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At the cathode, the one that is discharged is the less reactive one (compared to hydrogen) so why at the anode is the one that’s discharged more reactive (less stable) ?

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  • $\begingroup$ What is your criterion for less reactive - the reduction potential (or qualitatively - electrochemical series)? There is an oxidation process at the anode, so the half reactions and the order of activity are switched compared to the reduction half reactions. $\endgroup$ – Karsten Theis Apr 13 at 14:35
  • $\begingroup$ The reaction potential. $\endgroup$ – PERCIVAL Apr 13 at 14:45
  • $\begingroup$ The reactivity is mutually reversed on an anode and a cathode. On a cathode, preferred reduction is for the redox system with the highest potential. On an anode, preferred oxidation is for the redox system with the lowest potential. $\endgroup$ – Poutnik Apr 13 at 15:29
  • $\begingroup$ Thanks, but why is this the case? $\endgroup$ – PERCIVAL Apr 13 at 18:44
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The redox potential is a measure of tendency of redox system to exist rather in oxidized form ( negative potential values) or reduced form ( positive values ).

Therefore the higher the standard redox potential is, the stronger oxidation effect and weaker reduction effect have the respective oxidized/ reduced forms of the redox system.

And vice versa.

Particularly, on the opposite sites of the standard redox potential table are $$\begin{align}\ce{ Li &<=>> Li+ + e- \\ F2 + 2e- &<=>> 2 F- \\} \end{align}$$

The former is very strong reducting agent, while latter is very strong oxidating agent.

I will illustrate it on anode case, with the cathode case it is the same, but with the reversed potential order.

When the anode potential is externally forced to raise up, if first start to oxidate the reducted form of the redox system with lowest redox potential.

If the current is already saturated by diffusion rate of redox reagents, futher increase of potential does not increase the current until it reaches potential region of next easiest to oxidize redox system.

If the redox systems have similar values, the next system start to react on the electrode before the system

The result is, the curve current versus anode potential has a typical look of the rounded stepwise voltammetry curve.

See also Voltammetry, Polarography and example of the cathodic polarographic curve.

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