Learn about Senior Chemistry, Redox 10, in this comprehensive video by bannanaiscool.
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Rob Lederer: Did my best, a cell diagram sets up for you how the electrons move from the anode to the cathode. Now if you are going to write a test and you have to draw a cell diagram or something, you probably get almost half marks for -- you can totally mess it up, but still get pretty good if you just draw the cell, you draw a connection between the two. Now you can use a porous cup, which actually permits ions to flow from one solution to another, that's important, that's coming or you can use something called a U-tube and you may have seen that in the chemistry lab, it's just a piece of glassware that has a solution in it, that has stoppers at either end, stopper with cotton, so ions can still flow through, and inverted, tickling the solutions to provide a connection between the two half cells; very important to do, otherwise it just won't work. So we always write down anode and cathode, doesn't matter, I'd like to go from A to C here, following the alphabet. Electrons will always go from the anode, which is undergoing oxidation to the cathode undergoing reduction. So I draw from this bar electrons leaving and going to the cathode. Then that's an initial diagram with a Voltmeter attached to the middle. Now to put the chemicals in; in the half reactions we saw previously, the anode was the tin reaction or the tin reaction occurs at the anode, so what we would have as this piece of metal in solution is the tin, the tin is the anode, the anode is the electrode here and so that is Sn salt. Now what about the cathode? What is the solid there? Well dichromine and a hydrogen ion, those two guys, they don't have a solid there, so we actually put an inert carbon electrode. You don't have to use carbon, you can use platinum but that's pretty expensive. The inert carbon electrode is the one that allows electrons to come down in and a reaction will take place in solution with the chemicals that pick up the electrons from the rod. So the carbon is the cathode, the electrode in here, that's the carbon. Now what was the solution in here? The Sn2 positive is actually breaking down, electrons are going through and the Sn2 positive is turning into Sn or the Sn solid is turning into Sn2 positive in solution. This solution is already a solution of tin tube and we'll say probably tin tube nitrate. There was a -- put a nitrate in there with a positive ion if at all possible. Now here is what's happening over here. There is dichromate in this one, remember Cr2O7 2-negative, that's forming around with NH-positive. While they go into the bar, gain those electrons from the half reaction and turn into the products of their reaction, Cr3 positive and water. So the electrons go from the anode to the cathode, these guys pick up those electrons on this rod that are provided by the tin. Now in order to complete the diagram, we need to complete an electrical circuit. We've got electrons flow in this way but what is the purpose connecting the solutions? We need migration of ions and here is the ones that go. This is an anode. It's called an anode, because it's the electrons that anions flow to. Anions migrate to the anode; migrate means it come from the other side. So what is an anion or a negative ion in this solution? Well, the dichromate is negative, it's also involved in a reaction, but remember this H-positive means that there is probably Cl-negative swimming around. This Cr2O7 2-negative probably meant that you use potassium dichromate to make this solution, see we've got a whole bunch of different ions and solution. The Cl-negative or the dichromate is an anion and they can go through the bridge. It's called a salt bridge. This U-tube is actually called a salt bridge because the chemical that you put into it could be the KNO3, something that is entirely soluble that won't precipitate, that's important, but you have to have a conducting solution as the ionic solution to let ions be able to be conducted in flow through, hey, okay. So you've got anio