23.1. Electrochemical Cells http://www.ck12.org
Anode (oxidation): Zn(s)+2OH−(aq)→Zn(OH) 2 (s)+2e−
Cathode (reduction): 2MnO 2 (s)+2NH+ 4 (aq)+2e−→Mn 2 O 3 (s)+2NH 3 (aq)+H 2 O(l)Alkaline batteries tend to have a longer shelf life, and their voltage does not decrease as much over time.
Lead Storage Batteries
Abatteryisa group of electrochemical cells combined together as a source of direct electric current at a constant
voltage. Technically, dry cells are not true batteries, since they consist of only one cell. The lead storage battery is
commonly used as the power source in cars and other vehicles. It consists of six identical cells joined together, each
of which has a lead anode and a cathode made of lead(IV) oxide (PbO 2 ) packed on a metal plate (Figure23.3).
FIGURE 23.3
A lead storage battery, such as those
used in cars, consists of six identical elec-
trochemical cells and is rechargeable.The cathode and anode are both immersed in an aqueous solution of sulfuric acid, which acts as the electrolyte. The
cell reactions are:
Anode (oxidation): Pb(s)+SO^24 −(aq)→PbSO 4 (s)+2e−
Cathode (reduction): PbO 2 (s)+4H+(aq)+SO^24 −(aq)+2e−→PbSO 4 (s)+2H 2 O(l)
Overall: Pb(s)+PbO 2 (s)+4H+(aq)+2SO^24 −(aq)→2PbSO 4 (s)+2H 2 O(l)Each cell in a lead storage battery produces 2 V, so a total of 12 V is generated by the entire battery. This electrical
potential is used to start a car or power a different type of electrical system.
Unlike a dry cell, the lead storage battery is designed to be rechargeable. Note that the forward redox reaction
generates solid lead(II) sulfate, which slowly builds up on the plates. Additionally, the concentration of sulfuric acid
decreases. When the car is running normally, its generator recharges the battery by forcing the above reactions to
run in the opposite, or nonspontaneous direction.
2PbSO 4 (s)+2H 2 O(l)→Pb(s)+PbO 2 (s)+4H+(aq)+2SO^24 −(aq)This regenerates the lead, lead(IV) oxide, and sulfuric acid needed for the battery to function properly. Theoretically,
a lead storage battery should last forever. In practice, the recharging is not 100% efficient because some of the lead(II)
sulfate falls from the electrodes and collects on the bottom of the cells.