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Let's say I had the reaction where I had some copper ions.

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They have a positive charge of 2.

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It's an aqueous solution.

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There could be some other negative

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ions in that solution.

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Actually we could talk about that in a little bit.

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But we know that when you have these ions in solution, they

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all disassociate.

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So you just have to know you have a bunch of ions just

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floating around in water.

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There could be some other negative ones to neutralize

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the entire solution, but let's just worry about the positive

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ones for now.

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And in that solution, I'm going to put a bar of zinc.

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And what happens is that they essentially switch places.

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The copper wants to jump out of your aqueous solution.

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I should probably draw this.

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Let me draw it.

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This is my solution.

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And in my solution, we make it blue, because it's aqueous, so

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most of it is water.

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I have some copper molecules here in the solution.

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They're a plus-2 charge.

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I could have some other ions that are negative, like some

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nitrate ions or whatever, but they're all disassociated.

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They're all mixing around with the water molecules that are

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polar, and that's why ions disassociate, or salts in

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general, disassociate well in water.

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And I stick a bar of zinc in it.

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Solid zinc. So let me do that.

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So I stick of bar of solid zinc into it.

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And so what this reaction says is that these copper ions, if

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they can get a hold of some electrons and essentially

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become neutral, they would rather go back

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into the solid state.

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And the molecules of zinc out here that are in the solid

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state, if they can lose some electrons, they want to jump

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into the solution.

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So let me write down the whole reaction.

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So if the copper can get some electrons, they're going to go

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into the solid state.

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Copper will go into the solid state.

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And if the zinc can lose some electrons, they'll become

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positive ions or cations.

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And then they will go into the solution, plus-- I'll do the

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same color-- zinc plus 2 plus aqueous.

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Actually just so, a little side note here, these are

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actual ions.

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We've been dealing with oxidation numbers.

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And when you want to know the oxidation number of an ion,

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00:02:39,069 --> 00:02:41,859
the oxidation number is the same thing as its charge,

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because it's no longer hypothetical.

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Remember oxidation numbers were hypothetical charges if a

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bond were ionic.

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In this case they are ionic, so the oxidation state, or the

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oxidation number is the same as the charge.

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And just as a side note, the convention, if it's written 2

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plus, this means that this is a real charge.

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If it's written plus 2-- at least this is the way I

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learned, and maybe there's different conventions in

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different places-- then it would be an oxidation state,

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which says, oh, it's not necessarily an ion, but this

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is what it would be if it was an ion.

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So if someone writes plus 2, this is oxidation state.

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If they write 2 plus, they say, hey

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this is a real charge.

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And likewise, if you just have a regular ion that has, like,

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a charge of 1-- so let's say you have Na plus-- this means

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that it's an actual charge.

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If you wanted to write its oxidation state, you would

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write plus 1.

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Small little note, it doesn't probably make a big difference

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in the whole scheme of things, but sometimes it can get

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confusing whether something is an oxidation state or an ion.

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But depending on where you're going to school and what your

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teachers or conventions are, you might want to clarify that

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and just make sure that they use the same conventions that

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at least I remember when I first learned it.

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00:03:50,289 --> 00:03:52,389
So anyway, let's go back to this reaction

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that's happening here.

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00:03:53,469 --> 00:03:57,009
So let's do the half reactions for this.

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So we start here, and actually we can just look at the

85
00:03:59,229 --> 00:04:00,750
oxidation states to begin with.

86
00:04:00,750 --> 00:04:01,719
This guy has an oxidation.

87
00:04:01,719 --> 00:04:05,889
He has a charge of 2, which is also an oxidation number of 2.

88
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And he goes to a neutral charge.

89
00:04:07,990 --> 00:04:12,760
So clearly, he gains two electrons, which means that he

90
00:04:12,759 --> 00:04:13,829
was reduced.

91
00:04:13,830 --> 00:04:16,449
He went from a positive charge to a zero

92
00:04:16,449 --> 00:04:18,170
oxidation state or charge.

