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In the electrolysis of Copper (II) Sulfate solution with inert graphite electrodes, the electrons from the power pack will first go to cathode. I understand what happens at the cathode - the copper ions gets reduced and solidifies.

However, I'm unsure exactly what happens at the anode, specifically with it being made of graphite - as in the redox half equation. I know oxygen forms, but is it a hydroxide ion that gets oxidised, or is it water? Why?

The hydrogen cations then join with the sulfate to make sulfuric acid. From my understanding the sulfate cannot be oxidised because it cannot exist as a neutral atom.

Would anything change if it were a copper electrodes?

Thank you so much.

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  • $\begingroup$ For more than what is in the Maurice's answer, you may try to search copper sulfate electrolysis, with and without site:stackexchange.com OR site:libretext.org. You should also include in questions a short conclusion from prior info searching, so others do not write things you already know, or should know if searched about it. If nothing was found, write what you have been searching for. $\endgroup$
    – Poutnik
    May 22, 2023 at 8:07
  • $\begingroup$ Sulfate can be oxidized on anodes at special conditions. Search for peroxysulfuric acid and peroxydisulfuric acid, resp. peroxysulfates and peroxydisulfates $\endgroup$
    – Poutnik
    May 22, 2023 at 13:25

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In the electrolysis of $\ce{CuSO4}$, the following half-equation occurs at the graphite anode : $$\ce{2 H2O -> O2 + 4 H+ + 4 e-}$$ This production of electrons is due to $\ce{H2O}$ molecules and not to $\ce{OH-}$ ions, because $\ce{H2O}$ is more than $\pu{10^8}$ more frequent in solution than $\ce{OH-}$. If the anode is made of copper, the production of electrons will be due to another half-equation, namely $$\ce{Cu -> Cu^{2+} + 2 e-}$$

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  • $\begingroup$ CuSO4 solution has reportedly pH typically 3-4, so 10^11 - 10^12. SO4^2- does not get oxidized either (but at high current density), as it has much higher redox potential than water oxidation. $\endgroup$
    – Poutnik
    May 22, 2023 at 8:13

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