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Let's figure out the oxidation states

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for some more 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,

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but let's look at the periodic table,

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because it never hurts to get 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,

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is one of the most electronegative atoms.

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It's so electronegative that

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oxidized 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

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and this guy wants 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,

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the oxidation 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

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-- it's hypothetical oxidation state positive.

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So my temptation is to say,

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hey, magnesium here 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,

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if it's a neutral compound,

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all of the oxidation states in it 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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Minus 2.

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And hydrogen, when it's bonded with an oxygen

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-- remember. In this case, the hydrogen is bonded

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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,

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you might want to say, hey,

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maybe it'll take the electrons

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and it'll have a negative oxidation state.

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But when hydrogen is bonded with oxygen,

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it 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. At first, you might say, hey,

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I'm adding up 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,

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but you have two of these hydroxides right here.

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So what you do is you figure out

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the sum of the oxidation 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,

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you have a minus 1 sum.

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And then you have two of them. Right?

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You have two hydroxide molecules here.

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So the contribution to the entire compound's

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oxidation 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

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on some of my 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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with water autoionizing into

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

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And it's in equilibrium

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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,

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because it's hogging 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

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without taking 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,

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because that's the actual 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 molecule,

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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

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on this entire molecule, which is the case.

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Now, my question to you is

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has any of the oxidation states changed for any of the atoms?

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All of the hydrogens here

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-- and we could call 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? 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

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where the water was formed,

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but in this reaction, it was not oxidized.

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Likewise, the oxygens

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-- we have two oxygen molecules, or 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,

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no electrons changed 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

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nothing here was oxidized or reduced,

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because their oxidation 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 it,

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but that's not always the case.

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You could have a reaction

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where that necessarily didn't happen.

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But the oxidation state for magnesium

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is definitely plus 2.

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And the oxidation state for the oxygen,

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or the oxidation number, is minus 2.

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But I think you know what I'm talking about

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when I say it was oxidized.

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At some point, it went from a neutral magnesium

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to a positively charged magnesium

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by losing two electrons.

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So it got oxidized.

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Now, let's do some harder problems.

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So hydrogen peroxide

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-- I've said multiple times already that

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oxygen 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. 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,

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water normally has a minus 2 oxidation.

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So you might be tempted to do

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-- 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

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-- let's see. 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

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total net oxidation state.

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And that's not the case

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because this doesn't have any charge.

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So there's a conundrum here.

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And the conundrum is because,

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if you actually look at the structure of hydrogen peroxide,

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the oxygens are 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,

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especially in a first-year chemistry course,

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the peroxide molecules, especially hydrogen peroxide,

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tends to be that one special case.

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There are others, but this is the one special case

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where oxygen does not have a minus 2 oxidation state.

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Let's look at this and try to figure out

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what oxygen's oxidation state would be

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in hydrogen peroxide.

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So in this case, the hydrogen-oxygen bond,

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oxygen is going to hog the electron

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and hydrogen is going to lose it.

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So it's going to have a plus 1 there.

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Same thing on the side.

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Oxygen, at least on this bond, is going to have a plus 1.

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It's going to gain an electron.

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What about from this other bond with the other oxygen?

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Well, there's no reason why one oxygen should

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hog the electron from the other oxygen.

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So it's not going to have any net impact

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on its oxidation state.

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So in this case, this oxygen's oxidation state is plus 1.

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This oxygen's oxidation state is also plus 1.

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So each of the hydrogens have an oxidation number of plus 1.

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You said the oxygens have an oxidation number of minus 1.

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And so you have a net of 0.

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2 times plus 1, plus 2 times minus 1, is 0.

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So that's just a special case.

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That's a good one to be familiar with.

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Let's do another one.

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Iron 3 carbonate.

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And now, for the first time

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-- I remember when we first encountered iron 3 carbonate.

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You probably thought, hey,

244
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why is it called iron 3 carbonate

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when there are only two iron molecules,

246
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or two iron atoms, here?

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And you're about to learn why.

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Let's look at the oxidation numbers.

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So oxygen.

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Oxygen's oxidation number tends to be minus 2.

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Minus 2.

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Now, if carbon is bonding with oxygen

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-- let's look at the periodic table.

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We have carbon bonding with oxygen.

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Carbon can go either way.

