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.

An electrochemical cell consists of a metallic zinc immersed in 0.1 M Zn(NO₃)₂ solution and metallic copper immersed in 0.2 M CuSO₄ solution. Then what is the cell reaction?

Zn + Cu²⁺ → Zn²⁺ + Cu

Anode reaction: Zn → Zn²⁺ + 2e^-

Cathode reaction: Cu²⁺ + 2e^- → Cu 

Overall Cell reaction: Zn + Cu²⁺ → Zn²⁺ + Cu