The Foundations of Chemistry

PRIMARY VOLTAIC CELLS

As any voltaic cell produces current (discharges),chemicals are consumed. Primary

voltaic cellscannot be “recharged.” Once the chemicals have been consumed, further

chemical action is not possible. The electrolytes or electrodes (or both) cannot be regen-

erated by reversing the current flow through the cell using an external direct current

source. The most familiar examples of primary voltaic cells are the ordinary “dry” cells

that are used as energy sources in flashlights and other small appliances.

DRY CELLS

The first dry cell was patented by Georges Leclanché (1839–1882) in 1866. The container

of this dry cell, made of zinc, also serves as one of the electrodes (Figure 21-15). The

other electrode is a carbon rod in the center of the cell. The zinc container is lined with

porous paper to separate it from the other materials of the cell. The rest of the cell is

filled with a moist mixture (the cell is notreally dry) of ammonium chloride (NH 4 Cl),

manganese(IV) oxide (MnO 2 ), zinc chloride (ZnCl 2 ), and a porous, inert filler. Dry cells

are sealed to keep the moisture from evaporating. As the cell operates (the electrodes must

be connected externally), the metallic Zn is oxidized to Zn^2 , and the liberated electrons

flow along the container to the external circuit. Thus, the zinc electrode is the anode

(negative electrode).

Zn88nZn^2  2 e (oxidation, anode)

The carbon rod is the cathode, at which ammonium ions are reduced.

2NH 4  2 e88n2NH 3 H 2 (reduction, cathode)

21-22

886 CHAPTER 21: Electrochemistry

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 21.8, Batteries.

Anode

 

Cathode

Insulating

washer

Steel cover

Wax seal

Sand cushion

Carbon rod

(cathode)

Porous

separator

Zinc can

(anode)

Wrapper

NH 4 Cl

ZnCl 2 ,

MnO 2 paste

(a) (b)

Figure 21-15 (a) The Leclanché cell is a dry cell that generates a potential difference of

about 1.6 volts. (b) Some commercial alkaline dry cells.