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Spontaneity of Cell Reaction


Suppose for a cell written on paper, ERHC is greater than the ELHC. This implies that the reduction tendency of an ion appearing on the right-hand cell is greater than that appearing on the left-hand cell. As mentioned earlier, the ion appearing on the right-hand side will be reduced, whereas that appearing on the left-hand side will be formed by oxidation of the corresponding atom. These are the very reactions (reduction on RHC and oxidation on LHC) which we consider according to the accepted convention. Hence, the cell reaction in this case will be spontaneous in nature, and the cell potential Ecell (= ERHC - ELHC) will have a positive value as ERHC > ELHC. Conversely, if the cell, as it is written, has a positive emf, we conclude that the reaction, which is supposed to take place in the cell, is spontaneous in nature.

On the other hand, now consider a written cell in which ELHC is greater than the ERHC. This implies that the reduction tendency of an ion appearing on the left-hand cell is greater than that appearing on the right-hand cell. This means that the ion appearing on the left-hand side will be reduced, whereas that appearing on the right-hand side will be formed from the oxidation of the corresponding atom. But these reactions are just opposite to those written on the basis of accepted convention. In other words, the cell reaction in the case will be non-spontaneous as the actual tendency for the reaction to take place is in the opposite direction. The cell potential Ecell (= ERHC - ELHC) in this case will have a negative value as ERHC < ELHC. Conversely, if a cell, as written, has a negative emf, we conclude that the reaction, which is supposed to take place in the cell, is non-spontaneous in nature.




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