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Cell potential is an intensive property and is not a state function. Thus, we cannot just simply add the values together and apply a method similar to that of Hess's Law for cell potential. Therefore, since, delta G is a state function, we can use the equation $\Delta G=-nFE$ to find the change in Gibbs free energy for that particular reaction, use the same equation for the other reaction, and add the two values to get the change in Gibbs free energy for the desired reaction. From this value and the previous equation, we can evaluate for the total standard potential of the reaction.