93
00:04:18,170 --> 00:04:19,699
So his charge was reduced.

94
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Just let me write that.

95
00:04:21,759 --> 00:04:23,829
So this guy was reduced.

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And let me write his half reaction.

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He essentially started off with some copper in the

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aqueous state, and he got two electrons.

99
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That's how you get reduced.

100
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So plus two electrons.

101
00:04:40,439 --> 00:04:45,035
And he ended up in the solid state of copper.

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00:04:45,035 --> 00:04:48,120
And his oxidation number is now 0.

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00:04:48,120 --> 00:04:52,930
Now let's see what happened to the zinc. The zinc, it starts

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00:04:52,930 --> 00:04:55,150
completely neutral in the solid state.

105
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No extra electrons or deficiency of electrons.

106
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But then once the zinc jumps into the solution

107
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here, he has a charge.

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00:05:07,000 --> 00:05:08,959
So he must have lost electrons.

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00:05:08,959 --> 00:05:13,060
So the half reaction for zinc-- in this situation you

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00:05:13,060 --> 00:05:19,610
have zinc as a solid and he reacts to produce two

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00:05:19,610 --> 00:05:28,830
electrons and he's in the water and he's now an ion.

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So let me put that there.

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An aqueous solution.

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So if we did this experiment right here-- and I will tell

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you right now this reaction moves forward.

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This is truly a reaction.

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We're going to talk about, in a second, how badly do the

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electrons essentially want to jump from-- where are they

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jumping from?

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They're jumping from the zinc to the copper.

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So we're going to talk a little bit about how strongly

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they want to jump in the voltage.

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And voltage is really just how badly does charge want to move

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or how much potential energy does charge have. But this

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whole reaction, you just have to think about what's

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happening, is that you have charge moving from-- as the

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zinc jumps into the water, as the zinc jumps into the

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aqueous solution, it's passing on some charge to the copper

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that jumps into the solid form.

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So electrons are moving here.

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And if somehow we could harness these moving

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electrons, we could create a current.

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In this situation, it's all chemical.

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The electrons are moving around, but they're not doing

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it in any organized way.

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Because the solution is all mixed up.

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But this is an interesting situation we're

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dealing with here.

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And we can even talk about how badly the electrons want to

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move in these directions.

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You can look at these half reactions.

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You can look it up in a table, and you can get these energies

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or the voltage is for these reactions.

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And this half reaction right here has a voltage.

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So they'll sometimes right E.

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Sometimes it's an E not right over there, but that's like

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kind of out of the scope of at least a

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first-year chemistry course.

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But they'll say it's 0.34 volts.

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And then this reaction in this direction has a potential

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difference.

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The energy is 0.76 volts.

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If I wrote this in the other way, if I wrote this reaction

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of zinc plus 2 electrons going to zinc solid, then you would

155
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just switch the sign.

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But what this says is that this state, this state right

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here, is a higher energy than that state.

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So it wants to go in that direction.

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And it wants to do that with an energy per coulomb.

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And you can watch the physics playlist on

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more details of volts.

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But volt units are joules per coulomb.

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This is how badly the electrons

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want to join the copper.

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That's the best way of viewing it.

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And this is a measure of how badly the electrons want to

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leave the zinc. So if you wanted to talk about this

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00:07:54,029 --> 00:07:58,369
whole reaction, so this whole reaction that we have up here,

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we could say it's an energy, so the energy of this entire

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reaction up here.

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We know that electrons really want to join the copper with

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this energy, and they really want to leave the zinc with

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this energy.

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So the energy of this entire reaction, how badly do

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electrons want to move from zinc to copper to form this

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over there, is just the sum of the two.

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So you get what's 1.1 volts.

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You're probably saying, hey Sal, all this talk of volts,

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this is interesting and current.

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Is there some way that we could construct a battery or a

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galvanic cell?

