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would be plus 2.

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So let me write that there.

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

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it's a neutral compound, all of
the oxidation states in it

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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

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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

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say, hey, maybe it'll take the
electrons and it'll have a

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negative oxidation state.

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But when hydrogen is bonded
with oxygen, it

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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

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the oxidation states.

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Plus 2 minus 2 is 0 plus 1.

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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.

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And what you to remember is oh,
no, but you have two of

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these hydroxides right here.

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

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states of the hydroxide.

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So that's minus 2 plus 1.

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So for the entire hydroxide
molecule, you

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have a minus 1 sum.

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And then you have two of them.

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Right?

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You have two hydroxide
molecules here.

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So the contribution to the
entire compound's oxidation

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state will be minus 1
for each hydroxide.

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But then you have two of them.

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So it's minus 2 and then plus
2 from the magnesium.

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And it all adds up to 0.

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So that worked out.

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

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I want to go back to doing
some problems again.

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But I want to do a little bit
of an aside on some of my

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terminology.

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Because I've kind of used
oxidation state, and oxidized,

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and reduced interchangeably,
to a certain degree.

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But, we've done so many
problems with water

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autoionizing into-- actually,
let me do 2 moles of water.

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And it's in equilibrium with 1
mole of H30 plus OH minus.

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And obviously, everything is
in an aqueous solution.

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Now, let's look at the water.

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What are the oxidation states
in this water right here?

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Well, we've done this already
in the previous video.

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Oxidation state of oxygen is
minus 2, because it's hogging

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the two electrons from
the two hydrogens.

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Each hydrogen is giving
up an electron.

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So it has an oxidation
state of plus 1.

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And we see this molecule.

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Everything adds up.

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Because you have two hydrogens
with a plus 1.

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So that's plus 2.

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Plus 2 minus 2 for that one
oxygen, and you get to 0.

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And it's a neutral compound.

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Now here, what are the
oxidation states?

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So one of these hydrogens left
one of these water molecules

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and joined the other of the
water molecules without taking

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its electron with it.

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So it left the electron
over here.

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So that oxygen still has a
minus 2 oxidation state.

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And this hydrogen still
has a plus 1.

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And that's why you do
minus 2 plus 1.

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You get minus 1.

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And this time, it works out,
because that's the actual

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charge on this hydroxide ion.

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Now, here, what are the
oxidation states?

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Each of the hydrogens have
a plus 1 oxidation state.

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And then this oxygen
has a minus 2.

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And so if you look at the
charge for the entire

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molecule, plus 1 on three
hydrogens, so that's plus 3.

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I just added them up.

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Minus 2.

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So plus 3 minus 2, I should have
a plus 1 charge on this

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entire molecule, which
is the case.

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Now, my question to you is has
any of the oxidation states

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changed for any of the atoms?

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All of the hydrogens here--
and we could call

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this 2 moles of water.

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Or maybe I just have two
molecules of water.

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But I have four hydrogens
here.

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Right?

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And all of them had an
oxidation state of 1.

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On the right-hand side,
I have four hydrogens.

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All of them have an oxidation
state of 1.

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So although their oxidation
state is 1, in this reaction--

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and you can pick either
direction of the reaction--

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hydrogen has not
been oxidized.

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Its oxidation state
did not change.

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Maybe it was oxidized in a
previous reaction where the

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water was formed, but in this
reaction, it was not oxidized.

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Likewise, the oxygens-- we have
two oxygen molecules, or

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atoms, here.

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Each have a minus 2
oxidation state.

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Here, we have two oxygen
molecules.

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Each have a minus 2
oxidation state.

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Due to this reaction, at least,
no electrons changed

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hands in our oxidation
state world.

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So this is not an oxidation
or a reduction reaction.

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And I'm going to cover that in
detail in the next video.

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And I just want to be clear that
nothing here was oxidized

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or reduced, because
their oxidation

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states stayed the same.

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Because sometimes I'll
say, hey, look.

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Magnesium has an oxidation
state of plus 2.

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And oxygen has an oxidation
state of minus 2.

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Magnesium was oxidized.

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Two electrons were taken
away from it.

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And oxygen was reduced.

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Two electrons were
given to it.

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And I'll say that implying some
reaction that produced

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it, but that's not
always the case.

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You could have a reaction
where that

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necessarily didn't happen.

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But the oxidation state
for magnesium is

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definitely plus 2.

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And the oxidation state for the
oxygen, or the oxidation

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number, is minus 2.

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But I think you know what I'm
talking about when I say it

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was oxidized.

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

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positively charged magnesium
by losing two electrons.

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So it got oxidized.

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Now, let's do some harder
problems. So hydrogen

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peroxide-- I've said multiple
times already that oxygen

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tends to have a minus
2 oxidation state.

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This is minus 1.

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I think you see the pattern.

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These guys are plus 1.

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Hydrogen is plus or minus 1.

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These guys are plus 2.

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I think you see the pattern.

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It's whether you want to
lose or gain electrons.

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You might say, well see,
water normally

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has a minus 2 oxidation.

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So you might be tempted
to do-- OK.

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Hydrogen has plus 1, because
it's bonding with water.

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And oxygen has a minus 2.

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But when you do that, you
immediately have a conundrum.

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This is a neutral molecule--
let's see.

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Two hydrogens plus 2.

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Two oxygens at minus 2.

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Minus 4.

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So this would end up with
a minus 4 total

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net oxidation state.

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And that's not the
case because this

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doesn't have any charge.

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So there's a conundrum here.

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And the conundrum is because,
if you actually look at the

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structure of hydrogen peroxide,
the oxygens are

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

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That's where the peroxide
comes from.

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And then each of those are
bonded to a hydrogen.

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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.

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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.

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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.

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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.

00:10:35.610 --> 00:10:38.530


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.

00:13:38.835 --> 00:13:41.510


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.

00:13:47.530 --> 00:13:51.130


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

00:13:52.570 --> 00:13:56.270


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.

00:17:28.150 --> 00:17:28.267