WILLIAM D. HARKINS 149 



The change of state is 



Left 



— (10 atm.)-^ n+ (at concn. x) , 

 2 



Therefore, the total change is 



Right 



H+ (at concn. x)-^ — ^ (i atm.) 

 2 



^(10 atm.) -^ — ^(latm.) 



2 2 



The electromotive force ((5) of the cell is an energy quantity and may therefore 

 be calculated as the equivalent of the maximum work which would be obtained if § 

 mol of hydrogen gas expands from a pressure of 10 atm. to that of i atm.' 



Now the amount of work done by a force F in acting through a space 5 is 



W^FS . 



But since force equals pressure times area (A), 



W^pAS^pAv, 



a A V represents the increase of volume. Thus for a small amount of work 



dW^pdv , 

 but since 



dW^NRT'^, 



V 



or by integration between the limits V2 and Vi , 



W = NRT In ^-- = NRT In ^ . 



This gives the amount of mechanical work which is equal to the change of electrical 

 energy (S'DfJ, if '^l is the number of equivalents of electricity transferred. '^, the 

 value of one equivalent of electricity, is equal to 96,500 coulombs. So 



^m=NRTln^, 



p2 



or 



^NRlli^^ log t, 



^i 96,500 p2 ' 



' The small amount of work involved in the change of volume of the liquid is neglected for the 

 sake of simplicity. 



