722 EARNED 



ART. M 



Therefore Af for the chemical reaction involving quantities of 

 reactants and resultants corresponding to the passage of 96,500 

 coulombs or any multiple thereof may be measured at constant 

 pressure and temperature. If E is expressed in volts, Af is in 

 joules. Substituting this value of A^ in equations (29), (30), 

 and (31), we obtain 



where At; and Ay denote the finite changes of entropy and 

 volume respectively in the cell reaction, and 



±iViJF = AX±(<*^)1. (36) 



Thus, not only do we obtain the pressure and temperature 

 coefficients of electromotive force, but also the important 

 equations by means of which the changes of entropy and heat 

 content of chemical reactions can be obtained from measure- 

 ments of E. Equation (34) is equivalent to equation (25). 

 This method of measuring the entropy change in a reaction has 

 proved to be of great importance in obtaining the data necessary 

 for the verification of the so-called "third law of thermo- 

 dynamics."* 



Let us now consider two cells which are to be measured at 

 constant pressure and temperature: 



Pt I Ha (1 atm.) | HCl(wi) 1 AgCl 1 Ag; ±^i, 

 and 



Pt I H2 (1 atm.) I HC1(W2) | AgCl | Ag; zt^2, 

 and their corresponding reactions, 



^Ha (1 atm.) + AgCl -> Ag + HCl(w:), 



* Lewis and Randall, Thermodynamics and the Free Energy of Chem- 

 ical Substances, Chapter XXXI, McGraw-Hill Book Co., New York 

 (1923). 



