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 DrG is the Gibbs energy of the reaction, then
DrG = – nFE(cell)
(2 15)
It may be remembered that E(cell) is an intensive parameter but DrG
is an extensive thermodynamic property and the value depends on n Thus, if we write the reaction
Zn(s) + Cu2+(aq) ¾® Zn2+(aq) + Cu(s)
(2 1)
DrG = – 2FE(cell)
but when we write the reaction
2 Zn (s) + 2 Cu2+(aq) ¾®2 Zn2+(aq) + 2Cu(s)
DrG = – 4FE(cell)
If the concentration of all the reacting species is unity, then
E(cell) = 
(
)
o
Ecell
 and we have
DrGo = – nF
o
E(cell)
(2