Cell Potential Problems – Electrochemistry

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This video. Were gonna focus on calculating the cell potential of galvanic cells and and even electrolytic cells. So for a galvanic cell.
We need to know that the 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 a way to calculate a cell potential or the cell potential for a galvanic cell. That uses these two half reactions now we need to adjust it in such a way that the electrons are not on the same side of both reactions from one half reaction. Ask to be on the right side and for the other ask me on the left side.
And we need to switch the right v. Action. Such that the overall cell potential is positive so lets say if.
We switch the first one this would. Be. Negative.
08. Negative. 08.
Plus negative. Two point three seven will not give us a positive number. So that tells us that we need to reverse this reaction.
Because then well be adding. Two positive numbers instead of one. So lets go ahead.
And do that so im going to reverse the second reaction. And so the cell potential is now positive two point three seven and the first reaction. Im going to leave it the way it is but im going to multiply.
It by two so that the number of electrons will stay the same if you multiply this reaction by two the cell potential will not change you should have multiplied by two this is going to. Remain. Positive 08 its not going to be.
08 times 2 or 16. It doesnt work that way now. When we add these two equation to notice that the electrons will cancel because this is a redox reaction and one of the action has to be oxidation.
The other has to be reduction anytime. The electrons on the right side its oxidation. If its on the left side its reduction.
So the electrons cannot both be on the same side. It just doesnt work out that way so now if we add these two reactions on the left. We have the magnesium atom and a 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 its gonna be two point three seven plus. 08. So in this case.
Its positive three point one seven volts. So thats the cell potential for a galvanic cell. That uses these two half reactions now what about this one calculate the standard cell potential of a galvanic cell that uses the two half reactions shown below so how can we adjust these half reactions in such a 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.
Its gonna be positive. If we reverse the second reaction. This would be the cell potential is going to be positive point 23 and if you add that to negative 044.
Thats going to give you a negative cell potential for the overall reaction. So we dont 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 point. 4 4.
And the second reaction. Were 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 its going to be iron metal reacts with the nickel 2 plus cation to produce the iron two plus cation and nickel.
Metal so the cell potential is 044. Plus negative point. 23.
And so the cell potential for this reaction is positive point. 21. Volts.
And thats a simple way to calculate the cell potential of a galvanic cell. So you need to adjust the standard reduction potentials in such a way that the electrons cancel make sure that occurs. And when you add the cell potentials.
You need to get a positive answer or at least zero zero. Is the lowest you can get you shouldnt get a negative answer. Number three calculate the cell potential of the electrolytic cell.
According to the reaction shown below now as we mentioned before for a galvanic cell. The cell potential can be positive or zero. But for an electrolytic cell.
It could be negative positive or zero. Its based on the way. Its written so were just gonna have to calculate the cell potential.

According to what we see here now you need to look up the standard reduction potentials. Thats associated for this reaction. So let me give it to you bromine acquires.
Two electrons and turns into bromide and a cell potential for this reaction. The standard reduction potential is one point zero nine and for the next. One.
Fe. Three plus acquires one electron to turn into fe 2. Plus.
And a cell potential for that is positive point. Seven seven 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 a bromide. Any 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. 109. Now notice that we have fe 2.
Plus. Turning into. Fe.
3. Plus. So.
When fe 2. Plus. Becomes.
Fe. 3. Plus.
Notice. That its the reverse of this reaction. And so therefore.
This is going to be negative point seven seven and we need to multiply this by two. But it doesnt change this value. Its not like enthalpy will you have to multiply delta h.
By two for cell potential doesnt 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 its one point zero nine plus a negative 077. So for this example the cell potential is still positive but its positive 032. Volts and so for an electrolytic cell.
We need to calculate the cell potential based on the way. The reactions are in it could be positive or negative go ahead and try this one its very similar to the last problem and in order to get the answer. We really dont need to balance two half reactions.
Well just have to make sure that the electrons are opposite sides and also that it adds up to the reaction based on the way its written. But if you want to balance that you could now the first thing that we see is that we have iodide turnin into i2 and so we need two electrons on this side. The standard reduction potential for that reaction.
Its 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 05 for we have the reverse. Reaction.
So this is going to be. Negative 05. 4.
And then we have the aluminum cat i turn it into the aluminum metal and so it has to acquire 3 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. Its written and for aluminum.
Its a negative one point six six 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 3 to get. 6 electrons and this by 2.
So then well have six iodide ions. 3. I2 molecules and six electrons this number will not change it would still be negative point.
Five four and then well have two aluminum cations six electrons and two aluminum atoms. So we cancel the number of electrons. So the overall reaction.
And 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. Its negative let me do that again its negative two point two volts. So this reaction is a non spontaneous.
However in an electrolytic cell. You can put energy to drive this reaction for it and so the minimum voltage. That you need is two point two volts to give this going.
But you may have to apply a voltage thats higher because some energy will be lost due to friction. So thats how much voltage. You need the minimum you need to drive this reaction forward.
.

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