In electrolysis of aqueous solutions, I can loosely explain what happens at the cathode in terms of reactivity - any metal more reactive than hydrogen stays in solution, effectively because they're more attracted to water molecules. I could also explain this in terms of redox potential if I had to (I'm teaching this at GCSE level right now - to 16-year-olds - and we don't bring in redox potential or the electrochemical series until later).

But what happens at the anode is currently puzzling to me. We're told that the only anions which get oxidised in preference to hydroxide are the halides, but I can't explain that in terms of reactivity, and so far I'm not sure how to explain it in terms of standard reduction potential, either.

So - why the halides? And are they really the only anions oxidised in preference to hydroxide?

  • $\begingroup$ chemistry.stackexchange.com/questions/15435/… $\endgroup$
    – Mithoron
    Commented Dec 10, 2015 at 11:14
  • $\begingroup$ Thanks @Mithoron, I did see that, but if it answers my question, I'm not entirely following it. The fact that (e.g.) chlorine is oxidised comes down to the overpotential of oxygen, right? How do we generalise this observation to account for the fact that halides are the only ions oxidised in preference to oxygen (if indeed they are, and my GCSE course isn't lying to me)? $\endgroup$
    – Oolong
    Commented Dec 10, 2015 at 11:59
  • $\begingroup$ I think the statement is too general. Certainly because of the overwhelming concentration of water the redox reactions of water must be considered in any aqueous electrochemical cell. $\endgroup$
    – MaxW
    Commented Dec 10, 2015 at 23:12
  • $\begingroup$ I did wonder about that. But again I lack the in-depth chemistry knowledge to assess how much too general... $\endgroup$
    – Oolong
    Commented Dec 13, 2015 at 19:42


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