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The Leclanche cells are mostly used in which of the following option?

The Leclanche cells are mostly used in which of the following option? Correct Answer Both 1 & 2

The correct answer is 3 i.e. Both 1 & 2.

Primary battery:

  • Any battery (actually it may have one or more than one cell connected in series) or cell that we use as a source of electrical energy is a galvanic cell where the chemical energy of the redox reaction is converted into electrical energy. 
  • In the primary batteries, the reaction occurs only once, and after use over some 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.
  • The space between the electrodes is filled by a moist paste of ammonium chloride (NH4Cl) and zinc chloride (ZnCl2). 
  • The electrode reactions are complex, but they can be written approximately as follows:
    • Anode: Zn(s) → Zn2+ + 2e–  Cathode: MNO2 + NH4+ + e- → MnO(OH) + NH3
  • In the reaction at cathode, manganese is reduced from the + 4 oxidation state to the +3 state. 
  • Ammonia produced in the reaction forms a complex with Zn2+ to give 2+. The cell has a potential of nearly 1.5 V.
  • Mercury cells 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- → 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 and remains constant during its life as the overall reaction does not involve any ion in a solution whose concentration can change during its lifetime.

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