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You determine the Ecell value from the two half reactions you write from either the cell diagram given or the balanced chemical equation. The formula for calculating the Ecell is E(cathode) - E(anode) and these values you get from the chart of standard E values that is found in the back of the book. This calculated value determines is the cell is positive or negative overall.
For galvanic cells, which have to be spontaneous in order for it to run, E cell has to be positive. You can use the relationship deltaG = -nFEcell to explain this phenomenon. In order for delta G to be negative, which indicates that the reaction is a spontaneous one, E cell must be positive. For electrolytic cells, which are reactions that occur only with the input of an external energy source, E cell is negative because they are nonspontaneous.
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