714 HARMED ART. M 



which, at constant volume and temperature, becomes simply 



(7" - V')de = -#. (12) 



Thus, if the cell is maintained at constant volume and tem- 

 perature, the reversible electrical work done by cell discharge 

 equals the decrease in work content. 



In actual experimental studies, we are more likely to be con- 

 cerned with processes at constant pressure and temperature, 

 and for this reason Gibbs' thermodynamic potential f is of extra- 

 ordinary usefulness. This function is defined by 



^ = e-tv + pv (13) [91] 



and, consequently, at constant pressure and temperature, an 

 increment in ^ is given by 



d^ = de - tdrj + pdv. (14) 



Since equation (4) [694] may be written 



— dt -\- tdrj — pdv ,^ . 



Y" - y = ^^-^ ^ (15) 



de 



if we neglect dW a, we immediately obtain for the reversible cell, 



(F" - Y')de dr, (16) [699] 



and for any cell, 



(7" - Y')de ^ -dr. (17) [700] 



The reversible electrical work at constant pressure and tem- 

 perature is equal to the decrease in thermodynamic potential 

 due to the chemical reaction taking place in the cell. This 

 equation is of great importance since it affords a method of 

 evaluating directly the changes of thermodynamic potential 

 in many chemical reactions which otherwise could not readily 

 be obtained. 



These few considerations, deductions, and equations represent 

 Gibbs' explicit contribution to the thermodynamic theory of 

 the galvanic cell as contained in the "Equilibrium of Hetero- 



