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Implications behind +/- sign of anode in electrochemistry

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foxi

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In a spontaneous (Galvanic, concentration) cell, the sign of the anode is (+), as anions are attracted to it in order to balance out the newly oxidized charges. Opposite occurs in the cathode.

In a non-spontaneous (Electrochemical) cell, the sign of the anode is (-)... however, since oxidation still occurs at the anode, anions flow to the anode here too.

In both spontaneous & non-spontaneous electrochemical cells, electrons flow from anode to cathode, and anions flow into the anode (and cations into the cathode). Therefore, what is the implication of the sign switching? As far as I can tell it is nothing more than terminology... Can you yay or nay my thinking?
 

foxi

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The switching is what drives a non-spontaneous reaction forward.
>> Could you elaborate? What does the + or - mean? Nothing changes between these two different cell types besides the signs of the electrodes. Therefore is it not simply terminology?
 

foxi

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>> Could you elaborate? What does the + or - mean? Nothing changes between these two different cell types besides the signs of the electrodes. Therefore is it not simply terminology?

The switching is what drives a non-spontaneous reaction forward.
>> whether anode is + or -, it is still the site from where electrons are released... And it is still the site to where anions flow..
 

aldol16

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It's not a simple convention. It carries implications. In a galvanic cell, the cathode is positive because current (which conventionally is positively charged) moves from there to the anode, thereby completing the circuit. This is like a discharging battery. Now in an electrolytic cell, you're trying to force current to move in a direction that it doesn't want to move in. So basically if discharging a battery is favorable, then recharging it is unfavorable. In other words, forcing current to move from the anode to the cathode would be unfavorable.
 

NextStepTutor_3

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I strongly advise thinking about this like @aldol16 described it and not simply memorizing the signs of each electrode in each case (which tends to be a common approach). If you think about this in relation to spontaneity, it becomes simple. Electrons always move from anode to cathode, regardless of the type of cell. Galvanic cells are spontaneous, so which way must the electrons be moving? From a negative environment (which repels them) toward a more positive one (which attracts them). So the anode in a galvanic cell must be negative, while the cathode must be positive.

Now, electrolytic cells are nonspontaneous, so the anode can't be negative and the cathode positive. Why? Because in such a case, electrons would prefer to move from anode to cathode, and they would do so without requiring energy input. Instead, the anode must be positive and the cathode negative. It always helped me to imagine physically picking up electrons from the positive anode and dragging them against their will to the negative cathode. Of course such a process would be nonspontaneous!

I think a lot of confusion on this topic comes from the idea that electrons must "always" move toward something relatively positive. This is no more true than saying that an ion will "always" travel down its concentration gradient. It'll always move in that direction spontaneously, but if you put in energy, you can elicit movement in the opposite (nonspontaneous) direction. This topic is particularly important to master with regard to amino acids / isoelectric focusing, by the way, where you'll often hear that "amino acid X traveled toward the cathode" and will be expected to know what that means.

Good luck :)
 
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