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I am reading about electrolysis and I have trouble to understand why an electrolyte must be added in order for electrolysis to occur. I have read this question but still can't get it. The definition of electrolyte is a substance that when dissolved produces an electrically conducting solution.

Electrolytes in electrolysis

Why we need an electrolyte in order for electrolysis to occur? In electrolysis of water the water is consumed not the electrolyte. So why we need the electrolyte? Couldn't the reaction happen without the electrolyte?

After reading the comments I have modified the question. As I said before I can't understand the purpose of the electrolyte. For example in the electrolysis of water with sulfuric acid why we add it? Water molecules are in contact with both anode and cathode. So why we need to add the electrolyte?

What happens at molecular level when the electrolyte is added? How the ions move in the solution (if they move at all)?

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  • $\begingroup$ You are confusing between electrolysis and electrochemical cell. Both are opposite processes. The bulb glows when electrodes are inserted in an electrochemical cell and not in a electrolysis vessel. $\endgroup$ May 13 at 10:15
  • $\begingroup$ For reference read this: Electrolytic Cells and Electrolysis chem.libretexts.org/@go/page/266 (accessed May 13, 2021). $\endgroup$ May 13 at 10:16
  • $\begingroup$ I don't understand if you need the remark in the comments above or this one will suffices: in order for the electrolysis to occur, the current entering one side of the cell need to escape from the other one. This requires electrical conductivity, and this is why you need an electrolyte in solution (as per the same definition that you have mentioned). $\endgroup$
    – Alchimista
    May 13 at 10:34
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    $\begingroup$ The answers are good, but maybe this is giving you pause: say you apply 9 V to a pair of platinum electrodes in pure water. The 9 V will be divided into three portions: 1) the potential required to oxidize whatever at the anode, 2) the potential required to reduce whatever at the cathode, and 3) the potential drop across the water. Pure water has high resistivity, so, assuming typical cell electrode separation and electrode areas, the water’s resistance is very high. Hence almost all the applied 9V is uselessly dropped across the water and resultant current flow is very small. Small current $\endgroup$
    – Ed V
    May 13 at 21:48
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    $\begingroup$ means very little useful electrolysis happens. Adding an electrochemically inactive electrolyte greatly increases the solution conductivity, which is why it is called an electrolyte, so the voltage drop across the water is greatly reduced and the oxidation at the anode and reduction at the cathode are no longer strangled by the water resistance. The resistance of the pure water is just an obstruction that is greatly mitigated by adding the electrolyte, even if the electrolyte does no chemistry per se. $\endgroup$
    – Ed V
    May 13 at 21:51
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Sometimes the electrolyte directly takes part in the chemistry, by being oxidized or reduced at an electrode. In those cases, it is easy to understand why the concentration would have an influence on the kinetics of the redox reactions (and with that, on the observed electrical current).

For water electrolysis, the "active" electrolytes are hydrogen ions and hydroxide ions. If you change the pH, you change their concentration, and would expect to affect the rate of reaction. You are asking about other electrolytes such as sodium sulfate, which don't participate in electron transfer.

To explain their role, take a look at this animated GIF. It shows electrolysis of water in the presence of universal indicator and sodium sulfate:

enter image description here Source: CarolinaBiological

With the 9 volts applied to the graphite electrodes, you can see that the electrolysis causes local changes in pH (because one half reaction consumes hydrogen ions and the other hydroxide ions). It takes quite a bit of effort to mix the solution (with the plastic tube) to get the pH homogeneous again. This shows you that the diffusion of hydrogen and hydroxide ions takes a while. In the absence of other electrolytes, this would lead to local charge accumulation as well, stopping the reaction. With the electrolyte (at much higher concentration than the hydrogen and hydroxide ion), we get a pH gradient only.

This pH gradient also slows things down, and increases the voltage that needs to be applied (cell potential is 1.3 V at constant pH, but according to Nernst law, it would increase because of the pH differences). Because we are applying 9 volts, this still works.

What an electrolyte “really” does in electrolysis?

By moving in the appropriate directions, the ions of the "passive" electrolyte prevent large accumulations of charge, and allow the electrolysis to proceed at higher rates than without their presence. If you decrease the gap between electrodes down to sub-Debye length, they are no longer neccessary.

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An electrolyte (in your case water) keeps charge carriers (in your case $\ce{Na+}$ and $\ce{Cl-}$) separate from each other and allows them to travel to the electrodes. There is no constraint that this has to be a liquid phase, or a solid (e.g., a polymer).

I speculate when you say

«In electrolysis of water the water is consumed not the electrolyte.»

you accidentally skip the notion of the electrochemical window equally relevant e.g., to cyclic voltammetry. This means, if the electrode potential applied is too high, or too low, the same compound acting as electrolyte simultaneously reacts and may be oxidized / reduced.

Maybe the new form still is capable to be an electrolyte. Maybe the e.g., the electrochemical reduction of the electrolyte triggers the formation of bonds and a network representing an obstacle for the charge carriers to move; or products of reaction of the electrolyte leave the condensed phase (e.g., as hydrogen gas), which gradually depletes the electrolyte's ability about the charge carriers, too.

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  • $\begingroup$ My main problem is that I can't get why in first place water molecules can't be oxidised/reduced without the electrolyte If they are in contact with both anode and cathode. $\endgroup$
    – ado sar
    May 13 at 17:05
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    $\begingroup$ @ado sar They can be, but it's really slow. $\endgroup$ May 13 at 18:02

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