Application to Electrical Work

An example in which the relationships can be explored in some detail is the formation at constant T and P of one mole of aqueous HCl from the gaseous elements H2 and CI2  

If the gaseous mixture is exposed to a photochemical stimulus in the absence of any other constraints, the reaction proceeds spontaneously and irreversibly. Thus, 

When electrical work is obtained from the reaction under reversible conditions, that is, against a counterpotential only infinitesimally smaller than that of the cell, then 

The potentiometer also can be kept at a finitely lower potential than the cell. In this case, the reaction would proceed spontaneously, but the work obtained would be less in magnitude than the value given in Equation (7.83). If the potentiometer is kept at a potential greater than that of the cell, the reverse of Equation (7.82) will occur, irreversibly, with AG positive and W et positive. Then Wnet,rev is the minimum work required to reverse the process in Equation (7.82). 

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