ELECTROCHEMICAL THERMODYNAMICS 711 



to the silver-silver chloride electrode within the cell, and the 

 chemical changes corresponding to the passage of one faraday 

 of electricity F, when summed up will correspond to the 

 reaction 



iH2 (1 atm.) + AgCl -> Ag + HCl (w), 



which will take place from left to right. To measure the 

 reversible electromotive force, E, and the reversible electrical 

 work, NEF, corresponding to the equation of the reaction, the 

 electromotive force of the cell is exactly balanced against an 

 outside electromotive force just sufficient to prevent its dis- 

 charge and not sufficient to charge it. This is the electromotive 

 force of the cell when no current is passing through the cell, or 

 when the entire system is in equilibrium. If we imagine the 

 cell to discharge against this electromotive force until the quan- 

 tities specified in the equation have reacted, the cell process 

 will have taken place reversibly. The electrical work, NEF, 

 will then be the maximum, and will be denoted the reversible 

 electrical work. 



We shall now follow Gibbs in determining the total energy 

 increase of the cell. Four kinds of changes are possible (Gibbs, 

 1,338): 



"(1) The supply of electricity at one electrode and the 

 withdrawal of the same quantity at the other. 



(2) The supply or withdrawal of a certain amount of heat. 



(3) The action of gravity. 



(4) The motion of the surfaces enclosing the apparatus, as 



when the volume is increased in the liberation of 

 gases." 



In the cell just described, there will be a contraction in volume 

 due to the disappearance of one-half mol of hydrogen at a con- 

 stant pressure of one atmosphere. These changes are neces- 

 sary and sufficient for the evaluation of the energy change 

 accompanying cell action. Indeed, the third is usually negli- 

 gible. 



Since, according to the first law, the increase in energy is 

 equal to the algebraic sum of the work and heat effects received 



