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 anodePbSO4 + 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 anodeNi (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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