The Foundations of Chemistry

Addition of the half-reactions gives the overall cell reaction

Zn2NH 4 88nZn^2 2NH 3 H 2 Ecell1.6 V

As H 2 is formed, it is oxidized by MnO 2 in the cell. This prevents collection of H 2 gas
on the cathode, which would stop the reaction.

H 2 2MnO 2 88n2MnO(OH)

The ammonia produced at the cathode combines with zinc ions and forms a soluble
compound containing the complex ions, [Zn(NH 3 ) 4 ]^2 .

Zn^2 4NH 3 88n[Zn(NH 3 ) 4 ]^2 

This reaction prevents polarization due to the buildup of ammonia, and it prevents
the concentration of Zn^2 from increasing substantially, which would decrease the cell
potential.
Alkaline dry cellsare similar to Leclanché dry cells except that (1) the electrolyte is
basic (alkaline) because it contains KOH, and (2) the interior surface of the Zn container
is rough; this gives a larger surface area. Alkaline cells have a longer shelf life than ordi-
nary dry cells, and they stand up better under heavy use. The voltage of an alkaline cell
is about 1.5 volts. During discharge, the alkaline dry cell reactions are

Zn(s)2OH(aq) 88nZn(OH) 2 (s) 2 e (anode)

2MnO 2 (s)2H 2 O(
) 2 e88n2MnO(OH)(s)2OH(aq) (cathode)

Zn(s)2MnO 2 (s)2H 2 O(
)88nZn(OH) 2 (s)2MnO(OH)(s) (overall)

21-22 Dry Cells 887

Outer steel
case

Steel outer

top

Tin-plated

inner top Sealing and

insulating

gasket

Zn (anode)

KOH saturated with ZnO in

absorbent material (electrolyte)

HgO mixed with

graphite (cathode)

Inner steel case

Gas vent

Barrier

The mercury battery of the type frequently used in watches, calculators, and hearing aids is

a primary cell. Although mercury in the water supply is known to cause health problems, no

conclusive evidence has been found that the disposal of household batteries contributes to

such problems. Nevertheless, manufacturers are working to decrease the amount of mercury

in batteries. In recent years, the amount of mercury in alkaline batteries decreased markedly;

at the same time, the life of such batteries has increased dramatically.

The buildup of reaction products at an

electrode is called polarizationof the

electrode.