[INeedSerotonin]

The discharge reaction of the NiCad battery is as follows:

Cd_(S) + 2 OH^-_(aq) ---> Cd(OH)_2(s) + 2 e^-

NiO_2(s) + 2 H₂O_(ℓ) + 2 e^- ---> Ni(OH)_2(s) + 2 OH^-_(aq)

Cd_(s) + NiO_2(s) + 2 H₂O_(ℓ) ---> Cd(OH)_2(s) + Ni(OH)_2(s)

The insoluble hydroxides of nickel and cadmium deposit themselves on the eletrodes, and thus the reactions are easily reverted. The cell potential of each semi-reaction above (the first two ones), when written as reduction, are as follows:

The first: - 0.815 V
The second: + 0.490 V

Specify which one is the anode reaction and which is the compound that acts on the anode; do the same for the cathode; discover the cell potential.

Could you guys give me a hint on how to start here? I have absolutely no idea. If it were just simple metals and ions, it would be easy for me. But with all those bases??? No way!

It seems to me that Cd does not want to oxidize (because the voltage is negative); and the nickel wants to reduce. But I'm stuck here.

[mjc123]

It seems to me that Cd does not want to oxidize (because the voltage is negative);

The voltage is negative for what? As you yourself say, these are reduction potentials. You've written the Cd half-reaction as an oxidation. What's its potential?

[INeedSerotonin]

It seems to me that Cd does not want to oxidize (because the voltage is negative);

The voltage is negative for what? As you yourself say, these are reduction potentials. You've written the Cd half-reaction as an oxidation. What's its potential?

Thank you! I think I've got it now!

The Cd's half-reaction is an oxidation one, and its reduction voltage is negative. This means that Cd wants to oxide, so it goes to the anode.

Ni reduces, and it wants to reduce (because the reduction voltage is positive), so it goes to the cathode.

I remember reading that ΔV = greatest(V_reduction) - smallest(V_reduction), so that would be 0.490 - (-0.815) = 1.350 V

But let me ask you one more thing: the exercise asks me which compound/species goes to the anode and to the cathode. The answer is "Cd goes to the anode; NiO2 goes to the cathode"

Why is it "NiO2" and not just "Ni"?

[mjc123]

Where is Ni in your reaction?

[INeedSerotonin]

Where is Ni in your reaction?

Well, I guess there isn't a lonely Ni in the global reaction 

I still find this concept very strange. I find it hard to imagine a whole NiO2 molecule going to the cathode. I thought only ions or metallic elements could be attracted to the eletrodes.

[AWK]

You literally understand "goes".
Cd(OH)₂ covers the surface of the cadmium electrode with a thin layer, and during charging it "goes" to metallic cadmium. Ni(OH)₂ is even inside the nickel electrode (because the Ni electrode is porous) and is transformed into NiO₂ during charging.

[INeedSerotonin]

Thank you!

I am still having a hard time trying to understand this battery. I tried to draw it to show you how my imagination views it.

I think it looks this:



... or perhaps



I'm confused 

Nickel has an oxidation number of +4 on NiO2. And on Ni(OH)2 it has an oxidation number of +2. So it receives electrons. I probably should put it on the cadmium tank, so it would get their electrons. Or maybe not. I'm lost here.

[AWK]

Both: Ni(OH)₂ and NiO₂ are insoluble.
Like in lead battery: PbSO₄ and PbO₂.

[mjc123]

I probably should put it on the cadmium tank, so it would get their electrons.

What do you think the wire is for?

[INeedSerotonin]

Both: Ni(OH)₂ and NiO₂ are insoluble.
Like in lead battery: PbSO₄ and PbO₂.

Thank you! 

I probably should put it on the cadmium tank, so it would get their electrons.

What do you think the wire is for?

I think that it is connecting both the nickel and the cadmium, so there is a difference of voltage (U = R.i) and movement of electrons. The cadmium is oxidating, so it is leaving lots of electrons on the anode. These electrons will travel to the cathode, where nickel will take those electrons.

I guess if they were in the same tank it wouldn't work. You're right. The cathode and the anode would just "glue" together and there would be no circuit.

GREETINGS, THE CONGRATULATIONS FOR THE LEVEL OF KNOWLEDGE THAT HANDLE AND THIS PERFECT THE FORMULA, I AM A CHEMICAL ENGINEER AND I RECOGNIZE THE LEVEL OF KNOWLEDGE THAT MANAGE, CONGRATULATIONS.

Thank you, sir, but these conclusions are not mine. This is a chemistry exercise from a Brazilian federal university admission test. All the credit is theirs. 

[AWK]

Wikipedia describes the construction of Ni-Cd batteries - see the last paragraph of History (from Today ...). The electrolyte is ~ 20% KOH. As separator is used non-woven fabric made of synthetic polymers.
Sometimes it's worth looking at Wikipedia or a good manual.