Cell Potential Problems Electrochemistry

in this video we're going to focus on calculating the cell potential of galvanic cells and even electrolytic cells so for galvanic cell you need to know that the cell potential has to be positive so knowing that if the standard cell potential has to be positive how can we adjust these two equations in such way to calculate cell potential or the cell potential for galvanic cell that uses these two half reactions now we need to adjust it in such way that the electrons are not on the same side of both reactions for one half reaction it has to be on the right side and for the other it has to be on the left side and we need to switch the right reaction such that the overall cell potential is positive so let's say if we switch the first one this would be negative 0.8 negative 0.8 plus negative 2.37 will not give us positive number so that tells us that we need to reverse this reaction because then we'll be adding two positive numbers instead of one so let's go ahead and do that so i'm going to reverse the second reaction and so the cell potential is now positive 2.37 and the first reaction i'm going to leave it the way it is but i'm going to multiply by 2 so that the number of electrons will stay the same if you multiply this reaction by 2 the cell potential will not change you shouldn't multiply by 2. this is going to remain positive 0.8 it's not going to be 0.8 times 2 or 1.6 it doesn't work that way now when we add these two equations notice that the electrons will cancel because this is redox reaction one reaction has to be oxidation the other has to be reduction anytime the electrons on the right side it's oxidation if it's on the left side it's reduction so the electrons cannot both be on the same side it just doesn't work out that way so now if we add these two reactions on the left we have the magnesium atom and the silver ion on the right we have silver metal and magnesium ion so to calculate the cell potential for this overall reaction all we need to do now is simply add those two numbers so it's going to be 2.37 plus 0.8 so in this case it's positive 3.17 volts so that's the cell potential for galvanic cell that uses these two half reactions now what about this one calculate the standard cell potential of phonic cell that uses the two half reactions shown below so how can we adjust these half reactions in such way that one of the reactions have the electrons on the right and the other have the electrons on the left and when we add the cell potentials it's going to be positive if we reverse the second reaction this would be the cell potential is going to be positive 0.23 and if you add that to negative 0.44 that's going to give you negative cell potential for the overall reaction so we don't want that therefore we need to reverse the first half reaction so it becomes fe and that turns into the fe 2 plus ion and so the cell potential is going to be positive 0.44 and the second reaction we're just going to rewrite it exactly the way it was and so the cell potential will not change so now we can add these two half reactions so these will cancel and so it's going to be iron metal reacts with the nickel two plus cation to produce the iron two plus cation and nickel metal so the cell potential is point four four plus negative point twenty three and so the cell potential for this reaction is positive 0.21 volts and that's simple way to calculate the cell potential of galvanic cell so you need to adjust the standard reduction potentials in such way that the electrons cancel make sure that occurs and when you add the cell potentials you need to get positive answer or at least zero zero's the lowest you can get you shouldn't get negative answer number three calculate the cell potential of the electrolytic cell according to the reaction shown below now as we mentioned before for galvanic cell the cell potential can be positive or zero but for an electrolytic cell it can be negative positive zero it's based on the way it's written so we're just gonna have to calculate the cell potential according to what we see here now you need to look up the standard reduction potentials that's associated for this reaction so let me give it to you bromine acquires two electrons and turns into bromide and cell potential for this reaction the standard reduction potential is 1.09 and for the next one fe 3 plus acquires one electron to turn into fe 2 plus and the cell potential for that is positive 0.77 volts so using those reduction potentials go ahead and calculate the cell potential for the reaction shown above so what we need to do is take this overall reaction and break it into half reactions so we can see that bromine turns into bromide and the only way for that to happen is if it acquires two electrons and so we have this reduction potential and so the cell potential for that is 1.09 now notice that we have fe2 plus turning into fe3 plus so when fe2 plus becomes fe3 plus notice that it's the reverse of this reaction and so therefore this is going to be negative 0.77 and we need to multiply this by 2. but it doesn't change this value it's not like enthalpy where you have to multiply delta by 2. the cell potential doesn't work so these are the two half reactions that make up this reaction if you add them these will cancel and then you get these two on the left and these two on the right side so now all we need to do is add the cell potentials so it's 1.09 plus negative 0.77 so for this example the cell potential is still positive but it's positive 0.32 volts and so for an electrolytic cell you need to calculate the cell potential based on the way the reaction is written it can be positive or negative go ahead and try this one it's very similar to the last problem and in order to get the answer we really don't need to balance the half reactions we just have to make sure that the electrons are on opposite sides and also that it adds up to the reaction based on the way it's written but if you want to balance it you could now the first thing that we see is that we have iodide turning into i2 and so we need two electrons on this side the standard reduction potential for that reaction it's written this way and you can look this up too you can go to google images and type in standard reduction potentials this is positive 0.54 we have the reverse reaction so this is going to be negative 0.54 and then we have the aluminum cation turning into the aluminum metal and so it has to acquire three electrons to do that and so this notice that the electrons are already on the left side you can look up the reduction potential based on the way it's written and for aluminum it's negative 1.66 and so if we want to balance it we need to get the least common multiple of these two numbers so basically we need to multiply this by three to get six electrons and this by two so then we'll have six iodide ions three i2 molecules and six electrons this number will not change it would still be negative 0.54 and then we'll have two aluminum cations six electrons and two aluminum atoms so we cancel the number of electrons so the overall reaction in its balanced form is six iodide ions two aluminum cations three iodine molecules and two aluminum atoms and this cell potential has not changed either so for this reaction it's negative let me do that again it's negative 2.2 volts so this reaction is non-spontaneous however in an electrolytic cell you can put energy to drive this reaction forward and so the minimum voltage that you need is 2.2 volts to get this going but you may have to apply voltage that's higher because some energy will be lost due to friction so that's how much voltage you need the minimum you need to drive this reaction forward you