- The reaction in a primary battery occurs only once.
- The battery becomes dead after used onceand cannot be reused
- Example of primary cell is dry cell and mercury cell.
- The dry cell consists of a zincanode and the carbon (graphite) cathode surrounded by powdered manganesedioxide and carbon.
Fig. Dry cell
- The space between the two electrodes is filled by a soaking paste of ammonium chloride(NH4Cl) and zinc chloride (ZnCl2).
Anode: Zn(s) ⎯ -->Zn2+ + 2e–
Cathode: MnO2+ NH4++ e–⎯ -->MnO (OH) + NH3
- At the cathode manganese is reduced from + 4 oxidation state to +3 oxidation state.
- As per the equation, Ammonia producedforms a complex with Zn2+ giving rise to [Zn (NH3)4]2+.
- The cell has a potential of nearly 1.5 V.
- Mercury cell consists of zinc – mercuryamalgam as the anode and a paste of HgO and carbon as the
Fig. Mercury cell
- A paste of KOH and ZnO electrolyte used as an electrolyte.
- Theelectrode reactions for the cell are given below:
Anode: Zn(Hg) + 2OH– -->ZnO(s) + H2O + 2e–
Cathode: HgO + H2O + 2e– --> Hg(l) + 2OH–
- The overall reaction is represented by
Zn (Hg) + HgO(s) --> ZnO(s) + Hg(l)
- The cell potential is approximately 1.35 V that remains constant throughout its lifetime.