Concise Physical Chemistry

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

222 ELECTROCHEMICAL CELLS

H 2

H 2

Pt

H+

FIGURE 14.1 The hydrogen half-cell.

(or 1 bar) pressure so that the surface of the platinum electrode is alternately bathed

by H 2 (g) and an acid solution containing H+(aq)ion. The H 2 (g) is adsorbed onto

the Pt surface (Chapter 11) so that what the H+(aq) ion “sees” appears to be a solid

surface of H 2 (g). The electrode reaction is comparable to a metal ion reaction, except

that now it is a hydrogen–hydrogen ion reaction:

H 2 (g)→2H+(aq)+2e−

Hydrogen half-cells can be connected to other half-cells, and the resulting voltage

can be measured as before.

Half-cell potentials cannot be measured by conventional means. Instead, the

hydrogen half-cell is chosen as a standard and is arbitrarily assigned a potential

of 0.000 volts. Now cells can be constructed from the hydrogen half-cell and any

other half-cell. The whole-cell potential is measured and, the contribution from

H 2 (g); H+(aq) being zero by definition, the entire cell potential is assigned to the

other half-cell. In this way, anelectrochemical seriesof many half-cell potentials

is built up, going from active metals like Zn; Zn^2 + near the top to electrodes

like Cu; Cu^2 +=+ 0 .337 volts farther down. The Cu;Cu^2 +is more positive than

H 2 (g); H+(aq) and the Zn;Zn^2 + =−0.763 volts is more negative; hence the

difference between them is 0.337−(−0.763)=1.10 volts, which accounts for the

1.1 volts observed for the Daniell cell. All this, of course, requires a definition of the

concentration of the metal (or other) ions in question. Although other conventions are

sometimes used, the standard half-cell potentials are defined at ion activities of 1.00.

14.3 HALF-CELL POTENTIALS

Many half-cells have been studied in the long and rich history of electrochemistry.

They are, by convention, listed asreduction potentialsinvolving addition of one or