How do salt bridges work?

[combatwombat]

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I do not get how these work. I understand that in a battery, electrons flow from one side to the other, and that the salt bridge allows this to happen by preventing a buildup of charge on one side. Does this mean the cations in the salt bridge actually flow out of the bridge and into the half-cell that is losing electrons?

If this is true, wouldn't this mean that the salt bridge would eventually run out if there were an unlimited supply of oxidizable/reduceable substrates?

 

[plsfoldthx]

Anions flow into the side that is losing electrons because that side is left with cations. And yes, it would eventually run out just like a battery would run out.

 

[combatwombat]

nothing like a simple, straightforward answer. thx 🙂

 

[BerkReviewTeach]

I do not get how these work. I understand that in a battery, electrons flow from one side to the other, and that the salt bridge allows this to happen by preventing a buildup of charge on one side. Does this mean the cations in the salt bridge actually flow out of the bridge and into the half-cell that is losing electrons?

Without a salt bridge, a few electrons would flow from the anode into the cathode, but because of the negative charge that would build up, repulsion would get so large that no more electrons could flow. A salt bridge allows the anions in the cathode solution (the counterions to the cations in the cathode solution that are getting reduced) to migrate to the anode and thereby balance the charges.

If this is true, wouldn't this mean that the salt bridge would eventually run out if there were an unlimited supply of oxidizable/reduceable substrates?

The salt bridge is like the conducting metal wire; it's there just as a pathway for charges to flow. Salt bridges don't run out, cathodes and anodes do. Eventually you either run out of the reactant metal in the anode or the reactant cation in the cathode. Once the chemical reaction is complete, then the electrons stop flowing through the wire and the anions stop migrating through the salt bridge.

 

[plsfoldthx]

Couldnt a salt bridge run out in theory

 

[addo]

Couldnt a salt bridge run out in theory

It will eventually come to equilibrium, when there will be no net charge moving from one side to the other.

 

[BerkReviewTeach]

Couldnt a salt bridge run out in theory

The salt in the solution can run out, but a salt bridge is the path for the anions to travel. For every anion that adds on one end, another falls off the other end. Just like the wire can't run out of electrons, because they are transferring across and not being consummed, the salt bridge can't run out of anions.

 

[plsfoldthx]

The salt in the solution can run out, but a salt bridge is the path for the anions to travel. For every anion that adds on one end, another falls off the other end. Just like the wire can't run out of electrons, because they are transferring across and not being consummed, the salt bridge can't run out of anions.

Gotcha. it was in EK or Kaplan don't remember which they explained that the ions in the salt bridge actually leak into solution.

 

[loveoforganic]

They do, but they're replenished from the ions in solution, is my understanding.

 

[BerkReviewTeach]

They do, but they're replenished from the ions in solution, is my understanding.

Exactly! As one solution becomes richer in anions and the other becomes poorer, the gradient in the salt bridge changes, but the same number of anions will remain in the bridge. The only way this doesn't hold is if the salt in one of the solutions has a drastically different solubility than the salt in the other solution.

 

[loveoforganic]

But that would kill the ability of the salt bridge to buffer the charge buildup, wouldn't it? Hm, well maybe not, the ions still end up sitting at the bottom of that solution I guess. I feel like there would still be some functional prohibition from using a precipitating salt though.

 

[BerkReviewTeach]

But that would kill the ability of the salt bridge to buffer the charge buildup, wouldn't it? Hm, well maybe not, the ions still end up sitting at the bottom of that solution I guess. I feel like there would still be some functional prohibition from using a precipitating salt though.

The risk is that if both solutions had highly insoluble salts, then the ion concentrations would be so low that the reaction would be limited and the solutions would in fact leech the ions from the salt bridge. The choice of anion is critical in electrochemical cells, because you need them to be soluble (which would favor nitrates) but unreactive (which is not always the case with nitrates, as they can on occassion serve as oxidizing agents). My hope and thought is that these restrictions are beyond the scope of the typical test writer's questions.

As long as the salts are reasonable soluble, then the reduction half-reaction is not hindered by low cation concentration and the salt bridge is not being leeched on one side to the point that the migration rates of the anions is restrictive.

 

[loveoforganic]

Thanks, makes sense!

 

[SKation]

Gotcha. it was in EK or Kaplan don't remember which they explained that the ions in the salt bridge actually leak into solution.

Hey!

I was just looking over this in EK im not exactly sure what they mean by the reasoning that "there is come leakage of ions across the liquid junction , which causes the battery to lose its chemical potential over time"

How does this happen? I was confused on when the ions are leaking how do they cause the potential to be lost?

Thank You!