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Posted: Fri Mar 15, 2019 8:37 pm
How can the Ecell of a reaction be found by both adding the Ecells of its half reactions, like Dr. Lavelle did during the review on Friday, as well as subtracting the Anode Ecell from the Cathode Ecell? Both ways seem fundamentally opposite, but yet give you the same answer. Do you change the signs of one the Ecells at some point, and if so why?
Posted: Fri Mar 15, 2019 8:54 pm
There are two ways to find Ecell. One way is to use the equation Ecell=E(Cathode)-E(Anode). Remember that cathode is usually on the right whereas anode is on the left. You use the reduction E values of the half reactions and plug into Ecell equation
Another way is to not use the equation. Instead you determine which half reaction should be the anode and which half reaction should be the cathode. The one that should be the anode should have the sign of E reversed if the E given is the reduction of that half reaction. Then you just add the E values together to get Ecell
The difference between the 2 methods is that for the first method using the equation no signs need to be reversed as the equation reverses the sign at the anode. For the second method you just reverse the sign at the anode and add together.
Posted: Fri Mar 15, 2019 8:55 pm
When adding the Ecells of both reactions, we need to make sure that the electrons will cancel because we don't want electrons in the overall reaction. This is done by multiplying the equations by a least common multiple and by flipping one of the reactions so the electrons will cancel. By doing this, the signs of one of the Ecells will flip. In the problem from the review, the flipped reaction became an oxidation reaction. Therefore, adding the two Ecells (with one of them being negative) is the same thing as Ecathode - Eanode.
Posted: Sat Mar 16, 2019 1:17 am
You can find the Ecell by using two methods:
1) Ecell= ECathode-Eanode
2) Ecell=Eanode+Ecathode <-- for this method, you need to change the sign of the anode value from the reduction table
Posted: Sat Mar 16, 2019 1:30 am
Yes, if you calculate the Ecell by adding the Ecathode and the Eanode together, you need to change the sign of Eanode.