Gibbs energy and the cell potential

If the transformation is done reversibly, the work produced is equal to the decrease in the Gibbs energy = -AG. We have then, 

If both electrodes are in their standard states, az 2+ = 1 and ac i+ = 1, the cell potential is the standard cell potential, S°. Thus, after we divide by 2F, the equation becomes 

The result obtained for the Daniell cell in Eq. (17.20) is quite general. If the cell reaction as written involves n electrons rather than two electrons, the relation is 

Equation (17.23) is the fundamental relation between the cell potential and the Gibbs energy change accompanying the cell reaction. 

Observation shows that the value of S depends on the current drawn in the external circuit. The limiting value of S measured as the current goes to zero is called the electromotive force of the cell (the cell emf) or the reversible cell potential, 

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