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Carbon, sometimes it likes to give away electrons.

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Sometimes it likes to gain electrons.

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When carbon is bonding with oxygen,

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this right here is the electron hog.

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If we had to say who's taking the electrons,

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it's going to be oxygen.

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Right?

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So carbon is going to be giving away its electrons.

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But how many electrons can carbon give away?

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Well, let's see.

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It has 1, 2, 3, 4 valence electrons.

267
00:11:15,010 --> 00:11:16,070
So the most it can really do is

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give away four valence electrons.

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00:11:18,570 --> 00:11:21,450
So let's go back to the carbonate.

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00:11:21,460 --> 00:11:26,110
So the carbon could at most

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00:11:26,120 --> 00:11:29,370
give away its four valence electrons.

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00:11:29,390 --> 00:11:32,400
So what will be the net oxidation number

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00:11:32,420 --> 00:11:33,870
for the carbonate molecule?

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00:11:33,880 --> 00:11:36,390
For the CO3?

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00:11:36,400 --> 00:11:38,770
So this is a plus 4 oxidation,

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00:11:38,790 --> 00:11:40,290
because it only has four to give away.

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00:11:40,300 --> 00:11:42,680
If it's bonding with oxygen, it's going to give them away.

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00:11:42,690 --> 00:11:43,880
Oxygen is more of a hog.

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00:11:43,890 --> 00:11:46,270
Each oxygen has a minus 2.

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00:11:46,280 --> 00:11:48,790
So let's think about it.

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00:11:48,800 --> 00:11:54,130
I have plus 4 minus, 3 times minus 2.

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00:11:54,150 --> 00:11:55,210
Right?

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00:11:55,260 --> 00:11:57,220
I have 3 oxygen molecules.

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00:11:57,240 --> 00:12:00,350
So I have 4 minus 6 is equal to minus 2.

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00:12:00,370 --> 00:12:02,850
So we can kind of view it as the oxidation number

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00:12:02,860 --> 00:12:05,790
for the entire carbonate molecule is minus 2.

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00:12:05,810 --> 00:12:10,640
Now, if this entire carbonate molecule is minus 2,

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00:12:10,660 --> 00:12:14,370
its contribution to the oxidation state

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00:12:14,390 --> 00:12:16,940
for this whole kind of

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00:12:16,950 --> 00:12:18,730
-- the carbonate part of the molecule.

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00:12:18,740 --> 00:12:20,630
We have 3 carbonate molecules.

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00:12:20,640 --> 00:12:22,630
Each of them is contributing minus 2.

293
00:12:22,640 --> 00:12:24,680
So I have a minus 6 contribution.

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00:12:24,700 --> 00:12:28,360
If this is minus 6 and this is a neutral molecule,

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00:12:28,380 --> 00:12:32,210
then our 2 irons are also going

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00:12:32,220 --> 00:12:35,370
to have to have a plus 6 oxidation state.

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00:12:35,380 --> 00:12:36,820
Because it all has to add up to 0.

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00:12:36,830 --> 00:12:40,160
If both irons combined

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00:12:40,180 --> 00:12:43,380
have a plus 6 contribution to oxidation state,

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00:12:43,400 --> 00:12:46,410
then each of the irons must have a plus 3 oxidation.

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00:12:46,420 --> 00:12:49,600
Or that, in our hypothetical world, if this happens,

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00:12:49,620 --> 00:12:52,650
at least three electrons are going to

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00:12:52,670 --> 00:12:54,800
favor the carbonate from each of the irons.

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00:12:54,840 --> 00:12:58,120
So why is it called iron 3 carbonate?

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00:12:58,120 --> 00:13:00,400
I think you may have figured this out by now.

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Because this is iron in its third oxidation state.

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00:13:04,880 --> 00:13:06,070
Iron-- a lot of the metals,

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00:13:06,090 --> 00:13:07,550
especially a lot of the transition metals--

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00:13:07,560 --> 00:13:09,560
can have multiple oxidation states.

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00:13:09,570 --> 00:13:11,290
When you have iron 3 carbonate,

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00:13:11,300 --> 00:13:12,680
you're literally saying,

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00:13:12,690 --> 00:13:14,330
this is the third oxidation state.

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00:13:14,340 --> 00:13:17,810
Or iron's oxidation number in this molecule

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will be positive 3.

