Write the overall reaction that occurs during use (discharging) of nickel- cadmium cell. Is it a primary or a secondary cell? Mention its one merit over the lead storage cell.

Explanation

(i) Nickel-cadmium storage cell: It is another rechargeable cell. It consists of cadmium anode and the cathode is made of a metal grid containing nickel (IV) oxide. These are immersed in KOH solution. The overall reactions occuring:
Anode:
`Cd(s)+2OH^(-) (aq) to Cd(OH)_2(s)+2e^(-)`
Cathode:
`NiO_2(s)+2H_2O(l)+2e^(-) to Ni(OH)_2(s)+2OH^(-) (aq)`
Net reaction
`Cd(s)+NiO_2(s)+2H_2O(l)to Cd(OH)_2(s)+Ni(OH)_2(s)`
discharging Ni-Cd cell is :
`Cd(s)+2Ni(OH)_3(s)to CdO(s)+2Ni(OH)_2(s)+H_2O(l)`
(ii) Primary/Secondary: It is a secondary cell.
(iii) Merit over lead storage cell: As is evident, there are no gaseous products, the products formed. adhere to the electrodes and can be reconverted by recharging process.