Page 222 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 14] OXIDATION AND REDUCTION 211

flow through the salt bridge toward the right and anions flow to the left. The salt bridge is filled with a solution of
an unreacting salt, such as KNO 3 . The oxidation-reduction reaction provides the potential to produce the current
in a complete circuit.

Wire
KNO 3
Cu electrode
Zn electrode
Salt bridge

Zn 2+ Cu 2+
Fig. 14-3. Daniell cell

One such combination of anode and cathode is called a cell. Theoretically, any spontaneous oxidation-
reduction reaction can be made to produce a galvanic cell. A combination of cells is a battery.
We are used to the convenience of cells (more usually in the form of batteries) to power flashlights, cars,
portable radios, and other devices requiring electric power. Many different kinds of cells are available, each with
some advantages and some disadvantages. The desirable features of a practical cell include low cost, light weight,
relatively constant potential, rechargeability, and long life. Not all the desirable features can be incorporated into
each cell.
The lead storage cell (six of which make up the lead storage battery commonly used in automobiles) is
discussed as an example of a practical cell. The cell, diagrammed in Fig. 14-4, consists of a lead electrode and a
lead(IV) oxide electrode immersed in relatively concentrated H 2 SO 4 in a single container. When the cell delivers
power (when it is used), the electrodes react as follows:
Pb + H 2 SO 4 −→ PbSO 4 (s) + 2H + 2 e −
+
+
−
2H + 2 e + PbO 2 + H 2 SO 4 −→ PbSO 4 (s) + 2H 2 O
The solid PbSO 4 formed in each reaction adheres to the electrode. The H 2 SO 4 becomes diluted in two ways:
some of it is used up, and some water is formed. The electrons going from anode to cathode do the actual work
for which the electric power source is employed, such as starting the car. Both electrodes are placed in the same
solution; no salt bridge is needed. This is possible because both the oxidizing agent and the reducing agent, as
well as the products of the oxidation and reduction, are solids, so they cannot migrate to the other electrode and
react directly.

Pb
PbO 2

PbSO 4

H 2 SO 4

Fig. 14-4. Lead storage cell
The lead storage cell may be recharged by forcing electrons back the other way. Each electrode reaction is
reversed, and Pb, PbO 2 , and H 2 SO 4 are produced again. In a car, the recharge reaction takes place each time
the car is driven and the alternator changes some of the mechanical energy of the engine to electric energy and
distributes it to the battery, and much less often when the battery is recharged at a gasoline station.