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Video transcript

let's say that we're given this reaction and we're asked to find the standard cell potential we can do that by using our standard reduction potentials so if we look at this first half reaction we see that silver ion if you add an electron to silver ion you get solid silver so gain of electrons is reduction so this is a reduction half-reaction if we look at the reaction on the right we can see that we are reducing silver ions so here's silver ion which is being reduced to solid silver so we need this half reaction as it's written so this is going to be our reduction half-reaction so we have AG plus so we have AG plus plus an electron alright giving us a solid silver here the standard reduction potential for this half-reaction is positive 0.8 volts alright so this is positive 0.8 volts next let's look at what else is happening we're turning solid zinc into zinc two plus ions how do you turn solid zinc into zinc two plus solid zinc would need to lose two electrons to turn into zinc two plus loss of electrons is oxidation so if we go over here to this half reaction alright we can see that this half reaction is written as a reduction half-reaction zinc 2 plus is being reduced it's gaining two electrons to turn into solid zinc we need to write this as an oxidation half-reaction so it's given to us as a reduction we need to write it as an oxidation half-reaction we simply need to reverse it so we need to start with solid zinc so we start with solid zinc here and for solid zinc to turn into zinc two plus ions we need to lose two electrons so loss of electrons is oxidation so we need to find the standard oxidation potential next and we can do that by looking at our table once again the standard reduction potential for this half-reaction as its is negative 0.76 volts since we reversed this half-reaction right we reverse this half reaction we need to change the sign from negative to positive so for the reduction potential it's negative 0.76 for the oxidation potential it's positive 0.76 so the standard oxidation potential for this half-reaction is positive 0.76 volts next we need to look at moles so if we look at our reaction we have two moles of silver ions being reduced to two moles of solid silver down here we have one mole of silver ions being reduced to one mole of solid silver so we need to multiply everything by two here so we get two moles of saw of silver ions which require two moles of electrons to turn into two moles of solid silver now it might be really tempting to say oh well then don't we just multiply our reduction potential by two as well and we don't for the reason that we talked about in the previous video voltage is an intensive property so it doesn't matter if you're forming one mole of silver or two moles of silver the voltage is the same so we're going to leave the voltage at point eight volts all right next we have the number of electrons equal so we can add our two half-reactions together to get our overall reaction so if you add the reduction half-reaction and the oxidation half-reaction we together we get our overall reaction so the electrons lost by zinc these two electrons here are the same electrons that are gained by the silver by the silver ions so those cancel out and on the left side we would have two AG plus plus solid zinc so let's write that in here to a G plus plus solid zinc and for our products so over here on the right we have two AG plus zinc two-plus so we have to a g plus zinc - plus notice this is our reaction alright so if I if I box this here this overall reaction that we've just found is the same as what we were given in our problem right so we found our overall reaction but remember they wanted us to find the standard cell potential so how do we find the standard cell potential we talked about how to do this in an earlier video to find the standard cell potential alright you can think about how we found the overall reaction we added the reduction half-reaction and the oxidation half-reaction together to get the overall reaction so to find the standard cell potential we need to add the standard reduction potential and the standard oxidation potential together so if we add the standard reduction potential and the standard oxidation potential together we should get the standard potential for the cell so let's go ahead and do that the standard cell potential is equal to this would be positive 0.8 volts so positive 0.8 volts plus positive 0.76 volts plus positive 0.76 volts which is equal to positive one point five six volts so the standard cell potential is positive one point five six volts remember a positive value for your potential means a spontaneous reaction so this is how to use your reduction potentials to figure out a standard cell potential
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