# JEE Main & Advanced Chemistry Electrochemistry Some Commercial Cell (Batteries)

Some Commercial Cell (Batteries)

Category : JEE Main & Advanced

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.

 In charged Lead storage cell Alkali cell Positive electrode Perforated lead plates coated with PbO2 Perforated steel plate coated with Ni(OH)4 Negative electrode Perforated lead plates coated with pure lead Perforated steel plate coated with Fe Electrolyte dil. H2SO4 20% solution of KOH + 1% LiOH During charging Chemical reaction At anode :  PbSO4 + 2H+ + 2e– ® Pb + H2SO4 At cathode :  PbSO4 + SO4– – + 2H2O – 2e– ® PbO2                                                                         + 2H2SO4 Specific gravity of H2SO4 increases and when specific gravity becomes 1.25 the cell is fully charged. Emf of cell: When cell is fully charged then E = 2.2 volt Chemical reaction At anode :  Ni (OH)2 + 2OH+ – 2e– ® Ni(OH)4 At cathode : Fe(OH)2 + 2K+ + 2e– ® Fe + 2KOH Emf of cell : When cell is fully charged then         E = 1.36 volt During discharging Chemical reaction At anode :  Pb + SO4– – – 2e– ® PbSO4 At cathode : PbO2 + 2H+ + 2e– + H2SO4 ® PbSO4 +  2H2O Specific gravity of H2SO4 decreases and when specific gravity falls below 1.18 the cell requires recharging. Emf of cell : When emf of cell falls below 1.9 volt the cell requires recharging. Chemical reaction At anode : Fe + 2OH­– – 2e– ® Fe(OH)2 At cathode : Ni(OH)4 + 2K+ + 2e– ® Ni(OH)2 +  2KOH Emf of cell : When emf of cell falls below 1.1 V it requires charging. Efficiency 80% 60%

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