Salt Bridge Purpose in Galvanic Cells

Elisa_Rojas Palato

Hi! After searching and googling, I couldn't find a good answer for why the salt bridge is important in the Galvanic Cells. From what I have found, all answers mention salt bridge prevent the unbalanced charge difference caused by electrons (e) moving from the anode to the cathode where they interact with the ion that has a higher reduction potential. However, I don't understand how this charge difference forms in the first place. Wouldn't it be the case that as e-s move from the anode to the cathode, they reduce the cation (e.g., Cu2+ + 2e-s → Cu (s) ) such that there aren't flowing e's in the solution because they have all been hypothetically used to reduce the cation? So I don't see how the charge difference develops and becomes a problem. And how the common anion (e.g. sulfate) can alleviate it. Does the anion induce the oxidation of the metals on the anode? Please feel free to provide an example (e.g. Cu2+ and ZN2+) to illustrate this. Thank you in advance to anyone/people who answer this question!!!!!

Aleksandra_Katiliu

Hey Elisa, great question! And I just saw this be asked on one of my practice MCATs so it must be important to know. Let's use your example of a galvanic cell with Cu and Zn.

Cu has a stronger reduction potential, meaning it will pull/attract electrons and is considered the cathode. (RED CAT). Zn has a weaker reduction potential, meaning it will actually lose electrons (oxidize) to Cu and act as the anode (An Ox). So we have established that electrons are flowing across the wire from Zn to Cu. Electrons release energy as they approach higher reduction potentials. Think of a (-) charge and a (+) charge approaching one another, they lose potential energy. This is how batteries release energy,

As this happens, negative charge begins to collect on the right, with Cu. As this charge grows, electrons will become repelled. This impedes the flow of electrons. To fix this, the stored ions in the salt bridge, in this case NO3- and K+ counteract the stored charges to allow electrons to continue to flow according to reduction potential. K+ enters the Cu bath to counteract the growing negative charge. NO3- enters the Zn bath to counteract the growing positive charge due to loss of electrons. Anions travel toward the Anode and Cations travel toward the Cathode. This works because reduction potentials are an intrinsic characteristic. They are not affected by the presence of charges, only the presence of particular species. Even when the battery is drained, Cu will still have a higher reduction potential because that is simply a property of Cu. This is also why a galvanic property will never run in reverse. The reduction potentials will never move, so electrons only have one possible path to travel.

A salt bridge can be a tube or filter paper soaked in salt. In a AA battery the salt bridge is a porous barrier. Again, this is key to allow the ions to counteract the buildup of charge due to movement of electrons. Also like typical AA batteries, once they are drained they cannot be recharged. The electrons can only move one way.(Unless specifically marked rechargable, in which case they are electrolytic cells, not galvanic).

I hope this helps!😊