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If you don't know what a galvanic cell is, it's what

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00:08:38,178 --> 00:08:39,668
we're about to construct right now.

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And the answer is obviously yes, because that's the whole

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point of this video.

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So how can we somehow harness that charge?

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So let's separate the two reactions.

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Let's separate the two.

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00:08:52,830 --> 00:09:01,310
So let me put my zinc bar here.

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00:09:01,309 --> 00:09:04,589
I'll try to stay true to the colors that I used earlier.

191
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And let me put my copper bar there.

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Actually maybe I should copy and paste

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what I wrote up here.

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Just so we remember the reaction itself.

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Actually, yeah let me do that.

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Never hurts.

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OK.

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00:09:22,640 --> 00:09:24,759
I'll paste it down here.

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00:09:24,759 --> 00:09:26,825
Ignore all the squiggly lines around it.

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00:09:26,825 --> 00:09:31,330
And let me put but both of my bars in some aqueous solution.

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00:09:31,330 --> 00:09:34,310
And there could be some other dissolved ions in there to

202
00:09:34,309 --> 00:09:37,629
help neutralize the different-- so I put them both

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00:09:37,629 --> 00:09:40,230
in an aqueous solution.

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00:09:40,230 --> 00:09:41,480
But they're separate.

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00:09:41,480 --> 00:09:46,159


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00:09:46,159 --> 00:09:49,059
And let me label them appropriately.

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00:09:49,059 --> 00:09:55,209
So this is my zinc. Zinc right here, a bar of solid sync.

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00:09:55,210 --> 00:09:57,490
And this is my bar of solid copper.

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00:09:57,490 --> 00:09:58,960
I should do that in a different color.

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00:09:58,960 --> 00:10:00,980
So this is copper right here.

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00:10:00,980 --> 00:10:02,269
My bar of solid copper.

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00:10:02,269 --> 00:10:04,470
And then I have an aqueous solution.

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00:10:04,470 --> 00:10:07,050
So what happens if we were to connect

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00:10:07,049 --> 00:10:08,639
these two metals, right?

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00:10:08,639 --> 00:10:09,230
They're metals.

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00:10:09,230 --> 00:10:10,480
If you look at the periodic table they're

217
00:10:10,480 --> 00:10:13,659
both transition metals.

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00:10:13,659 --> 00:10:15,569
They both have the sea of electrons.

219
00:10:15,570 --> 00:10:17,350
So they both conduct electricity well.

220
00:10:17,350 --> 00:10:21,879
What happens if you connect these two with a wire?

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00:10:21,879 --> 00:10:27,370
So if I connect them with a wire right here like that.

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00:10:27,370 --> 00:10:30,779
So we already know that the zinc is just dying to jump

223
00:10:30,779 --> 00:10:33,879
into the water and lose its electrons, right?

224
00:10:33,879 --> 00:10:36,669
So let's say that happens.

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00:10:36,669 --> 00:10:40,429
So the zinc jumps into the aqueous solution, and it turns

226
00:10:40,429 --> 00:10:44,719
into a positive ion of zinc. And then its electrons are

227
00:10:44,720 --> 00:10:47,029
left behind, right?

228
00:10:47,029 --> 00:10:48,980
So it essentially has two electrons.

229
00:10:48,980 --> 00:10:51,300
I did the half reaction right here, right?

230
00:10:51,299 --> 00:10:54,409
The zinc jumps into the solution and it leaves behind

231
00:10:54,409 --> 00:10:56,850
two electrons right here.

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00:10:56,850 --> 00:10:59,810
Two electrons here, they really want to jump off of the

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00:10:59,809 --> 00:11:00,469
zinc.

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00:11:00,470 --> 00:11:05,110
Now I have a lot of copper out of here, out here in the

235
00:11:05,110 --> 00:11:09,850
solution, that really want to jump on to this bar of copper.

236
00:11:09,850 --> 00:11:12,050
But right now they're all ions.

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00:11:12,049 --> 00:11:13,339
These are all plus 2 ions.

