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Solution


Q.

The reaction at cathode in the cells commonly used in clocks involves:           [2024]
1 Oxidation of Mn from +3 to +4  
2 Reduction of Mn from +7 to +2  
3 Oxidation of Mn from +2 to +7  
4 Reduction of Mn from +4 to +3  

Ans.

(4)

The cell commonly used in clocks is dry cell. It consists of a zinc container that also acts as anode and the cathode is a carbon (graphite) rod surrounded by powdered manganese dioxide and carbon. The space between the electrodes is filled by a moist paste of ammonium chloride and zinc chloride.

   Anode half cell reaction:  Zn(s)→Zn2++2e-.

   Cathode half cell reaction:

   MnO2+NH4++e-→MnO(OH)+NH3

   Oxidation state of Mn changes from +4 in MnO2 to +3 in MnO(OH).