Concise Physical Chemistry

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

CELL DIAGRAMS 223

TABLE 14.1 A Few Selected Reduction Potentials.

Half-Cell Reaction E◦(volts)

K+;K(s) K++e−→K(s) −2.925

Zn^2 +;Zn(s) Zn^2 ++2e−→Zn(s) −0.763

Cd^2 +;Cd(s) Cd^2 ++2e−→Cd(s) −0.403

H+;H 2 (g)2H++2e−→H 2 (g) 0.000

Cl−; AgCl(s); Ag AgCl+e−→Ag(s)+Cl− 0.222

Cu^2 +;Cu(s) Cu^2 ++2e−→Cu(s) 0.337

Fe^3 +;Fe^2 +;Pt Fe^3 ++e−→Fe^2 + 0.771

Ag+;Ag(s) Ag++2e−→Ag(s) 0.799

more electrons causing reduction of the oxidation number, one for K+(aq)+e−→

K(s), two for Cu^2 +(aq)+2e−→Cu(s), and so on (Table 14.1).

14.4 CELL DIAGRAMS

Certain conventions serve to simplify the written description of electrochemical cells.

Following convention, the half-cell that is higher in the table goes on the left and the

half-cell that is lower in the table goes on the right. We write the right half-cell as it

appears in the table and reverse the left half-cell to give adiagramfor the whole cell.

In the case of the Daniell cell, this is

Zn(s); Zn^2 +(aq)||Cu^2 +(aq); Cu(s)

where the double line represents a salt bridge. Thecell reactionis the sum of half-cell

reactions with the left-hand half-cell reversed:

Zn(s)→Zn^2 +(aq)+2e−

Cu^2 +(aq)+2e−→Cu(s)

Zn(s)+Cu^2 +(aq)→Cu(s)+Zn^2 +(aq)

The two electrons cancel upon addition. The electrode at which reduction occurs

(remember that half-cell potentials are listed as reductions) is defined as thecathode.

The other electrode is theanode.

The voltage of the whole cell is the sum of the half-cell potentials with the left

half-cell once again reversed:

E◦=E◦ 1

2 R

−E◦ 1

2 L

= 0. 337 −(− 0 .763)= 1 .10 volts