238
00:11:13,340 --> 00:11:18,350


239
00:11:18,350 --> 00:11:20,970
And they really want to jump on to the bar and join all of

240
00:11:20,970 --> 00:11:23,190
these other copper ions here.

241
00:11:23,190 --> 00:11:25,530
But in order to jump on to that, they

242
00:11:25,529 --> 00:11:27,879
have to get some electrons.

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00:11:27,879 --> 00:11:30,269
Each of these guys have to grab two electrons.

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00:11:30,269 --> 00:11:34,579
And we know that kind of potential, or the

245
00:11:34,580 --> 00:11:37,170
electromotive force with which they want to grab those

246
00:11:37,169 --> 00:11:41,829
electrons is 0.34 volts.

247
00:11:41,830 --> 00:11:45,940
These guys want to jump here and grab electrons from

248
00:11:45,940 --> 00:11:49,260
someplace with plus 0.34 volts.

249
00:11:49,259 --> 00:11:52,350
These guys want to jump off of the bar and leave their

250
00:11:52,350 --> 00:11:59,350
electrons behind with 0.76 volts.

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00:11:59,350 --> 00:12:00,820
So what do you think's going to happen if we have a

252
00:12:00,820 --> 00:12:02,500
conducting wire here?

253
00:12:02,500 --> 00:12:05,960
Well those electrons are, once this zinc just jumps off,

254
00:12:05,960 --> 00:12:10,259
those electrons are going to travel through the wire.

255
00:12:10,259 --> 00:12:13,049
And then they're going to be available, because these are

256
00:12:13,049 --> 00:12:17,289
surplus electrons, to merge with these copper ions that

257
00:12:17,289 --> 00:12:19,669
want to jump back on to the bar.

258
00:12:19,669 --> 00:12:22,849
So these copper ions are going to jump back on to the bar.

259
00:12:22,850 --> 00:12:24,779
But from this reaction what's happening?

260
00:12:24,779 --> 00:12:27,379
You're having a current flow.

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Current is actually flowing.

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And what is the voltage across this?

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How badly does an electron want to go from the zinc on

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this bar to over here, join with some copper in the

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solution and let that copper join on to that bar?

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It wants to do it with plus 1.1 volts.

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Now the question is the electrons are flowing in that

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way, what is the cathode and what is the anode?

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So the cathode is the positive terminal of, well, you can

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already imagine, this is a battery of sorts, right?

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Because we are able to generate a current by having a

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voltage difference across two terminals.

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The cathode is the positive terminal.

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That's where the electrons want to go to.

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And then the anode-- and you can familiarize yourself with,

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you know, just a cation is a positive ion and anion is a

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negative ion.

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So the anode is a negative terminal.

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So what's the negative terminal here?

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The negative terminal is where the electrons come from.

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See, electrons are coming from the zinc jumping off.

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So is your negative terminal.

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And what's the positive terminal?

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That's where the electrons want to go to.

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They want to go down here and be available for these coppers

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that want to jump out of the solution.

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So this is our positive terminal, or our cathode.

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This is the cathode.

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This is the anode.

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And if someone says, hey, does oxidation or reduction occur

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at the cathode or the anode?

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Well what's occurring right here?

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We have zinc in its solid state right here.

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We have solid zinc jumping in-- and it's neutral-- and it

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jumps into the solution and it leaves two electrons behind.

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So it's losing electrons.

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When you lose electrons you're being oxidized.

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So this is where the oxidation is occurring.

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Likewise, on this side you have copper ions that are

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already in the solution, and they're gaining electrons.

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So their charge is being reduced.

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So the cathode is where reduction is taking place.

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Reduction.

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Now you're probably saying, oh Sal, this is a nice cute

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little, you know, battery-- or this is called a galvanic

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cell-- that, you know, you've created this current.

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But I see a problem here.

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Because as this happens, you're going to have more and

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more positive zinc ions jump into the solution here.

