Batteries

BATTERIES

Primary cell

  • In these cells, reactions occur only once and after use they become dead. Therefore, they are not chargeable.
  • Some common examples are dry cell, mercury cell.
  • Dry cells – They are used in transistors and clocks. They are also known as Leclanche cell.
  • Mercury cells – They are used in low current devices like hearing aids, watches, etc.

Dry cell

  • It is a type of primary battery.
  • 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 cell consists of a zinc container that also acts as an anode and the cathode is a carbon (graphite) rod surrounded by powdered manganese dioxide and carbon. 
  • The space between the electrodes is filled with a moist paste of ammonium chloride (NH4Cl) and zinc chloride (ZnCl2 ). 

 

Mercury cell :

  • It 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 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 solution whose concentration can change during its life a time.

Mercury cell

 Mercury cell, 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 solution whose concentration can change during its life time.