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00:13:19,610 --> 00:13:21,940
Now, let's do another one.

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00:13:21,960 --> 00:13:23,030
This is interesting.

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00:13:23,050 --> 00:13:24,200
Acetic acid.

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00:13:24,210 --> 00:13:25,560
And I think is the first time that

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00:13:25,570 --> 00:13:29,990
I've actually shown you the formula for acetic acid.

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00:13:30,010 --> 00:13:33,160
I won't go into the whole organic chemistry of it.

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00:13:33,170 --> 00:13:37,620
But let's try to figure out what the different charges are,

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00:13:37,640 --> 00:13:39,200
or the different oxidation states.

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00:13:41,570 --> 00:13:43,280
Sometimes you'll just see it written like this.

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00:13:43,290 --> 00:13:44,600
You'd say, OK.

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00:13:44,610 --> 00:13:48,050
Oxygens, each of those are going to have minus 2.

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00:13:50,920 --> 00:13:53,270
Hydrogens are each going to have plus 1.

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00:13:56,570 --> 00:13:57,600
So how are we doing so far?

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00:13:57,630 --> 00:14:01,520
So these oxygens are going to contribute minus 4.

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00:14:01,540 --> 00:14:03,290
And then the hydrogens

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00:14:03,300 --> 00:14:08,930
-- here you have plus 3. And then here you have plus 1.

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00:14:08,950 --> 00:14:12,070
You add these up and you get to 0. And you're like, oh.

332
00:14:12,090 --> 00:14:14,890
So the carbons must have no oxidation state.

333
00:14:14,900 --> 00:14:16,780
They must have an oxidation number of 0.

334
00:14:16,790 --> 00:14:20,480
Because we're already at 0,

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00:14:20,490 --> 00:14:23,040
if we just consider the hydrogens and the oxygens.

336
00:14:23,050 --> 00:14:25,670
So let's look at that and see if that's actually the case.

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00:14:25,680 --> 00:14:28,770
So when carbon is bonding with hydrogen,

338
00:14:28,780 --> 00:14:30,230
who's going to hog the electrons?

339
00:14:30,240 --> 00:14:34,670
When carbon is bonding with hydrogen.

340
00:14:34,680 --> 00:14:37,160
Electronegativity-- as you go to the right.

341
00:14:37,170 --> 00:14:39,330
Carbon is more electronegative.

342
00:14:39,350 --> 00:14:42,690
It likes to keep the electrons, or hog them,

343
00:14:42,700 --> 00:14:43,800
more than hydrogen.

344
00:14:43,810 --> 00:14:45,470
So hydrogen is going to lose the electrons

345
00:14:45,480 --> 00:14:46,980
in our oxidation state world.

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00:14:46,990 --> 00:14:50,600
It's actually a covalent bond, but of course,

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00:14:50,620 --> 00:14:51,870
we know that when we're dealing with oxidation states,

348
00:14:51,880 --> 00:14:53,260
we pretend that it's ionic.

349
00:14:53,280 --> 00:14:56,620
So in this case, your hydrogens are going

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00:14:56,630 --> 00:14:58,140
to lose electrons.

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00:14:58,160 --> 00:15:00,170
So they're each going to have an oxidation state of plus 1.

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00:15:00,190 --> 00:15:02,940
That's consistent with what we know so far.

353
00:15:02,950 --> 00:15:04,740
And actually, that's another thing.

354
00:15:04,760 --> 00:15:06,680
When I did this exercise, right here,

355
00:15:06,710 --> 00:15:07,750
I immediately assumed hydrogen has

356
00:15:07,760 --> 00:15:10,810
an oxidation state of plus 1.

357
00:15:10,820 --> 00:15:11,850
I did that because, oh,

358
00:15:11,860 --> 00:15:13,620
everything else in the molecule is carbon and oxygen,

359
00:15:13,630 --> 00:15:15,800
which are more electronegative than the hydrogen.

360
00:15:15,810 --> 00:15:17,950
So the hydrogen is going to go into its plus 1 state.

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00:15:17,960 --> 00:15:19,670
If, over here, I had a bunch of

362
00:15:19,680 --> 00:15:21,500
alkali and alkaline earth metals,

363
00:15:21,520 --> 00:15:22,910
I wouldn't be so sure.

