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This is a very basic question about which side reactions can take place if the electrodes are not shielded from each other during electrolysis of concentrated $\ce{NaCl}$.

So, this is the desired net reaction for this process: $$2\ce{NaCl(l) + 2H2O -> 2NaOH(l) + H2(g) + Cl2(g)}$$

At the anode, oxidation of chloride ions to chlorine gas: $$\ce{2Cl- -> Cl2 + 2e-}$$ At the cathode, reduction of water to form hydrogen gas and hydroxide ions: $$\ce{2H2O + 2e- -> H2 + 2OH-}$$

The section 'The need to keep all the products separate' on this page gives $ 2$ by-products that are formed when $\ce{ H2}$ and $\ce{Cl2}$ stay in the system: $$\ce{H2 + Cl2 -> 2HCl}$$ $$\ce{Cl2 + 2NaOH ->NaOCl + NaCl + H2}$$

I wonder if $ 2 $ additional side reactions can take place, simply with the gases diffusing to the other electrode, resulting in the reverse redox reactions:

At the cathode, reduction of chlorine gas to chloride ions: $$\ce{Cl2 + 2e- -> 2Cl-}$$ At the anode, oxidation of hydrogen gas to form hydronium ions: $$\ce{H2 + 2H2O -> 2H3O+ + 2e-}$$

  1. Aren't these reverse (and spontaneous, based on the standard potential of these reactions) redox reactions also likely to happen when the electrodes aren't shielded?
  2. Which one of all these side reactions (or maybe others?) are more likely to occur in this scenario? I think $\ce{ H2 + Cl2 -> 2HCl}$ as both are in gas form and are likely to encounter each other

Thanks a lot.

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  • $\begingroup$ The mixture of gas H2 + Cl2 is stable in the dark at room temperature. The reaction starts if an energetic enough photon enters the mixture : blue, violet or UV. So the main side reaction is the reaction between NaOH and Cl2 producing NaClO. Now if the temperature is nearly at boiling point, NaClO disproportionates into sodium chlorate NaClO3 and NaCl. That's all. – $\endgroup$
    – Maurice
    Apr 17 '20 at 10:13
  • $\begingroup$ @Maurice: thank you for the insight. Can you clarify why the reverse reactions on the opposite electrodes also won't happen? $\endgroup$
    – colicab
    Apr 17 '20 at 10:17
  • $\begingroup$ Which reverse reactions ? $\endgroup$
    – Maurice
    Apr 17 '20 at 11:17
  • $\begingroup$ At the cathode, reduction of chlorine gas to chloride ions: Cl2 + 2e- -> 2Cl- At the anode, oxidation of hydrogen gas to form hydronium ions: H2 + 2H2O -> 2H3O+ + $\endgroup$
    – colicab
    Apr 17 '20 at 12:26
  • $\begingroup$ Because of the potential applied to the system from the outside. If you stop applying the potential, then yes (provided the right electrodes and other setup), the reaction will run in the reverse direction, and so will the current. That's how rechargeable batteries work. $\endgroup$ Apr 18 '20 at 12:21

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