lectrochemistry: Primary and Secondary Batteries

A Primary Battery: The Leclanché Dry Cell (cont.)

The overall reaction occurring in the dry cell produces 1.5 volts:
anode				Zn(s)		Zn2+(aq)	+	2e–
cathode		2 e–	+	2 NH4+(aq)		2 NH3(g)	+	H2(g)
		2 NH3(g)	+	Zn+2 (aq)		[Zn(NH3)2]2+(aq)
		H2(g)	+	2 MnO2(s)		Mn2O3(s)	+	H2O(l)
Zn(s)	+	2 NH4+(aq)	+	2 MnO2(s)		[Zn(NH3)2]2+(aq)	+	Mn2O3(s)	+	H2O(l)

Note: Polarization is not the only disadvantage of a dry cell; these batteries tend to have poor shelf life because the zinc anode will slowly react with ammonium ions even when not connected to an external circuit. Storing a dry cell in a refrigerator will decrease the rate of this reaction and extend the shelf life of the battery significantly.

Another example of a common primary battery is the alkaline battery. The reactions that occur in this battery are shown below.

Assign the reactions to the correct electrode (cathode or anode).

unassigned   cathode  anode		Zn (s) + 2 OH– (aq) ZnO(s) + H2O + 2 e–
unassigned   cathode  anode		e – + MnO2(s) + H2O (l )                                 MnO(OH)(s) + OH–(aq)

As in the dry cell, zinc is used as the anode (and container). Look closely at the reduction reaction. What is a major difference between the dry cell and this battery? No gases are produced in the alkaline battery! Polarization is therefore not a problem and no significant decrease in current is observed under high loads. The main disadvantage of an alkaline cell is that it is relatively expensive to make.

Before moving to the next page, you should understand how a simple dry cell and a simple alkaline cell operates. You should also know what makes these primary batteries.

Primary and Secondary Batteries