ELECTROMOTIVE FORCE of VOLTAIC CELLS-CONVENTIONS as to SIGN R. C . CANTELO University of Cincinnati, Cincinnati, Ohio
The differences between the so-called American and European conventions as to the signs of the electrode potentials, and of the electromotive force of a voltaic cell are illustrated and com@red.
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TUDENTS of physical chemistry are always confused by the two apparently contradictory conventions as to sign of (a) the electrode potentials and (b) the electromotive force of a voltaic cell. This is the case especially when a student has had two
"courses" under different instructors each of whom uses a different convention. It is the purpose of this note to illustrate the differences between the so-called American and European conventions. The derivation of the expressions used may be found in textbooks of physical chemistry.' For example, M a c D o u c ~ ~''Thermodynamics t, and chemistry," 2nd ed., John Wiley and Sons, Inc., New York City, 1926, chap. 17; THOMPSON, "Theoretical and applied electrochemistry," rev. ed., The Macmillan Co., New York City, 1925, chap. 5.
CONVENTIONS
Ammican American Chemical Society; Lewis and Randall; Taylor; MacDougall. (1) Positiue electricity passes left to right through the cell. (2) The electrode reactions are mitten as oridationr. (3) If the electrode reaction is an oxidation, its electrode potential is given a positi~esign; if i t is a reduction, the electrode potentid is given a negative sign. (4) The cell readion is obtained by subtracting the cathode readion from the anode reaction. (5) E = El - E,
- 2 RT ... In onn aba. . .. (7) E 1 0 means that the positive current passes l ~ j tto right thmueh - the cell as written. (6) El = EL -
e ' ~ a 1 ~ .
(8) Example, the cell: Zn(s), Z n + + = 0.1 m), Cut+(a = 0.01 m) Cu(s) Anode reaction. Zn(s) 2 f3+ Znt+(e = 0.1 m) Cathode reaction, 2 @--t Cu++(a = 0.01 m) Cu(s) Cell reaction, Cuf+(a = 0.01 m) -+Cu(s) Zn++ (a = Zn(s) 0.1 m) Normal electrode potentials referred to the standard hydrogen half-cell, Ha(g. a. = 1atm.), H t (a = 1 m) Zn(s), Zn++ (a = 1 m), E = 1-0.758 Cu(r), Cu++ (a = 1m), E = -0.345 0.059 El = 0.758 - - log 0.1
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= -0.345
- 0.059 log 0.01
European American Electrochemical Society; Creighton; Thompson; McLewis; European Chemical Societies. (1) Positive electricity passes right to left through the cell. (2) The electrode reactions are written as oxidetions. (3) If the electrode reaction is an oxidation, its electrode potential is given a negative sign; if i t is a reduction, the electrode potential is given a positins sign. (4) The cell reaction is obtained by subtracting the cathode reaction from the anode reaction. ( 5 ) E = E, - EX RT a - ~ a f .. ~. - In (6) El = El nF a * r a b s . ... (7) E > 0 means that the positive current passes right to left throuxh - the cell as written. (8) Example, the cell: Cu' Zn(s), Znt+ (a Anode reaction, 2 63--+ Znt+ (a = 0.1 nz) Zn(s) . . Cathode reaction, Cds) 2 @-, Cut+ (a = 0.01 m) Cell reaction, Zn(s) Cut+ (a = 0.01 m) +Cu(s) Zn++ (a = 0.1 m) Normal electrode potentials referred t o the standard hydragen half-cell. HZ(g. a. = 1atm.), H + (a = I m) Zn(s), Zn++ (a = 1m), E = -0.758 Cu(s), C u t + (a = l m), E = +0.345 0.059 El = -0.758 log 0.1 2 = -0.7% 0.059 log 0.01 Ez = 0.345
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