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I know that as electrons move from anode to cathode through wire in a galvanic cell, there remains in the anode +ions (Zn for example) while - ions are created in the cathode. Afterwards a salt bridge is required to maintain electrical neutrality by moving opposite ions to both electrodes.

My question is why electrical neutrality is necessary to maintain the current flow? Is it because, the abilities of the elements in the electrodes to attract electrons relative to each other change as the starting charges of the elements(as they gain/lose electrons) in the electrodes change ? Does this affect the voltage ?

Secondly, what is meant exactly by maintaining electrical neutrality? Does maintaining electrical neutrality mean that both electrodes have to be perfectly neutral with no charges ?

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  • \$\begingroup\$ Yes, as you move charged particles from one body to another, a voltage develops between the two bodies. The more charges you move, the larger the voltage, and the more force required to move additional charged particles. \$\endgroup\$
    – mkeith
    Aug 13, 2017 at 19:53
  • \$\begingroup\$ I guess one more thing. Charges generally do not build up in metal, because it is a good conductor (high charge mobility). So the charges will be in the electrolyte solution or near the interface of the solution and the electrode. \$\endgroup\$
    – mkeith
    Aug 13, 2017 at 19:55

2 Answers 2

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A galvanic cell is driven by two reactions that occur, one at each of the electrodes. Each reaction creates a potential difference between the electrode and the surrounding electrolyte. The salt bridge functions as a conductor that keeps the electrolyte at both electrodes at the same voltage, which then forces the entire potential difference to appear across the external load.

Without the bridge, there would be no voltage across the load, and the entire potential difference would be between the two isolated batches of electrolyte. However, the bridge must not conduct electrons, because that would short out the cell internally, and the reactions would proceed without limit until the reactants were consumed.

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  • \$\begingroup\$ And how does this answer my question, sir ?? \$\endgroup\$
    – MECHA
    Aug 13, 2017 at 15:25
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    \$\begingroup\$ More importantly, how does it fail to answer your question? If it isn't sufficient, you'll have to provide additional details about what you don't understand. \$\endgroup\$
    – Dave Tweed
    Aug 13, 2017 at 15:32
  • \$\begingroup\$ This answer doesn't mention the effects and importance of electro neutrality between the electrodes of a cell, which is the main question \$\endgroup\$
    – MECHA
    Aug 13, 2017 at 15:37
  • \$\begingroup\$ "electro neutrality" isn't really a standard term. I'm​ assuming that it means essentially the same thing as "the same voltage". \$\endgroup\$
    – Dave Tweed
    Aug 13, 2017 at 15:40
  • \$\begingroup\$ No i wasn't talking about voltage. I was talking about the charges that build up at both electrodes due to the transfer of electrons (the redox reaction). \$\endgroup\$
    – MECHA
    Aug 13, 2017 at 15:45
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Consider a battery electrode as a system. That system cannot create charge (due to conservation of charge). Also, objects generally cannot acquire a net charge. Sure, they can acquire a small net charge due to static electricity or what have you. But generally, in circuit analysis, objects are not allowed to acquire a net charge. Charges can separate and move around, but they cannot be created. If electrons leave an atom, the atom becomes a positively charged cation. Thus, charge is conserved.

So, back to the electrode system. In order to respect conservation of charge, and avoid acquiring net charge, every negative charge that leaves the electrode through the wire must be balanced out by a positive charge leaving the electrode into the electrolyte solution. Likewise, every electron that enters the other electrode must be balanced out by a positive charge entering the electrode through the electrolyte solution.

The purpose of the salt bridge is to allow the positive charges to migrate from one electrolyte bath to the other. There must be a positive ion circuit between the two battery terminals in order for electrons to flow through the external circuit.

OK, an alternate explanation. Consider two systems. Each one is an electrolyte solution with a partially submerged electrode. There is no salt bridge. We add a wire connecting the dry portions of the electrodes (which now makes our two systems into a single system). Let us say that for a brief moment, a tiny current flows in the wire. This current actually imparts a very slight net charge to the electrolyte solutions (equal and opposite). This net charge changes the favorability of the electrochemical reaction which produces current in the battery, and shuts it down. When you add a salt bridge, it allows cations to escape the more positively charged solution, and thus allows current to flow through the wire on a more sustainable basis.

Net charge is self-arresting, because each charge you transfer makes it that much harder to transfer additional charges (because like charges repel each other).

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  • \$\begingroup\$ I know that already. I am asking about the scenario where ions don't get to move to the other electrode to cancel the transferred electrons. It is obvious that in this case the current will stop; I am asking why ?? Please re-read my 1st question again (in the body). Thanks:) \$\endgroup\$
    – MECHA
    Aug 13, 2017 at 19:06
  • \$\begingroup\$ I already answered. Current flow without a pathway for cations would lead to net charge accumulation. \$\endgroup\$
    – mkeith
    Aug 13, 2017 at 19:26
  • \$\begingroup\$ I added an alternate explanation. \$\endgroup\$
    – mkeith
    Aug 13, 2017 at 19:57
  • \$\begingroup\$ "This net charge changes the favorability of the electrochemical reaction which produces current in the battery, and shuts it down." Does this mean that the second portion of my first question, "the abilities of the elements in the electrodes to attract electrons relative to each other change as the starting charges of the elements(as they gain/lose electrons) in the electrodes change", is true ?? \$\endgroup\$
    – MECHA
    Aug 13, 2017 at 21:11
  • \$\begingroup\$ I think so, but I am also not a chemist. The basic ideas, that similar charges repel each other, and that charge is conserved, those are basic science principles which you may rely upon with confidence (except in quantum mechanics, but that is not relevant here). The details of how it occurs at the interface of the electrode and electrolyte I am not too sure about. \$\endgroup\$
    – mkeith
    Aug 13, 2017 at 21:33

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