Galvanic Cells vs Electrolytic Cells

[joey7]

Hey guys having a little bit of trouble getting my head around changes, when a cell is discharged opposed to being recharged, would appreciate it if someone could clarify.

Am I right in that when recharged

- the negative electrode in which oxidation occured (anode) stays the negative electrode but becomes the cathode in which reduction occurs.
-the positive electrode in which reduction occurs (cathode) stays the positive electrode but becomes the anode in which oxidation occurs.

[golden]

Galvanic cells: Negative = anode = oxidation, positive = cathode = reduction.
Electrolytic cells: Negative = cathode = reduction, positive = anode = oxidation.

[lzxnl]

I was initially confused about this too and thought about this for a long time before and this is what I came up with.

The positive electrode is at higher electric potential. Therefore electrons will spontaneously move to the higher electric potential if they can. This is what happens with a galvanic cell; no external influence => electrons move from negative to positive. As the electrons leave from the negative, the negative electrode is the side of oxidation, or the anode.

In an electrolytic cell, you have an extra voltage somewhere. The positive terminal of the external battery is connected to the positive electrode and the negative terminal of the battery is connected to the negative electrode.

Let us assume that both positive electrodes were initially at the same potential so that there is no tendency for electrons to move from the positive electrode to the positive terminal. As the power source must have a larger potential difference than the corresponding galvanic cell, the negative terminal of the battery has a larger potential drop. Therefore, the negative electrode is at a higher potential than the negative terminal of the battery. Electrons thus flow from the negative terminal of the battery to the negative electrode. While this is happening, the shortage of electrons at the negative terminal increases its electric potential. As the voltage of the battery is still the same, the potential at the positive terminal goes up too, now higher than the potential of the positive electrode, so electrons flow from the positive electrode to the positive terminal. You can also think of it as in order to replenish the lost electrons at the negative terminal. Thus, the net effect is that the electrons flow from the positive to the negative electrodes, meaning the positive electrode is now the anode.

Or, imagine what happens when the negative terminal and electrode are at the same potential. That means the positive terminal has to be at a higher potential than the positive electrode, so electrons flow from the positive electrode to the positive terminal etc.