JEE Main & Advanced Chemistry Electrochemistry / विद्युत् रसायन Some Commercial Cell (Batteries)

One of the main use of galvanic cells is the generation of portable electrical energy. These cells are also popularly known as batteries. The term battery is generally used for two or more Galvanic cells connected in series. Thus, a battery is an arrangement of electrochemical cells used as an energy source. The basis of an electrochemical cell is an oxidation – reduction reaction.

Types of commercial cells : There are mainly two types of commercial cells,

(1) Primary cells : In these cells, the electrode reactions cannot be reversed by an external electric energy source. In these cells, reactions occur only once and after use they become dead. Therefore, they are not chargeable. Some common example are, dry cell, mercury cell, Daniell cell and alkaline dry cell

(i) Voltaic cell                

Cathode : Cu  rod                        

Anode : Zn rod Electrolyte : dil. \[{{H}_{2}}S{{O}_{4}}\]    

Emf : 1.08 V

At cathode : \[C{{u}^{2+}}+2{{e}^{-}}\to Cu\]

At Anode : \[Zn\to Z{{n}^{2+}}+2{{e}^{-}}\]

Over all reaction :\[Zn+C{{u}^{2+}}\to Z{{n}^{2+}}+Cu\]

(ii) Daniel cell

Cathode : Cu  rod                        

Anode : Zn rod Electrolyte : dil. \[{{H}_{2}}S{{O}_{4}}\]    

Emf : 1.1 V At cathode : \[C{{u}^{2+}}+2{{e}^{-}}\to Cu\]

At Anode : \[Zn\to Z{{n}^{2+}}+2{{e}^{-}}\]

Over all reaction :\[Zn+C{{u}^{2+}}\to Z{{n}^{2+}}+Cu\]

(iii) Lechlanche cell (Dry cell)

Cathode : Graphite rod               

Anode : Zn pot Electrolyte : Paste of \[N{{H}_{4}}Cl+ZnC{{l}_{2}}\] in starch

Emf : 1.2 V to 1.5 V

At cathode : \[NH_{4}^{+}+Mn{{O}_{2}}+2{{e}^{-}}\to MnO{{(OH)}^{-}}+N{{H}_{3}}\]

At Anode : \[Zn\to Z{{n}^{2+}}+2{{e}^{-}}\]

Over all reaction : \[Zn+NH_{4}^{+}+Mn{{O}_{2}}\to Z{{n}^{2+}}+MnO{{(OH)}^{-}}+N{{H}_{3}}\]

(iv) Mercury cell           

Cathode : Mercury (II) oxide                       

Anode :    Zn rod Electrolyte :  Paste of \[KOH+ZnO\]         

Emf   :  1.35 V At cathode : \[Hg{{O}_{(s)}}+{{H}_{2}}{{O}_{(l)}}+2{{e}^{-}}\to H{{g}_{(l)}}+2OH_{(aq)}^{-}\]

At Anode : \[\underset{(\text{amalgam})}{\mathop{Z{{n}_{(s)}}}}\,+20H_{(aq)}^{-}\to Zn{{O}_{(s)}}+{{H}_{2}}{{O}_{(l)}}+2{{e}^{-}}\]

Over all reaction : \[Z{{n}_{(s)}}+Hg{{O}_{(s)}}\to Zn{{O}_{(s)}}+H{{g}_{(l)}}\]            

(2) Secondary cells : In the secondary cells, the reactions can be reversed by an external electrical energy source. Therefore, these cells can be recharged by passing electric current and used again and again. These are also celled storage cells. Examples of secondary cells are, lead storage battery and nickel ? cadmium storage cell.