what is the emf of this cell under standard conditions? This is a topic that many people are looking for. amritsang.org is a channel providing useful information about learning, life, digital marketing and online courses …. it will help you have an overview and solid multi-faceted knowledge . Today, amritsang.org would like to introduce to you Calculate the cell potential at standard and then at non-standard conditions. Following along are instructions in the video below:
This video. We are going to calculate the cell potential for a copper two and and copper cell. Reacting with the zinc and zinc plus two half cell.
We are to calculate the cell potential under standard conditions. Meaning that we have a 1 molar concentration of zinc nitrate. This is really the spectator ion.
So it does not matter. What that is and weve also got a 1 molar copper sulfate solution. Again sulfate would be a spectator ion.
So our zinc is dissolved in solution as well as the copper and remember copper. 2. If we made that cell in the lab.
Our copper 2 would be blue in color and then we in this beaker. We would put a solid piece of copper and in the other beaker. Wed have some zinc nitrate dissolved in solution and a solid piece of zinc and that solution.
So our standard condition. Assuming that we are at 298 degrees. Kelvin.
Or. 25. Degrees.
Celsius and our solutions are 1 molar. The e cell comes from those half cell reactions copper coppers potential measured against the hydrogen electrode is plus 033. 7.
Volts zincs reduction potential is negative point seven six 3. Volts. So we could reverse this reaction as weve written here and add that to our copper reduction reaction and so its really a matter of choice on the way wed like to do this if we right both reactions as reductions.
Were going to do subtraction. But if we reverse the order of the reduction. So that it shows that metal being oxidized will simply change the sign of that answer and then well use hesss law.
Well add these equations together the electrons are going to cancel and the potential that we get out of this standard cell. Would be a positive 110 volts. So the e cell.
It may be easier to remember big number minus small number. So what this minus sign serves to do why the lighting is so bad that minus sign means change the sign to reverse the reaction. So we need to decide if were just going to stick with the math trick definition or use hesss law in this case.
So if we reverse the sign of the reaction. Ourselves then well change the sign of that potential and in that case. Well add those two reactions.
If we just choose to take big number minus. Small number this minus sign is reversing the sign of the potential force so either way we do that well get 0337 volts minus negative point seven six three volts so under normal conditions standard state conditions that electrochemical cell would give us a potential of 110. Volts.
Now. What i would like to do is run this under non standard conditions. So thats where we use the nernst equation and our temperatures could vary.
What im going to vary are the concentration units. So the non standard conditions. Which is the nernst equation means the potential would be the same cell potential that we measured above using the appendix minus.
0059 two volts divided by in the number of electrons transferred and this uses. The log of q. So q.
Remember is always products over reactants. So if i think about the chateliers principle for a few minutes. If i want this reaction to continue to proceed toward the right if i make the copper solution.
More concentrated so lets do that if we add more reactants then the shatt liaison supposes that that will cause that equilibrium to shift to the right and if i make the zinc solution more dilute in a way thats similar to removing products. If i make the solution more dilute than one molar that will also cause the reaction to continue to proceed to the right so im going to have us calculate non standard conditions and lets assume our copper copper two cell and our zinc zinc plus two cell that we have again and well say that the concentration of the copper plus. Two lets make that more concentrated two five molar and well make our zinc solution that our zinc electrode is in maybe 05.
Molar and our net ionic equation is up here so recall that solids and liquids do not go in the equilibrium expression. Only solutions or gases. So if i write q.
Q. Is going to equal r. Zinc.
Plus. Two concentration divided by a copper plus two and if we do that were going to get. 05 on the top 50.
On the bottom. And that should give us a value of 10. So i probably should have worked out an example before i got started doing this because thats when we take the log of 10.
Were only going to get a value of 1. But thats alright so if q is equal to 10. Well plug that into.
Here we already know the standard cell. Potential which was 110. Volts and we know that two electrons were transferred in this process.
So thats really 2 moles of. Electrons so if i calculate. E thats going to be e.
Cell. 110. Volts minus point zero five nine two volts divided by two and then i take the log of 10 and i cant find my calculator thats not going to be 10 didnt look right.
Point. 5 divided by 5 is going to be 001. I believe point 5 divided by 5.
Yeah point 1. Not 10 went the wrong way with my decimal. So thats what was not sitting well with me so if we take the log of 01.
Thats going to give us a negative. 1. So if i take the log of my second answer ill get a negative 1 for that value so im.
Still calculating this 11. 10. Volts minus point zero 5.
9. 2. Volts divided by.
2 and we. Said the log of 01. That we get on the calculator was negative.
1. So this negative and that negative is actually going to cancel to a positive number and so our potential for the cell will go up slightly. Plus.
Now if i take point 0. 5. 9.
2 and divide that by to get point o 2. 9. 6 so plus.
002 9. 6. Meaning our cell potential at non standard conditions.
Im going to add the one point one volts to that so well have one point one three volts. So well get a little bit more potential out of these non standard conditions than we had at our standard condition state. So that was standard just like the equation that we used for non standard calculation of gibbs free energy.
All of this or i should have done it right here this term corrects for non standard conditions. So that q. The reaction quotient looks just like k.
Its always products over reactants. But if our concentrations are not one molar then that will affect the conditions. Here and as we stated earlier if we look at the net equation for this electrochemical cell.
If we had excess of one molar that is very similar to adding more reactants and if we had a solution. Less than one molar of our zinc ion in solution in effect. Thats the same as removing products.
So the chateliers principle just from our initial conditions. The shotley. A sprin.
Suppose says that this reaction will shift to the right at equilibrium and a shift to the right is good that means. Were shifting toward products and in this case. A shift to the right meant.
We got more potential out of that cell. So almost any time. Were asked to explain our results when were talking about free energy and equilibrium.
If we write the shot the as principal down for that explanation will be correct. So that is a big deal out of the equilibrium chapter. But anytime we upset the balance the reaction will shift to proceed back toward equilibrium.
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