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In an electrochemical cell that consists of two half-cells, there is an electrolyte and an electrode in each half-cell.

Example: $$ \begin{align} \ce{CuSO4(aq) + 2e- &-> Cu(s) + SO4^2-(aq)} \\ \ce{Zn(s) + SO4^2-(aq) &-> ZnSO4(aq) + 2e-} \end{align} $$

In the half-cell where the reduction takes place, $\ce{Cu^2+}$ gains two electrons. Here the electrode and the electrolyte serve a purpose, as it is the $\ce{Cu^2+}$ ion from $\ce{CuSO4}$ that goes through a reduction reaction.

However, in the half-cell where the oxidation takes place, it is $\ce{Zn(s)}$ that loses two electrons and forms $\ce{Zn^2+(aq)}$. Thus, the $\ce{ZnSO4}$ does not seem to play any role in the oxidation. Why is it included in the electrolyte in the zinc oxidation half-cell

The electrochemical cell with separated half-cells

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The primary role of the anode chamber electrolyte (the anolyte) in this case is to provide sufficient conductivity in that chamber.

If deionized water were used in the anode chamber, the conductivity of that liquid would be extremely low. Thus, it's important for there to be something dissolved there in order for any appreciable amount of current to pass through the overall electrochemical cell. The overall electrochemical circuit involves both electronic conduction (in the wires and electrodes) and ionic conduction (in the two electrolytes), and it will not function efficiently/effectively if any element of that circuit is too poorly conductive.

That said: It's not crucial that it be specifically $\ce{ZnSO4}$, however. $\ce{NaCl}$, $\ce{H2SO4}$, $\ce{NH4NO3}$, or a wide variety of other chemicals might work just as well (or even better!). However, a sulfate salt makes sense here because (at least):

  • It provides relatively high conductivity
  • Its salts with metal cations, including $\ce{Zn^{2+}}$, are relatively water-soluble.
  • In this specific cell configuration, using sulfate makes sense because it's the same anion as in the $\ce{CuSO4}$ used for the catholyte, and thus you don't have to worry about counter-diffusion of different anions across the salt bridge.

It further makes sense to use $\ce{ZnSO4}$ specifically as the anolyte salt because it provides the same cation as you'll be forming from your anode reaction. This will result in a more stable electrochemical cell, because having a different, potentially electrochemically-active cation in the anolyte would most likely change both the thermodynamic and the practical operating characteristics of the cell.

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  • $\begingroup$ If the anode and the cathode are connected through a metal wire, the electrons would go through the metal wire. In the anode chamber, the electrons (I assume) never interact with the electrolyte. Does then electrolyte serve a function? Or is the anode's ability to oxidize and move electrons through the metal wire dependent on the electrolyte? It seems in this case that not even the zinc ions, after oxidation, are dependent on the electrolyte, as it rather the solvent (water) that makes it able to neutralize with the ions from the salt bridge? (Not sure) $\endgroup$ – Bernard Bengtsson May 23 '18 at 17:15
  • $\begingroup$ @BernardBengtsson See my edit; does that help clarify at all? $\endgroup$ – hBy2Py May 23 '18 at 17:19
  • $\begingroup$ To clarify, as ZnSO4 has a high ionic conductivity, the negative ion in the salt bridge is better able at neutralizing Zn 2+ ions in the solution? $\endgroup$ – Bernard Bengtsson May 23 '18 at 18:12
  • $\begingroup$ @BernardBengtsson Mmmm, no, that really doesn't have anything to do with it. If the anode-side electrolyte had minimal dissolved ions in it, then it would allow very little current to pass, and the electrochemical cell would not function well. $\endgroup$ – hBy2Py May 23 '18 at 18:22

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