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When the half reactions are both reduction reactions or both oxidization reactions you change one of them by switching them around and changing the sign of the cell potentials right? How do you know which one to change so that they are no longer the same?
They'll most likely both be given with an associated standard reduction potential (Eº), so you'll identify the half-reaction with the more positive Eº as the reduction half reaction, and the other when (when flipped) will be the oxidation half reaction.
To find which one is the oxidation half and which one is the reduction half, you subtract the two Ecells and whichever one gives you a positive Ecell is the correct order. Then you analyze which numbers are in which position. The one on the left is the cathode and the one on the right is the anode.
You want the cell to transfer electrons from anode to cathode, from negative to positive (it's for this reason that the anode is always on the left and the cathode is always on the right). In order to do that, you usually want the potential difference to be positive. Therefore, the one with the greater magnitude (larger number) is the one that you want as your cathode. That's the one you flip.
Depends on what you want. By default, all the half-rxns should be presented in reduction formats. If you are trying to calculate E, flip the one with lower E and then simply add two cell potential together. Or instead of flipping any of the two, you can minus the lower half-rxn cell potential from the higher half-rxn cell potential. Either way works.
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