[retep128]

Hi all. A little context: high school redox chemistry; in particular primary galvanic cells.

I'm having a little trouble understanding why the typical galvanic cell you see in textbook can't be recharged. Textbooks often say primary cells can't be recharged because the products of the discharge reaction (spontaneous) don't remain in contact with the electrodes for the reverse reaction to occur.

If I think about the basic Daniel cell (Zn|Zn2+||Cu2+|Cu), the products of the discharge reaction should be Cu (s) and Zn2+(aq). Why couldn't I place a power source so that copper is connected to the positive terminal and Zn is connected to the negative terminal to restore/recharge the cell?

If one argues that the Zn2+ isn't in contact with the electrode, could I not argue the same for the forward reaction (i.e. if copper ions somehow find a way to make contact with the copper electrode during discharge, why can't zinc ions somehow find a way to find a way to make contact with the zinc electrode in the same fashion?)

If one argues that water would interfere, could you please explain why? Let's assume standard conditions. At the cathode of the cell, water looks to be a weaker oxidant than Zn2+ (so Zn2+ would reduce in preference to water). In a similar fashion, Cu is a stronger reductant than water so Cu would oxidise in preference at the anode.

If anyone could enlighten me on my mistake, I'd appreciate it!

[Borek]

Textbooks often say primary cells can't be recharged because the products of the discharge reaction (spontaneous) don't remain in contact with the electrodes for the reverse reaction to occur.

As written this is plain wrong, as most too far going generalizations are.

Unless I am completely wrong recharging Daniel cell won't work as instead of reducing Zn²⁺ to Zn you will reduce H⁺ to H₂. Changing pH to higher won't help, as it will also remove Zn²⁺ (precipitation, complexation, conversion to zincate anion) shifting the reduction to Zn potential as well.

Think about it this way: some galvanic cells can't be recharged for whatever reason (typically thermodynamics or side reactions related, but it is not like there is a single, universal reason). Instead of calling them "unreachargeable cells" we call them primary cells.