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I understand how a galvanic cell works and the purpose of erverything such as the ssalt bridge and the electrodes, however I don't understand why an electrolyte is required. For example, consider this scenario.

I have a copper electrode in one beaker, connected by a wire with a voltmeter to a graphite electrode in another beaker. The copper electrode is immersed in water while the graphite electrode is immersed in lead ions. There is also a salt bridge connecting the two beakers. Now my question is, will there be a reading in the voltmeter?

I believe that there should be one since copper is more reactive than lead so copper ions will form while solid lead will form on the graphite electrode. However my school teacher said it won't work as there is no electrolyte in the first beaker. Why does it make a difference if there is an electrolyte or not?

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    $\begingroup$ Pure water without any electrolytes doesn't conduct electricity (or only does so extremely poorly), so how can you expect a current to flow? $\endgroup$ – orthocresol Jan 30 '16 at 12:15
  • $\begingroup$ @orthocresol why does the current need to flow through the water. Forgive me for sounding stupid, but doesn't the current only need to flow from the copper electrode to the lead solution? $\endgroup$ – user19300 Jan 30 '16 at 12:20
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    $\begingroup$ You can't have a current going from A to B that doesn't go back to A. This is simple physics - you need a closed circuit for any electricity to flow $\endgroup$ – orthocresol Jan 30 '16 at 12:45
  • $\begingroup$ Just because you have a potential difference (i.e., a voltage reading) does not mean you'll get much current. Without supporting electrolyte, the water will be too resistive and (as @orthocresol said) very little will happen. $\endgroup$ – Geoff Hutchison Jan 30 '16 at 13:53
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    $\begingroup$ Because if your electrons only move from point A to point B, there will very soon be a buildup of excess electrons and negative charge at B. Electrons don't want to move to a place where there's too much negative charge, so you need some way for negative charge (in a galvanic cell - this role is performed by anions in solution) to go back from point B to point A. If there are no charge carriers, then that doesn't happen. $\endgroup$ – orthocresol Jan 31 '16 at 0:29
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Two issues are at play here.

1) You will measure a potential without any supporting electrolyte. As soon as you have everything connected, a potential will develop. Because a self-respecting voltmeter doesn't require much current (ideally zero), the potential will be measured.

2) As soon as you try to pass any current, as comments have mentioned, that current has to be carried from the electrode through the solutions and the salt bridge. Without any electrolyte in the solution, the solution resistance will be huge(as Geoff mentioned) and the amount of current passing will be cell voltage/cell resistance. This will essentially limit the current passing through the solution to very low values.

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  • $\begingroup$ Thanks for your explanation, however I still don't get why the electrons need to come back to the anode to form a complete circuit if the anode electrode is continuously supplying electrons. Also as the reaction proceeds wouldn't the solution in beaker A become an electrolyte anyway as Cu ions are being formed? $\endgroup$ – user19300 Feb 1 '16 at 12:01
  • $\begingroup$ Electrons don't need to come back to the electrode, but overall electroneutrality has to be sustained. Otherwise, charge will buildup, and the current will not flow. $\endgroup$ – Burak Ulgut Feb 2 '16 at 14:11
  • $\begingroup$ @Ulgut But as I mentioned before, isn't that why we have the salt bridge. It is dipped in KNO3, and the K ions go to the cathode solution while the nitrate ions go the anode solution to prevent charge build up (this is what my teacher told me). $\endgroup$ – user19300 Feb 3 '16 at 12:41
  • $\begingroup$ But as told before, that would take a very long time, hence the very high resistance. $\endgroup$ – Burak Ulgut Feb 4 '16 at 8:11

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