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I understand that an electrolyte does not conduct electrons, but rather that the positive and negative dissolved ions are attracted by electric charge to the cathode and anode where they receive and release electrons, and thereby turn into their elemental atom form and possibly form new compounds; e.g. NaCl in water would dissolve into Na+ and Cl- and then possibly after gaining/losing an electron turn into sodium hydroxide and chlorine gas. Am I correct in assuming that this means (at least for this electrolyte) that eventually there would be no more ions left to conduct the electricity? And if so would it be more of a gradual (possibly linear) diminishing capacity? Or perhaps more of a sudden stop? And finally, would this pattern apply to all electrolytes?

Thank you

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    $\begingroup$ Yes/maybe/kind-of to all your questions. See Peukert's Law and Google "C rate battery" for the rest. $\endgroup$ Commented Apr 28, 2022 at 23:43

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Note that there are two types of electrochemical cells: galvanic, also called Voltaic, and electrolytic. Galvanic/Voltaic cells derive energy from spontaneous redox reactions, while electrolytic cells involve non-spontaneous reactions and thus require an external electron source like a DC battery or an AC power source. Ref 1.

If the cell generates its own voltage (i.e, we are looking at a galvanic/Voltaic cell), when the reactants get used up, current flow through an external circuit will gradually decrease to zero, or an immeasurably small amount. This is the case in a typical 1.5V battery (or a 3 V mercury cell). But you are surely familiar with the situation of batteries getting used up, either partially or completely.

The conduction of electricity may be stopped in an electrolytic cell if you disconnect the power source from the cell completely, or if you reduce the voltage applied to the cell to such a low level that the anions and cations cannot discharge.

Perhaps you wonder if electrolyzing a solution of NaCl uses up the NaCl and could bring about a complete stop of current flow. No, not really, but things do change. If you could completely flush out the chlorine gas as soon as it is produced, the sodium ion will still remain. If Na$^+$ were reduced to metal, it would react immediately to give H2, but that doesn't even happen (although, if you used a mercury cathode, you could get sodium amalgam, which would slowly decompose to Na$^+$ and H2). The Na$^+$ just carries 4 or 5 or 6 H2O molecules close to the cathode where an electron turns a water molecule into a proton and a hydroxide ion. Eventually, all the NaCl could be converted to NaOH - actually, the conductivity would increase! I suppose you could make the current go to zero by electrolyzing all the water away, to O2 and H2, but the electrolyte would still work if you add more water.

Similar situations occur with most of the electrolytes you will encounter, like NaCl, KCl, CuSO4, Zn(NO3)2, etc.

Now it is conceivable that some electrolytes could degrade by leaving the solution, as in the case of chloride ions. Organic acids could be degraded to paraffins and CO2; nitrates could be flushed out as nitrogen oxides, noble metals (Cu, Ag, Au, Pt) could plate out, but carrying electrons to or from electrodes does not require that the ions die or become ineffective.

Ref 1. https://www.khanacademy.org/test-prep/mcat/physical-processes/intro-electrochemistry-mcat/a/electrochemistry

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An electrochemical cell requires a cathode reaction, an anode reaction, a conductive path for electrons to flow from the anode to the cathode and an ion path to neutralize charge movement from anode to cathode. IF any one of these is missing or not operative the cell will not function regardless of chemical conditions[the flashlight is turned off, batteries might be good]; If they are all operative and the cell voltage is zero the cell has reached equilibrium [the flashlight is turned on but the battery is dead, no light].

If the electrolyte is involved in the reaction such as the quantitative removal of copper from a solution the voltage must be chosen that the Copper ions remaining must be essentially zero; as copper ions are removed the current flow lowers and if no other reduction reaction is taking place reaches zero at equilibrium. If this were a copper plating bath a copper anode would be used to replace copper ions hopefully on a one to one basis. If copper ion is being removed by reduction there must be a different oxidation reaction at the anode, perhaps oxidation of water to O2 or maybe a sacrificial anode such as zinc isolated to prevent direct reaction.

The conditions are the same whether a galvanic, producing electricity, or an electrolytic, being charged by an external electrical source. The reactions are simply reversed.

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  • $\begingroup$ Why was this answer given a downvote? $\endgroup$
    – jimchmst
    Commented May 2, 2022 at 5:47

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