Kaplan vs. UWorld Rechargeable Batteries

[thedolphin]

Hello,

I’m confused on electrode conventions for charging rechargeable battery electrodes. Kaplan says the anode for a Ni-Cd battery is Cd when discharging and Cd(OH)2 when charging, which means the anode assignation stays with the same electrode whether the battery is charging or discharging.

UWorld, however, says that the anode when charging becomes the cathode when discharging for a lead-acid battery. I don’t understand how this is consistent with what Kaplan says on Ni-Cd batteries - aren’t they both rechargeable batteries?

Thanks in advance!

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[Instance_Variable]

When discharging, the cadmium electrode is the anode, as cadmium metal is oxidized to Cd(OH)2. In other words, oxidation at the cadmium electrode produces electrons that flow to the nickel electrode where NiOOH is reduced to Ni(OH)2. The nickel electrode would then be the cathode. When charging, this process is reversed. Electrons flow from the nickel electrode to the cadmium electrode. Therefore, the cadmium electrode becomes the cathode, and the nickel electrode becomes the anode.

I'm not sure what Kaplan is getting at, but that doesn't sound correct. By definition, anode and cathode are named according to electron flow. If the anode were Cd(OH)2 when the battery is charging, that would mean electrons were flowing away from the cadmium electrode (or were coming out of Cd(OH)2) during charging. This obviously can't be right, because Cd(OH)2 can't be oxidized further. The only option is for Cd(OH)2 to be reduced back to cadmium metal, in which case the cadmium electrode would be the cathode (as electrons are flowing towards it to supply the reduction taking place there).

What UWorld says is correct. As I said before, the naming is done according to electron flow. Electrons always flow towards the cathode (to supply reduction there), and they flow away from the anode (as a product of an oxidation reaction at the anode).

 

[thedolphin]

When discharging, the cadmium electrode is the anode, as cadmium metal is oxidized to Cd(OH)2. In other words, oxidation at the cadmium electrode produces electrons that flow to the nickel electrode where NiOOH is reduced to Ni(OH)2. The nickel electrode would then be the cathode. When charging, this process is reversed. Electrons flow from the nickel electrode to the cadmium electrode. Therefore, the cadmium electrode becomes the cathode, and the nickel electrode becomes the anode.

I'm not sure what Kaplan is getting at, but that doesn't sound correct. By definition, anode and cathode are named according to electron flow. If the anode were Cd(OH)2 when the battery is charging, that would mean electrons were flowing away from the cadmium electrode (or were coming out of Cd(OH)2) during charging. This obviously can't be right, because Cd(OH)2 can't be oxidized further. The only option is for Cd(OH)2 to be reduced back to cadmium metal, in which case the cadmium electrode would be the cathode (as electrons are flowing towards it to supply the reduction taking place there).

What UWorld says is correct. As I said before, the naming is done according to electron flow. Electrons always flow towards the cathode (to supply reduction there), and they flow away from the anode (as a product of an oxidation reaction at the anode).

That makes a lot more sense, thank you! Perhaps Kaplan made a mistake with the concept check solutions.