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If I understand correctly, the salt bridge in a battery is used to move positive ions towards the side that is becoming more negative, and negative ions towards the side that's becoming positively charged, to prevent the reaction from quickly reaching equilibrium. The ions in the salt bridge never leave the salt bridge.

But if we removed the salt bridge, wouldn't the forces between the $\ce{SO_4^{2-}}$ ions or $\ce{Zn^{2+}}$ ions cause the reaction to go in reverse and cause them to reattach to the electrodes?

Another.Chemist
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Abcd
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  • Why is this up for a bounty? Related: https://chemistry.stackexchange.com/questions/12332/is-there-always-a-need-for-a-salt-bridge-to-let-a-galvanic-cell-work-continuousl – Buck Thorn Dec 13 '18 at 14:17

2 Answers2

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The ions in the salt bridge never leave the salt bridge.

That's not true. The ions literally have to move to balance the charge of the gain / loss of the electrons at the electrodes. (You even allude to this in your question.) One of the ways that a battery can run out is that the bridge is used up. You could views this by using a deeply colored ions (e.g. MnO4-) to see the migration.

While the salt bridge is in place, there is no build up of charge. Electrons moving through the wires are offset by the ions moving through the salt bridge and into the respective half-cell. Thus, if you remove the salt bridge, there is no net charge that will cause anything to reverse; there is no 'force' that will cause ions to revert.

If you want to reverse the reaction (and charge the battery), you have to force electricity back in the other direction, using more voltage than what you got out of the battery. This doesn't work for all batteries. (I.e. this is why only some batteries are rechargeable). As you force electricity backwards through a battery, any movement of ions from the salt bridge means that the half-cell might now be contaminated with chemicals other than the ones you want in your half-reaction (unless you designed your salt bridge just right). Thus, the "right" ions may not go back to being uncharged (or vice versa).

Van
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  • How the bridge used up? I think the solution ions moving across the bridge. – Adnan AL-Amleh Dec 14 '18 at 05:54
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    "used up" was a bit inexact. Salt bridges have to be designed in such a way as the ions don't interfere with the half-cell reactions. If the ions in salt bridge are depleted (all migrated into the half-cells), ions from the reactions can migrate between the half-cells. If these ions interfere with the reaction in the new container, voltage will change. Most intro chemistry texts just barely go over salt bridges and their construction, b/c they are more concerned with the basic idea of batteries. But, in the construction of long lasting batteries, salt bridges have to be carefully crafted. – Van Dec 14 '18 at 14:39
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I think you are misunderstanding the purpose of the salt bride in the voltaic cell. It acts to prevent a charge imbalance as the electrons are sucked over to the cathode. The negative ions flow through the electrolyte to complete the circuit. Just like cutting a wire to break the circuit, removing the salt bride would stop the flow of electrons. The battery wouldn't recharge because the ions would stay in solution and the solid metals would stay as they are. Nowhere for the electrons to go without the connection.

124c41
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