Electrical neutrality between electrodes of galvanic cell: why is it necessary for current flow?

Question

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 ?

Answer 1

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.

Answer 2

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).