[galpinj]

Hey guys,

I have always had difficulty understanding an aspect of the half cell, and I'm hoping someone here can clear it up for me.

Regardless of the type of cell, we think of the cathode as gaining electrons. For example, in the common zinc/copper galvanic cell (cell diagram: Zn l Zn²⁺ ll Cu²⁺ l Cu) we have zinc at the anode and copper at the cathode.

My question is twofold:
1. Why do we need copper at the cathode, and
2. Why don't we consider the Cu²⁺ the "cathode", as it is the cation that is actually being reduced.

Basically, why couldn't I just replace every cathode in the world with a platinum bar? Unlike the anode, which is obviously participating, it seems like the cathode is doing nothing important.

Thank you for your help

[Arkcon]

There are a number of phenomena at work here.  And you can't just handwave them away.  That's an argument, not science.  Lets tackle some of them:

Can you tell me, why copper, and why now wood or glass?

Will you, personally, give me the platinum to replace all the electrodes I use?

Does nothing, in fact, happen at the cathode?  Do they always endure, unchanged, forever?  Did you try it?

[galpinj]

I would have said cooper (or platinum) because they allow for the transfer of electrons (being metals). If I could in fact give you platinum for such reactions, would they continue to function the same? Certainly the cathode would begin to be platted by copper, but this doesn't really answer my question on the theory.

If it is the copper cation that is reacting, why don't we consider it the cathode? Beyond it being capable of transferring electrons, why do we care what the "cathode" is?

[mjc123]

Because it has to be capable of transferring electrons to/from the external circuit. The electrode is the interface between the electrolyte (ionic conduction) and the circuit (electronic conduction). The cathode is the electrode at which reduction takes place - not the species that is being reduced.
And the cathode doesn't gain electrons - it gives them up to the species being reduced. (If that results in deposition of the reduced species, like copper, you could think of it as "gaining cations".) In open circuit this results in it becoming positive - it needs a circuit connection to replenish itself with electrons released at the anode.

[galpinj]

Because it has to be capable of transferring electrons to/from the external circuit. The electrode is the interface between the electrolyte (ionic conduction) and the circuit (electronic conduction). The cathode is the electrode at which reduction takes place - not the species that is being reduced.
And the cathode doesn't gain electrons - it gives them up to the species being reduced. (If that results in deposition of the reduced species, like copper, you could think of it as "gaining cations".) In open circuit this results in it becoming positive - it needs a circuit connection to replenish itself with electrons released at the anode.

Okay, that makes a lot of sense. Thank you!