The Foundations of Chemistry

21-12 The Zinc–SHE Cell 865

1.The initial potential of the cell is 0.763 volt.

2.As the cell operates, the mass of the zinc electrode decreases. The concentration of

Zn^2 ions increases in the solution around the zinc electrode.

3.The Hconcentration decreases in the SHE. Gaseous H 2 is produced.

We can conclude from these observations that the following half-reactions and cell reac-
tion occur.

E^0

(oxidation, anode) Zn88nZn^2  2 e 0.763 V
(reduction, cathode) 2H 2 e88nH 2 0.000 V (by definition)

(cell reaction) Zn2H88nZn^2 H 2 E^0 cell0.763 V (measured)

The standard potential at the anode plusthe standard potential at the cathode gives the
standard cell potential. The potential of the SHE is 0.000 volt, and the standard cell poten-
tial is found to be 0.763 volt. So the standard potential of the zinc anode must be 0.763
volt. The Zn
Zn^2 (1.0 M)

H(1.0 M), H 2 (1 atm)
Pt cell is depicted in Figure 21-9.
Note that in thiscell the SHE is the cathode,and metallic zinc reduces Hto H 2. The
zinc electrode is the anodein this cell.

e– e–

H 2 (g)

Zn

Cl– K+

Salt bridge

1 M ZnCl 2 1 M HCl

Zn2+ H+

Pt

black

2H+ + 2e– → H 2

Reduction, cathode

Zn → Zn2+ + 2e–

Oxidation, anode

Voltmeter

• 
  
  
  
  • e–
  
  
  
  

H 2 gas

bubble

H+(aq)

solution

SHE as

cathode

Chloride

ion, Cl–

Zinc atom,

Zn

Zinc ion,

Zn2+

e–

Figure 21-9 The Zn
Zn^2 (1 M)

H(1 M); H 2 (1 atm)
Pt cell, in which the following net

reaction occurs.

Zn(s)2H(aq)88nZn^2 (aq)H 2 (g)

In this cell the standard hydrogen electrode functions as the cathode.