What is the change in free energy of the cell reaction?

Electrical work done in one second is equal to electrical potential multiplied by total charge passed. If we want to obtain maximum work from a galvanic cell then charge has to be passed reversibly. The reversible work done by a galvanic cell is equal to decrease in its Gibbs energy and therefore, if the EMF of the cell is E and nF is the amount of charge passed and ∆_rG is the Gibbs energy of the reaction, then

∆_rG = – nFE_cell

It may be remembered that E_cell is an intensive parameter but ∆_rG is an extensive thermodynamic property and the value depends on n. From the measurement of E^o_cell , we can obtain an important thermodynamic quantity, ∆_rG^o , standard Gibbs energy of the reaction.