Electrolysis when E_cell = 0

I'm a bit confused by the electroplating of Zinc onto Copper, with Zinc (anode) and Copper (cathode) electrodes in a Zn(NO3)2 solution.

The reduction reaction is: Zn2+ + 2e- ---> Zn (s) (E_rxn = -0.76 V)

The oxidation reaction is: Zn(s) ---> Zn2+ + 2e- (E_rxn = 0.76 V)

Thus, in the overall reaction, all elements "cancel out" and E_cell = 0 V. Does this mean the reaction occurs spontaneously (although electrolysis is generally non-spontaneous)? Also, what is the overall cell reaction? I'm confused about this and can't seem to find any clarifying information online.

Thank you!

• You don't mention which electrode is the positive pole and which is the negative. Both choices are possible. But the chemistry is not the same. Anyway it is not a good idea to electrolyze a zinc nitrate solution. Zinc sulfate would be a better choice. Electrolyzing zinc nitrate produces side reactions due to the nitrate ion, with the reduction of nitrate ion to $\ce{NO2^-}$ ions and even to $\ce{N2O}$. May 28, 2022 at 16:15
• Sorry about that, although I believe it could be inferred from my equations! I just updated my question: zinc is the anode (positively charged) and copper the cathode (negative charge). Ok, I will think about using zinc sulfate instead.
– cim
May 28, 2022 at 18:05

You are confused because you are mixing two different concepts. Half-cells belong to the field of potentiometry when no electric current is passing through the circuit. So that calculations you do above, do not apply in electroplating.

Electroplating is just the opposite and it belongs to the field called voltammetry. As the name indicates, you play with "volts" and "ammetry" i.e., the current. One variable is fixed and the other is varied. You can have constant current electrolysis or a constant potential electrolysis. Both cannot be arbitrarily controlled. Herein are forcing a non-spontaneous or even a spontaneous process to proceed by providing electrons from an external source like a battery. The moment electric current passes through a solution, electrolysis must occur. Nature does not provide a choice to matter in that case. Either the solvent like water will decompose, or the cation or the anions must undergo a redox process and if there is no choice, the electrode material itself has to participate in a redox process. The key point is that electroplating needs a threshold voltage.

• Thank you! How would the threshold voltage you mention be calculated?
– cim
May 28, 2022 at 16:10
• @cim, This information can come from a voltammetric experiment. You scan the voltage in a given solution and observe current changes. The moment redox process starts, current rises rapidly. May 28, 2022 at 16:16