Finding the emf of a Cell

The emf of a cell can be calculated from the standard electrode potentials of the half-reactions. In order to find the emf, we have to look at the two halfreactions involved in the reaction. Then, set up the two half-reactions so that when they are added we will get the net reaction. Once we have set the equations properly and assigned the prpper potentials to those half-reactions, we can add the standard electrode potentials. A common mistake that students make is that they forget the fact that the standard electrode potentials are given in terms of reduction reactions. Redox reactions involve both oxidation and reduction. If one half-reaction is reduction, the other should be oxidation. So we must be careful about the signs of the half-reaction potentials, before we add the two half-reaction potentials to get the emf value. Do the next example. 

we have to write the half-reactions as indicated below. 

From Table 12-1, we can take the standard electrode potential values. The cell containing the copper electrode has a standard potential value of 0.34 V. For the other half-cell, the reaction is oxidation. Since the value given in the table is in terms of reduction half-reactions, we have to reverse the sign of the standard 

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