Oxidation and reduction

This is the first time i have come across a question like this:The following equation summarises the reactions that occur during discharge and
charging of the nickel-cadmium battery.

2NiOOH + Cd + 2H2O <(charge) --- (discharge)> 2Ni(OH)2 + Cd(OH) 2


Which of the following occurs during discharge?
(A) Ni2+ is oxidised to Ni3+

(B) Ni2+ is reduced to Ni3+

(C) Ni3+ is oxidised to Ni2+

(D) Ni3+ is reduced to Ni2+


Ok, I KNOW that it cannot be b or c because of OIL and RIG

What happens during the charging and discharging of a battery in terms of redox reactions

Please explain

p.s. the answer is D

i've asked the answer before, so please dont just tell me the answer is D, that is not being helpful

i need someone TO EXPLAIN what happens during discharging and charging a battery.

and i know that NiOOH is converted to Ni(OH)2 - but how do i know what the valency is for each??

During discharge, the forward feasible reaction occurs where NiOOH is reduced to Ni(OH)2 and Cd is oxidised to Cd2+ in Cd(OH)2.

For charging the backward non-feasible reaction requires energy (hence charging) to make it feasible where Ni (OH)2 gets oxidised to NiOOH and Cd2+ gets reduced to Cd

To calculate O.s of Ni in NiOOH, recall valency of O is 2- and OH- is -1. Since NiOOH is neutral , o,s of Ni is 0-(-2)-(-1)=+3.
O.s of Ni in Ni(OOH)2 is +2 by the way.

This battery uses nickel oxide in its positive electrode (cathode), a cadmium compound in its negative electrode (anode), and potassium hydroxide solution as its electrolyte. The Nickel Cadmium Battery is rechargeable, so it can cycle repeatedly. A nickel cadmium battery converts chemical energy to electrical energy upon discharge and converts electrical energy back to chemical energy upon recharge. In a fully discharged NiCd battery, the cathode contains nickel hydroxide [Ni(OH)2] and cadmium hydroxide [Cd(OH)2] in the anode. When the battery is charged, the chemical composition of the cathode is transformed and the nickel hydroxide changes to nickel oxyhydroxide [NiOOH]. In the anode, cadmium hydroxide is transformed to cadmium. As the battery is discharged, the process is reversed, as shown in the following formula.
Cd + 2H2O + 2NiOOH —> 2Ni(OH)2 + Cd(OH)2


Cathode: 2NiO(OH) + 2H2O+ 2e-→2Ni(OH)2+ 2OH-
Anode: Cd + 2OH-→Cd(OH)2+ 2e-
Overall: 2NiO(OH) + Cd + 2H2O→2Ni(OH)2+ Cd(OH)2