In my textbook it is given that for electrolysis of dilute sulfuric acid at anode following reactions can occur:

At moderate concentrations

$\ce{2H2O -> O2 + H+ +4 e-}$

And for high concentrations

$\ce{2SO4- -> S2O8^2- +2 e-}$

SRP value for first reaction is less than second and hence the first reaction should take place. But why does that not hold for high concentrations?

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    $\begingroup$ I was taught that at high concentrations, there are too many sulphate ions surrounding the anode. Hence, they get oxidised even if their SRP values disagree. But, I am not sure if this is correct. $\endgroup$ – Gaurang Tandon Mar 8 '18 at 15:58
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    $\begingroup$ Why anyone would think that "standard" value has importance in highly non-standard environment? $\endgroup$ – Mithoron Mar 8 '18 at 15:59
  • $\begingroup$ Do you mean it becomes kinetically preffered? and we have something like overpotential? $\endgroup$ – Karmanya GB Mar 8 '18 at 16:00
  • $\begingroup$ There's not much water around in conc Sulfuric acid $\endgroup$ – Waylander Mar 8 '18 at 16:01
  • $\begingroup$ Related: chemistry.stackexchange.com/q/75108/5026 $\endgroup$ – Gaurang Tandon Mar 8 '18 at 16:07

This is because in concentrated sulfuric acid solution equilibrium condition will favour more water molecules than oxidation of water into protons. So there will be an over potential required (to go against the equilibrium) , that is extra potential beyond the theoretical reduction potential derived from thermodynamics to complete the reaction. So the total potential required will be theoretical Plus overpotential. In case of oxidation of sulphate reduction potential will be much less that for water ,thus oxidation of sulphate happens.


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