Class 12 Chemistry Electrochemistry Primary battery

Primary battery:

  • 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.

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