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You have more and more positive zinc ions in here as

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this progresses.

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And you're going to have fewer and fewer copper ions in here.

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So if you assume that there were some other, let's say

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there was, I mean, maybe to keep the solution neutral--

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the copper is positive-- maybe you had some chloride in

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there, chloride anions that were keeping

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this solution neutral.

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And maybe you had some chlorine anions here that were

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keeping, well, it started negative but then it maybe

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gets a little bit more neutral as the zinc jumps in.

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But once this happens a lot then it becomes

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very positive, right?

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This is becoming more and more positive as more and

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more zinc jumps in.

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And this is becoming more and more negative as more copper

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jumps on to this bar.

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So what can we do to solve that problem?

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What they do is-- and this is just one way to solve the

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problem, there's actually many but this probably what you

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might encounter in your first-year chemistry class--

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is they create something called a salt bridge.

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And what they do is they create a, I guess we could

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call it a pseudo-aqueous solution of salt.

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Maybe it's potassium chloride.

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So in here we have potassium.

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Potassium has a plus charge.

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It's aqueous, plus chlorine with a

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negative charge aqueous.

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And I say pseudo-aqueous because it's not really

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super-liquid water.

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What they do is they try to make it more viscous so it

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just doesn't mix completely.

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If this was pure water, then all of it would just fall into

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the solution, and you would really have nothing up here.

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It would be a useless device.

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But what they do is they create a gel that's still

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aqueous enough that the ions can move around.

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The sodium and chloride ions can move around.

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So let's say that those are the-- sorry, not the sodium,

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the potassium-- let's say that's the potassium.

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And let's say the chlorine ions are-- I already did it in

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that blue-- so chlorine are the minus.

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And what happens is as we have this oxidation reduction

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reaction that's causing these electrons to travel from the

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anode to the cathode, this becomes positives, this

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00:17:08,880 --> 00:17:10,089
becomes negative.

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The salt bridge allows the negative components of the

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00:17:13,740 --> 00:17:17,240
ionized salt and just waste the chlorine to go into the

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00:17:17,240 --> 00:17:18,588
solution to neutralize it.

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So they'll end up having more and more chlorine as

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the zinc pops off.

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00:17:24,130 --> 00:17:26,020
If you were to evaporate the solution you'd end up with a

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00:17:26,019 --> 00:17:27,500
bunch of zinc chloride.

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And on this side, to keep the balance here, you'll have a

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00:17:32,039 --> 00:17:36,609
lot of the positive parts, components, of the salt.

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So the sodium would jump in to replace the copper that's

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jumping out.

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00:17:41,609 --> 00:17:43,809
So when you have this whole thing it enables the current

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to keep flowing without having a bad effect on the charge.

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Because, you know, what happens if this

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00:17:48,490 --> 00:17:50,140
becomes really positive?

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If this becomes really positive-- I'll do it in

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vibrant color because there's no salt bridge-- at some point

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00:17:56,309 --> 00:17:58,649
the electrons are going to say, hey, I can either go over

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00:17:58,650 --> 00:18:01,120
here and drive this reaction or, hey, I got a lot of

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00:18:01,119 --> 00:18:02,859
positive stuff hanging out here.

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00:18:02,859 --> 00:18:04,429
Maybe I don't want to leave it all.

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00:18:04,430 --> 00:18:07,289
So you have to do something to neutralize the charge here.

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00:18:07,289 --> 00:18:10,309
Likewise, if this becomes really negative from all the

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00:18:10,309 --> 00:18:13,779
chlorine atoms jumping out, electrons aren't going to want

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00:18:13,779 --> 00:18:14,389
to go here.

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Because like charges repel each other.

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So that's what having the salt bridge neutralizing the

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00:18:19,670 --> 00:18:20,650
solution does for you.

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00:18:20,650 --> 00:18:22,600
It keeps it from getting negative, and then our

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00:18:22,599 --> 00:18:25,500
galvanic cell can keep operating.

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