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Water competes with any ions in solution for both oxidation and reduction. There are $1000\ \mathrm{g\ l^{-1}}/18\ \mathrm{g\ mol^{-1}}\approx55.56\ \mathrm{mol\ l^{-1}}$. That is truly huge in comparison to any amount of salt we can dissolve. Why doesn't water dominate any electrolysis reaction since even if a small percentage of the collisions of water can accomplish the reaction? The sheer number of collisions on its side will ensure it wins. Also, if hydrogen ions and hydroxide ions alone participated, a concentration of $10^{-7}$ means that it's several orders of magnitude away from the dissolved salt's ion concentration, which means that water can never dominate. Yet, we use a comparison of the thermodynamic feasibility in the form of reduction potentials to decide which competing ion gives the product. How is it that the concentration effects become comparable enough to allow this in most cases?

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    $\begingroup$ Do you know Nernst equation ? $\endgroup$ – Maurice Jul 11 '20 at 19:57
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    $\begingroup$ Two down votes on a very reasonable valid question and surely enough the down voters have no clue of the answer. $\endgroup$ – M. Farooq Jul 12 '20 at 5:27
  • $\begingroup$ @M.farooq just a theoretical question. $\endgroup$ – Jack Rod Jul 12 '20 at 12:19
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It is good question given the fact that this idea is not commonly discussed. The main source of your confusion is the following idea

Why doesn't water dominate any electrolysis reaction since even if a small percentage of the collisions of water can accomplish the reaction? The sheer number of collisions on its side will ensure it wins.

Unlike chemical reactions, electrode reaction do not solely require collisions. It requires an additional process: electron transfer. No matter how much collisions are occurring, the electrode is unable to transfer or withdraw electron from water in high salt concentration. Thermodynamics tells us (from the Nernst equation) how easy it would be to remove or add an electron to a water molecule at a given salt concentration. It is silent as to why this happens and what is the mechanism.

Electrode processes are very complicated. Marcus got a Nobel Prize on electron transfer.

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