vary appreciably during its use. There are mainly two types of batteries. In the primary batteries, the reaction occurs only once and after use over a period of time battery becomes dead and cannot be reused again. The most familiar example of this type is the dry cell (known as Leclanche cell after its discoverer) which is used commonly in our transistors and clocks.
The cell consists of a zinc container that also acts as anode and the cathode is a carbon (graphite) rod surrounded by powdered manganese dioxide and carbon (Fig. . ). The space between the electrodes is filled by a moist paste of ammonium chloride (NH Cl) and zinc chloride (ZnCl ).
The electrode reactions are complex, but they can be written approximately as follows : Anode: Zn(s) ¾® Zn + + 2e – Cathode: MnO + NH + + e – ¾® MnO(OH) + NH In the reaction at cathode, manganese is reduced from the + oxidation state to the + state. Ammonia produced in the reaction forms a complex with Zn + to give [Zn (NH ) ] + . The cell has a potential of nearly . V.
Mercury cell, (Fig. . ) suitable for low current devices like hearing aids, watches, etc. consists of zinc – mercury amalgam as anode and a paste of HgO and carbon as the cathode.
The electrolyte is a paste of KOH and ZnO. The electrode reactions for the cell are given below: Anode: Zn(Hg) + 2OH – ¾® ZnO(s) + H O + 2e – Cathode: HgO + H O + 2e – ¾® Hg(l) + 2OH –