364
00:15:22,930 --> 00:15:24,540
I'd say, oh, maybe hydrogen would take electrons from them.

365
00:15:24,550 --> 00:15:26,470
But anyway.

366
00:15:26,490 --> 00:15:30,270
So these all gave an electron to this carbon.

367
00:15:30,290 --> 00:15:32,270
So just from these hydrogens,

368
00:15:32,290 --> 00:15:39,200
that carbon would have a minus 3 oxidation state, right?

369
00:15:39,220 --> 00:15:41,450
These lost electrons.

370
00:15:41,460 --> 00:15:43,170
This guy gained three electrons,

371
00:15:43,190 --> 00:15:44,620
so his charge goes down by 3.

372
00:15:44,630 --> 00:15:47,450
The carbon-carbon bond. Well, there's no reason

373
00:15:47,470 --> 00:15:49,620
one carbon should take electrons from another carbon.

374
00:15:49,630 --> 00:15:51,400
All carbons are created equal.

375
00:15:51,410 --> 00:15:53,770
So there should be no transfer here.

376
00:15:53,800 --> 00:15:56,120
So this carbon's oxidation status is 3.

377
00:15:56,140 --> 00:15:57,490
Now what about on this side?

378
00:15:57,500 --> 00:16:00,130
So we know that this hydrogen is going

379
00:16:00,140 --> 00:16:02,750
to have a plus 1 oxidation state.

380
00:16:02,770 --> 00:16:04,830
It's going to give its electron to this oxygen.

381
00:16:04,850 --> 00:16:07,880
This oxygen, like most oxygens,

382
00:16:07,890 --> 00:16:09,680
are going to take up two electrons.

383
00:16:09,690 --> 00:16:12,960
One from this carbon, and one from this hydrogen.

384
00:16:12,970 --> 00:16:15,800
So it's going to have a minus 2 oxidation state.

385
00:16:15,810 --> 00:16:19,100
This oxygen is also going to take two electrons.

386
00:16:19,110 --> 00:16:20,700
In this case, both of them are going

387
00:16:20,720 --> 00:16:22,100
to be from this orange carbon.

388
00:16:22,120 --> 00:16:24,490
So it's going to have a minus 2 oxidation state.

389
00:16:24,510 --> 00:16:26,720
So what's the oxidation state of this carbon?

390
00:16:26,730 --> 00:16:30,190
It lost two electrons to this guy up here,

391
00:16:30,200 --> 00:16:35,350
and it lost one electron to this oxygen down here.

392
00:16:35,360 --> 00:16:37,700
Remember, this guy got one electron from the carbon

393
00:16:37,710 --> 00:16:38,830
and one from the hydrogen.

394
00:16:38,840 --> 00:16:41,600
So it lost one electron here, two there.

395
00:16:41,610 --> 00:16:43,390
It lost three electrons.

396
00:16:43,410 --> 00:16:47,570
So in that reality, it would have a plus 3 charge.

397
00:16:47,590 --> 00:16:52,010
So it turns out that the average oxidation state

398
00:16:52,020 --> 00:16:54,990
for the carbon in acetic acid is 0.

399
00:16:55,010 --> 00:16:56,990
Because if you average minus 3 and plus 3,

400
00:16:57,000 --> 00:16:58,300
you get to 0.

401
00:16:58,320 --> 00:17:00,520
And that's why I said, oh, maybe these are a 0.

402
00:17:00,530 --> 00:17:03,180
But if you actually write out their oxidation numbers,

403
00:17:03,200 --> 00:17:07,300
this green C has a minus 3 oxidation state.

404
00:17:07,310 --> 00:17:10,020
And this orange C, this orange carbon,

405
00:17:10,030 --> 00:17:12,640
has a plus 3 oxidation state.

406
00:17:12,660 --> 00:17:14,050
If you got this one,

407
00:17:14,070 --> 00:17:16,060
and I don't think it's overly complex,

408
00:17:16,080 --> 00:17:21,880
you will be an oxidation state jock.

409
00:17:21,890 --> 00:17:23,750
So I think you're all set now.

410
00:17:23,760 --> 00:17:25,150
In the next video, we're going to start exploring

411
00:17:25,160 --> 00:17:28,150
oxidation reduction reactions.