0:00:00.580,0:00:03.690
In the last video, we saw that[br]if you took some solid zinc

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and stuck it in a solution[br]of copper sulfate,

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that the zinc will essentially[br]give electrons to the copper.

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So then you have zinc cations[br]that are in the solution.

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So essentially, it'll become[br]a solution of zinc sulfate.

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And the copper, once it[br]gets those two electrons

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is going to go into[br]it's solid state,

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and it's going to precipitate[br]out of the solution.

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And we saw the reaction[br]right over here-- solid zinc

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plus copper sulfate[br]in solution and water.

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It's an aqueous solution.

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You have the solid[br]copper precipitating out.

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And now it's a solution[br]of zinc sulfate,

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that the zinc has[br]essentially been oxidized.

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It lost two electrons.

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It went from[br]neutral to positive.

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And the copper went from[br]positive to neutral.

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So the copper took[br]those two electrons.

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Zinc was oxidized by copper.

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It lost electrons to the copper.

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Copper was reduced by zinc.

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Its charge was reduced by zinc.

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It gained electrons from zinc.

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Now, this by itself[br]is interesting.

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It's an interesting[br]redox reaction.

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Something was oxidized,[br]something was reduced.

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But wouldn't it be interesting[br]is if we could somewhat

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separate these[br]two half reactions

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and make these electrons[br]travel over a wire.

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Now, why would that be[br]interesting to make electrons

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travel over a wire?

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Well, electrons traveling[br]over a wire, that's a current.

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And you could make current[br]do useful things, like power

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a motor or a light or[br]whatever it might be.

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And so essentially,[br]if we can do that,

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we would have constructed[br]something of a battery.

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And if we can keep[br]that going, if we

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can keep the current[br]flowing, we would

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have constructed[br]something like a battery.

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And what I have here,[br]this is a picture

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of a galvanic-- sometimes[br]called a voltaic-- cell.

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And this is doing exactly that.

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It's separating these[br]two half reactions

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and separating them with a wire.

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So zinc can gave[br]copper its electrons,

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but it forces the electrons[br]to go along this wire

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and produce an actual current.

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So let's think about[br]why this is working.

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So you have solid[br]zinc right over here.

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We've already said that[br]look, the solid zinc

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wouldn't mind giving[br]its electrons to copper.

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Copper wouldn't mind taking it.

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Copper is more electronegative.

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And so you have a reality where[br]the solid zinc could give away

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its two electrons and[br]become the cation zinc, so

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a positive charge, and then[br]it dissolves in the water.

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Once it has a[br]positive charge, it's

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easy to dissolve into a[br]polar solvent like water.

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And then you have[br]those two electrons.

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Where are those two[br]electrons going to go?

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Those two electrons can then[br]go and be given to the copper.

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And both zinc and copper[br]are great conductors

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of electricity.

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They're transition metals.

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They have these[br]seas of electrons.

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So electrons can travel[br]within them fairly easily.

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And so you have[br]your two electrons.

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So those are your[br]two electrons that I

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showed traveling in green.

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And they can come all the way to[br]the bottom of where this copper

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bar is in contact with the[br]copper with the copper sulfate

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solution.

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And now you're going to have[br]a cation, an ion of copper,

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that when it comes into[br]contact with those electrons,

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it's going to nab them[br]up and become neutral.

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And when it becomes[br]neutral, it's

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going to precipitate[br]out of the solution.

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It's going to precipitate[br]onto that bar.

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Now, you might be saying,[br]look, if more and more positive

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things, if more and more[br]of this positive zinc

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is flowing in this, wouldn't[br]this make this an imbalance?

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And if this solution[br]becomes too positive,

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then the electrons wouldn't[br]want to leave as much anymore.

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So if this starts becoming[br]very, very, very, very positive,

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and similarly, if all the[br]positive stuff, all the copper

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cations are capturing[br]the electrons,

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the solution is going to[br]become more and more negative.

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It's going to have more sulfate[br]and less of the positively

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charged copper ions.

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So what can we do to[br]make sure that doesn't

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happen too quickly?

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Well, what we do is we use[br]something called a salt bridge.

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And the salt bridge[br]right over here, this

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helps neutralize that effect[br]that we just talked about.

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And with a salt bridge,[br]you can view it.

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It's not going to be[br]liquid, because then

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everything inside of[br]it would just fall out.

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You can view it as[br]a goo of a salt.

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In this diagram, we picked[br]sodium sulfate as our salt.

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So for every sulfate molecule,[br]you have sulfate anion.

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You have two sodium cations.

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And so what's going to[br]naturally happen here?

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Well, as this becomes more[br]and more positively charged,

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as more and more positive zinc[br]ions go into the solution,

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the negative sulfate[br]ions are going

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to want to come out of here.

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So the negative[br]sulfate ions are going

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to want to leave all of[br]their negative friends

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right over here, go[br]into the salt bridge,

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and then the ones that are[br]already in the salt bridge

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are going to want[br]to come out here.

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Similarly, the sodium[br]right over here

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will be tempted to[br]help neutralize.

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The sodium-- let me[br]do it this way-- could

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go in this direction[br]and help neutralize

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any negativity that's[br]happening there.

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And so that will keep[br]each of these solutions

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from becoming too[br]positive or too negative

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and allow this[br]current to continue

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to flow and do useful things.