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If a substance is going from Fe3+ to Fe2+, you know it gained 1 electron because the charge/oxidation # went up by 1. Then you balance the two half reactions to make sure both transfer the same amount of electrons.
To determine the number of electrons being transferred you look at the oxidation numbers of the substances. Then after writing and balancing the half reactions (since the number of electrons must be balanced) that is the number transferred.
The easiest way to do it for me, is write down what's being reduced or oxidized. From there, see if you need to add e- to the reactants for a reduction (gaining e-) or add e- to the products for a oxidation (losing e-). When you combine them and balance them out the number of e- transferred should be the same. That would be the number you use for n.
Once you are done balancing all the elements on both sides. calculate net charge on both sides and then determine which side would need electrons such that the netcharge on both sides is equal. One way to make sure you're writing it correctly is to check if there you added electrons on the LEFT for your reduction half reaction and on the RIGHT for your oxidation half reaction.
Balancing the charge of a half reaction is the last step so once you have everything but the charge balanced you should sum up both sides and then add electrons to the side of the reaction that has the more positive charge.
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