Electrochemical cell salt bridges

[TokenMonkey]

My final chemistry exam is in about a week, and I still don't know EXACTLY how salt bridges of electrochemical cells work. None of my three textbooks explain it in-depth. I know the salt bridge's functions, but what exactly happens?

 

For example, let's say we have a Cu anode and Ag cathode, in their respective half-cells. The electrolytes are Cu(NO3)2 and AgNO3, respectively, in the half cells. Let's say there's MgSO4 in the salt bridge.

 

I understand that without the salt bridge, the half cells would become charged and the electrons wouldn't flow. That's all fine. What I don't get is which ions go from where to where to maintain the overall neutrality of the cell.

[jdurg]

Well, in your example as the electrode is run, silver ions come out of solution and from silver metal, and copper metal comes out of solution and forms copper ions. Therefore, on the silver side of the cell there will be a shortage of positive ions. On the copper side, there's be a shortage of negative ions. The salt bridge will then supply the proper ions to each side in order to keep the charges balanced in the cells. (At least this is how I believe it works. Any experts can feel free to correct me).

[TokenMonkey]

That's what I was thinking but then isn't it a waste if the ions in the salt bridge run out before the anode is eaten away?

 

The other thing I was thinking is this: Why do you even need a solution in the salt bridge. Here's my reasoning, and I'd love it if someone could explain why it's wrong:

 

At the anode, the following reaction occurs: Cu -> Cu2+ + 2e-. The electrons flow to the cathode through the wire. That's all fine. At the cathode: Ag+ + e- -> Ag. That basically means that the anode has an overall positive charge (excess of Cu2+ ions) and the cathode becomes negatively charged (excess of NO3- ions). Couldn't the Cu2+ ions just flow through the U-tube to neutralise the negative charge, and NO3- goes to neutralise the positive charge. But this then also means that the electrons themselves could flow through the water and not through the wire. So obviously the solution in the salt bridge separates the two half-cells.

 

But why? How?

[TokenMonkey]

Aaarrrgh! Please! Someone's gotta know about this stuff!