[Ranowa]

First topic here, though I've found this site very helpful before to lurk as a guest and find hw answers! I'm having trouble understanding one point in the process of galvanic cells, specifically, why one electrode is oxidized and the other is reduced. My textbook uses the example of a Zn, Cu cell and says that Zn acts as the reducing agent when paired with Cu, but not necessarily with other electrodes. That makes logical sense and I know you can determine which is the reducing agent and which is the oxidizing agent by looking at the half reaction reduction potentials, but my question is why does Zn just 'know' that it's the reducing agent and reacts accordingly? The two compartments of the cell are separated, so I'm looking at them as two entirely separate reactions. It doesn't make any sense to me that when put with ZnSO4 the Zn electrode grows increasingly negative when in a cell with Cu but if I changed Cu to something else it could grow increasingly positive instead when the two reactions are entirely separate. A video I saw explaining this said that the point is that the two reactions would both be oxidation, but at different rates which creates the charge discrepancy- but that didn't make any sense either because my book very clearly has one compartment labelled as positive and the other as negative.

I hope that's not too confusing... electricity's always been a weak point for me :/ Any help would be appreciated!

[Big-Daddy]

Zn would be impelled to mostly give its electrons up to Cu²⁺ (forming Zn²⁺ and Cu) by the thermodynamic favourability of this reaction. That's all you learn from the galvanic cell (along with the extent to which this is the case).

In the actual cell, no reaction is happening, because, as you said, the two compartments of the cell are separated. Actually they are separated by, say, a salt bridge in which certain ions may be able to flow (but not Cu²⁺!) - this does not allow passage of electrons so no reaction occurs here - and also they are connected by a voltmeter which allows passage of a tiny amount of current. It is in this current (the transfer of electrons) that all reaction of one compartment's species with another must occur, because the species are not directly in contact so they cannot exchange electrons directly.

Therefore, there is a very slight reaction occurring, but it is negligible (if the voltmeter were ideal, none would occur). The Zn electrode and Cu electrode will not change in charge. The point is that one is labelled "positive" and the other "negative" because this is the potential difference being considered.