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 likehearing aids, watches, etc.

Dry cell

• It is a type ofprimary battery.
• In the primary batteries, the reactionoccurs only once, and after use over a period of time battery becomes dead and cannot be reused again.
• The cell consists of azinc 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 ofammonium 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 ofKOH and ZnO.
• The overall reaction is represented by Zn(Hg) + HgO(s)→ ZnO(s) + Hg(l)
• The cell potential is approximately 1.35 Vand 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.