5. Batteries

Batteries

Battery :

A battery is a an electrochemical cell, a device for interconverting chemical and electrical energy. A battery takes the energy relased by a spontaneous chemical reaction and uses it to produce electricity.

Primary Batteries : 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 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. All substances used are either pure solids or pure liquids.

Types of primary batteries :

(i) Dry cell %                            

   Anode  : Zn(s)

  Cathode : MnO₂(s)

Electrolyte : Paste of NH₄Cl + ZnCl₂ in starch

  Cathode :

 MnO₂ + NH₄+ + e–  MnO(OH) + NH₃

(Oxidation state of Mn changes from +4 to +3)

 Anode :

  Zn  Zn²⁺ + 2e^–

(ii) Mercury cell :

Anode: HgO(s)

 Cathode : Zn(Hg)

 Electrolyte : Paste of KOH + ZnO 

Cathode :

Zn(Hg) + 2OH^–  ZnO(s) + H₂O(l) + 2e^– 

                                                              (amalgam)

Anode  :

HgO(s) + H₂O(l) + 2e^–   Hg(l) + 2OH^–

Secondary Batteries : A secondary cell afer use can be recharged by passing current through  it in the opposite direction so that it can be used again. A good secondary cell can udnergo a large number of discharing and charging cycles. The most important secondary cell is the lead storage battery commonly used in automobiles and inverters.

  Anode          : Pb(s)

Cathode : PbO2(s)

Electrolyte : 38 % Conc. H₂SO₄ solution Ecell = 2.05 V. 

 

During the working of the cell discharge H₂SO₄ will be consumed so it's concentration in the solution hence density of the solution will decrease during charging of the cell PbSO₄ will get converted into Pb(s) and, PbO₂(s) and H₂SO₄ will be produced.

Nickel – cadmium battery.

Ecell = constant as cell reaction has pure solide/liquids only.

Anode : Cd(s)

Cathode : NiO₂(s)

Electrolyte : KOH

Cd + 2OH^– Cd(OH)₂ + 2e^–

2e^– + NiO₂ + 2H2O Ni(OH)₂(s) + 2OH^–

 Cd(s) + NiO₂(s) + 2H₂O(λ)Cd(OH)₂(s) + Ni(OH)₂(s)