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Current time:0:00Total duration:6:00

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 will become a solution of zinc sulfate and the copper once it gets those two electrons is going to go into its neutral its 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 in water it's an aqueous solution you have the copper the solid copper precipitating out and now it's a solution of zinc sulfate that the zinc has essentially been oxidized it gave 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 somehow somewhat separate these two half reactions and make 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 can 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 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 give 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 you have solid zinc right over here we've already said that look you know 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 charges its it's easy to dissolve into a polar solvent like water and then you have those two electrons where those two electrons going to go those two electrons can then those two electrons can then go and be given to the copper can then be given to the copper and both zinc and copper are great conductors of electricity their transition metals they have the C's of electrons so electrons can travel with it 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 what where this copper where this copper bar is in contact with the copper with the copper sulphate solution and now you're going to have some you're going to have an atom a cation I should say an ion of copper that when it comes into contact with those electrons it's going to NAB them up it's going to NAB them up and become neutral so it's going to NAB them up and become neutral and it becomes neutral it's going to precipitate out of the solution it's going to precipitate on to that bar now you might be saying look if more and more positive things if more and more of this is zinc that 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 you just have a bunch of if you if this starts to becoming very very very very positive and similarly if all of the positive stuff all the copper if all of the copper cations are are capturing the electrons the solution is going to become more and more negative it's going to have more sulfate and less of this of and less of the positively charged copper ions so what can we do to make sure that that doesn't happen too quickly well what we do is we produce something we use something called a salt bridge and this salt bridge right over here salt salt bridge this helps equalize this helps neutralize that effect that we just talked about and what a salt bridge you can kind of view it it's not just it's not going to be liquid because then everything inside of it would just would just fall out you can kind of view it as a goo of a salt and this one in this in this diagram we picked sodium sulfate as our as our as our salt so for every sulfate molecule you have sulfate anion you have two sodium two sodium cations and so what's going to naturally what's going to naturally happen here well as this becomes more and more positively charged as more and more positive positive ink 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 go are going to want to leave them all of it's their negative friends right over here go into the salt bridge and the ones that are already in the salt bridge you're going to want to come out out here similarly the sodium the sodium right over here will be tempted to help neutralize the sodium could go in this correct let me do it this way could go could go in this direction and help neutralize any net 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