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Main content
Current time:0:00Total duration:4:33
At the bottom right, Sal wrote "SO4^-" but meant "SO4^2-."
At the top left, Sal wrote "Cu(q)" but meant "Cu(aq)."

Video transcript

in the last video we talked about how we could create a voltaic cell or galvanic cell or essentially a battery by separating the oxidation and the reduction reactions and connecting them with the wire which forces the electrons that zinc is losing to go through the wire to go to copper to go to the copper ion so that they can get them so that they the copper ions can be reduced now that might have raised some questions if this is a battery well what is the positive terminal what is the negative terminal if this is a battery what is the voltage of this battery well I'll encourage you first of all when you think about the terminal what's the positive and what's the negative terminal encourage you to pause this video and think about that on your own where is the current coming from and where is it going well the negative terminal of a battery is where the electrons are coming from so the electrons are coming from the zinc bar right over here so this is the negative electrode of the battery that right over there or the negative electrode is often called the anode that is the anode of this battery and on the other side the copper bar this is where the electrons are going to this is the positive that's the positive electrode or sometimes called or what tends to be called the cathode now the next question is what is the voltage across they said that voltage is going to depend on what's the concentration of zinc ions you have the concentration of copper ions you have it'll depend on the pressure it'll depend on the temperature but all of that has been standardized and you can actually look up standard electrode potentials I encourage you you could do a web search for standard electrode potentials and you'll see a bunch of voltages for different ions and it was just essentially a measure of relative to hydrogen and this is all relative to hydrogen how much does this ion want to grab its electrons and so if you were to look that up you would get it for this reaction right over here for the copper for the copper ion with an oxidation state a tube with an oxidation number of positive two to grab these two electrons and turning into cop solid copper relative to relative to a the when they call the standard hydrogen electrode that has a 0.34 voltage which means that it's it's it's it's it's more likely to happen than in the case with a standard with a standard hydrogen electrode and and don't worry too much about that we're really just going to compare the voltages and seeing and seeing well what is the total electro-motive force or the total voltage with which this this redox reaction is going to happen and or the total electro-motive force with which we're going to push these electrons across the wire if you were to look up the zinc reaction in a table of standard electrode potentials you might see you might see negative 0.76 now you have to be careful though because they're going to give you they're usually good this if they're giving you this number they're giving you the opposite reaction here they're giving the reaction from going from zinc ions grabbing some electrons and becoming and becoming solid zinc we want the other reaction this is the reaction that we need to occur for our galvanic cell so this reaction right over here is going to be the negative of that so it's going to be positive zero or is positive 0.76 volts so one way to think about it is this wants to happen with an electro-motive force or kind of the energy per Coulomb relatively of 0.76 volts this wants to happen with the electro-motive force here is 0.34 volts so combined this entire reaction is going to happen with an electro-motive force or you could say that the potential per Coulomb difference between this side and that side is going to be the sum of these two things and so that's going to be if we if we have the standard concentration which would be one one molar which is one mole per liter of the ions in the in there aqueous solution for a standard temperature standard pressure then we would have consensually constructed a 1.1 volt battery and i have just added these two things together and of course there's other ways you could do it you could literally I mean you could literally just take a voltmeter and measure what is the voting you could literally just measure what is the voltage difference between these two terminals just as you would do in a traditional battery