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Video transcript
In the last video, we saw that if you took some solid zinc and stuck it in a solution of copper sulfate, that the zinc will essentially give electrons to the copper. So then you have zinc cations that are in the solution. So essentially, it'll become a solution of zinc sulfate. And the copper, once it gets those two electrons is going to go into it's solid state, and it's going to precipitate out of the solution. And we saw the reaction right over here-- solid zinc plus copper sulfate in solution and water. It's an aqueous solution. You have the solid copper precipitating out. And now it's a solution of zinc sulfate, that the zinc has essentially been oxidized. It lost two electrons. It went from neutral to positive. And the copper went from positive to neutral. So the copper took those two electrons. Zinc was oxidized by copper. It lost electrons to the copper. Copper was reduced by zinc. Its charge was reduced by zinc. It gained electrons from zinc. Now, this by itself is interesting. It's an interesting redox reaction. Something was oxidized, something was reduced. But wouldn't it be interesting is if we could somewhat separate these two half reactions and make these electrons travel over a wire. Now, why would that be interesting to make electrons travel over a wire? Well, electrons traveling over a wire, that's a current. And you could make current do useful things, like power a motor or a light or whatever it might be. And so essentially, if we can do that, we would have constructed something of a battery. And if we can keep that going, if we can keep the current flowing, we would have constructed something like a battery. And what I have here, this is a picture of a galvanic-- sometimes called a voltaic-- cell. And this is doing exactly that. It's separating these two half reactions and separating them with a wire. So zinc can gave copper its electrons, but it forces the electrons to go along this wire and produce an actual current. So let's think about why this is working. So you have solid zinc right over here. We've already said that look, the solid zinc wouldn't mind giving its electrons to copper. Copper wouldn't mind taking it. Copper is more electronegative. And so you have a reality where the solid zinc could give away its two electrons and become the cation zinc, so a positive charge, and then it dissolves in the water. Once it has a positive charge, it's easy to dissolve into a polar solvent like water. And then you have those two electrons. Where are those two electrons going to go? Those two electrons can then go and be given to the copper. And both zinc and copper are great conductors of electricity. They're transition metals. They have these seas of electrons. So electrons can travel within them fairly easily. And so you have your two electrons. So those are your two electrons that I showed traveling in green. And they can come all the way to the bottom of where this copper bar is in contact with the copper with the copper sulfate solution. And now you're going to have a cation, an ion of copper, that when it comes into contact with those electrons, it's going to nab them up and become neutral. And when it becomes neutral, it's going to precipitate out of the solution. It's going to precipitate onto that bar. Now, you might be saying, look, if more and more positive things, if more and more of this positive zinc is flowing in this, wouldn't this make this an imbalance? And if this solution becomes too positive, then the electrons wouldn't want to leave as much anymore. So if this starts becoming very, very, very, very positive, and similarly, if all the positive stuff, all the copper cations are capturing the electrons, the solution is going to become more and more negative. It's going to have more sulfate and less of the positively charged copper ions. So what can we do to make sure that doesn't happen too quickly? Well, what we do is we use something called a salt bridge. And the salt bridge right over here, this helps neutralize that effect that we just talked about. And with a salt bridge, you can view it. It's not going to be liquid, because then everything inside of it would just fall out. You can view it as a goo of a salt. In this diagram, we picked sodium sulfate as our salt. So for every sulfate molecule, you have sulfate anion. You have two sodium cations. And so what's going to naturally happen here? Well, as this becomes more and more positively charged, as more and more positive zinc ions go into the solution, the negative sulfate ions are going to want to come out of here. So the negative sulfate ions are going to want to leave all of their negative friends right over here, go into the salt bridge, and then the ones that are already in the salt bridge are going to want to come out here. Similarly, the sodium right over here will be tempted to help neutralize. The sodium-- let me do it this way-- could go in this direction and help neutralize any negativity that's happening there. And so that will keep each of these solutions from becoming too positive or too negative and allow this current to continue to flow and do useful things.