Concise Physical Chemistry

c14 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

224 ELECTROCHEMICAL CELLS

This is consistent with our treatment of thermodynamic potentials, for example, we

have

G◦=G◦(B)−G◦(A)

for the reaction

A→B

The cell reaction, which involves reduction at one electrode and oxidation at the other,

is called a reduction–oxidation reaction, or simply aredoxreaction. Redox reactions

are spontaneous if written according to the conventions agreed upon.

14.5 ELECTRICAL WORK

A joule is a volt coulomb, so an electrochemical cell operating at 1.0 volt does 1.0 J

of work for every coulomb of electricity it produces. Don’t forget that the coulomb

Cis an amount of charge placed in a capacitor by a specific current in a specific

timeC=It. Therefore the number of coulombs is proportional to the number of

electrons driven through a resistance or motor by the cell reaction. If one electron

is exchanged in the cell reaction and one mole of reactant is used up, one mole of

electrons is exchanged, and one faraday of charge is produced. The work produced

is 96,485 joules. In general,nelectrons are exchanged and the cell potential is

Evolts, so

w=nFE=−G

because the amount of work done per mole of reactant consumed is the molar

decrease in the Gibbs free energy. This leads to the important connection between

thermodynamics and electrochemistry:

G=−nFE

14.6 THE NERNST EQUATION

For ideal solutions we have

G=G◦+RTlnQ

whereQis the equilibrium quotient, not to be confused with the charge Q. Therefore

nFE=nFE◦−RTlnQ