The Foundations of Chemistry

THE ZINC–COPPER CELL

Consider a standard cell made up of two half-cells, one a strip of metallic Cu immersed
in 1 Mcopper(II) sulfate solution and the other a strip of Zn immersed in 1 Mzinc sulfate
solution (see Figure 21-6). This cell is called the Daniell cell. The following experimental
observations have been made about this cell.

1.The initial voltage is 1.100 volts.

2.The mass of the zinc electrode decreases. The concentration of Zn^2 increases in

the solution around the zinc electrode as the cell operates.

3.The mass of the copper electrode increases. The concentration of Cu^2 decreases

in the solution around this electrode as the cell operates.

The Zn electrode loses mass because some Zn metal is oxidizedto Zn^2 ions, which
go into solution. Thus the Zn electrode is the anode.At the cathode,Cu^2 ions are reduced
to Cu metal. This plates out on the electrode, so its mass increases.

21-9

21-9 The Zinc–Copper Cell 859

Metallic

zinc

electrode

Cl– K+

Salt bridge

1 M ZnSO 4 1 M CuSO 4

Zn2+ Cu2+

Cu2+ + 2e– → Cu

Reduction, cathode

Zn → Zn2+ + 2e–

Oxidation, anode

Voltmeter

K+ Cl–

(5% agar)

Electrons Electrons

Metallic

copper

electrode

Zinc atom,

Zn

Zinc ion,

Zn2+

e–

e–

Copper

atom, Cu

Copper

ion, Cu2+

Sulfate

ion, SO 42 

Figure 21-6 The zinc–copper voltaic cell utilizes the reaction

Zn(s)Cu^2 (aq)88nZn^2 (aq)Cu(s)

The standard potential of this cell is 1.10 volts. The standard cell can be represented as
Zn
Zn^2 (1 M)

Cu^2 (1 M)
Cu.