[nj_bartel]

Ok, I think I understand the basic idea of the salt bridge - it's a U filled with a gelatin that contains cations and anions.  It's immersed in the electrolytic solutions with the electrodes.  As the redox reaction runs, cations in the salt bridge migrate toward the cathode, anions migrate toward the anode, but they don't leave the salt bridge.  This is because the electrostatic force between like charges is so large.

Assuming this is correct, what would happen if you were to remove the conducting wire, then pull the salt bridge out of the solutions?  I'd assume nothing spectacular, but that doesn't make sense based on my understanding.

Any clarification would be great, thanks.

[nj_bartel]

Second question:  Just read this - "In the Cu/Ag cell, the Cu electrode become electron rich (negative) due to the tendency of Cu to be oxidized to Cu²⁺, whereas the the Ag electrode becomes electron deficient (positive) because of the tendence for Ag⁺ to be reduced to Ag."

Why would the electrode being oxidized pick up a negative charge if it's losing electrons?

[o1ocups]

I am also really confused about the idea of a salt bridge. "A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate" But what are the ions? Are they the ions of the electrolyte in the salt bridge? I was once under the impression that a salt bridge sort of transfers anions from the cathode to the anode, but this is probably not true, right?

Second question:  Just read this - "In the Cu/Ag cell, the Cu electrode become electron rich (negative) due to the tendency of Cu to be oxidized to Cu²⁺, whereas the the Ag electrode becomes electron deficient (positive) because of the tendence for Ag⁺ to be reduced to Ag."

Why would the electrode being oxidized pick up a negative charge if it's losing electrons?

Where did you read this? What this sentence is saying is completely opposite of my understanding. The Cu electrode (the anode) is where the oxidatoin occurs, so there should be a reduction in electrons (because it's going to the cathode), and the Ag electrode (cathode) is where the reduction occurs, so there should be an increase in electrons (because it's receiving the electrons from the anode). Speaking of this, I know that in a galvanic cell, the sign for anode is - and the sign for cathode is -, but I still don't really understand why, especially when comparing to the signs in an electrolytic cell. My textbook says that in a galvanic cell, because the anode is giving away the electrons, it has a negative sign, and because the cathode is receiving the electrons, it has a positive sign. But, in an electrolytic, cell, the anode is + because it is connected to the positive end of the battery, and the cathode is - because it is connected to the negative end of the battery. My question is: why do you use two different kind of standard when assigning the signs of galvanic cells and electrolytic cells? And what is the purpose of assigning these signs in the first place?

I am also confused about the role of battery in an electrolytic cell, but I think I am going to ask this question in a new post.

[nj_bartel]

I believe the (-) (+) terminology is just convention.  The end that the electrons flow FROM is termed the (-) end.

Salt bridge don't actually transfer electrons to the solution.  The ions in the bridge just make their way through the agar toward the solution - they can't cross into it.

[o1ocups]

The end that the electrons flow FROM is termed the (-) end.

But in an electrolytic cell, the electrons flow from the anode, too, don't they?  Why is the anode +?

Salt bridge don't actually transfer electrons to the solution.  The ions in the bridge just make their way through the agar toward the solution - they can't cross into it.

So the ions just stay in the part of the agar that is immersed in the electrolyte? Because I don't know how else the salt bridge can balance the charge 

[nj_bartel]

But in an electrolytic cell, the electrons flow from the anode, too, don't they?   Why is the anode +?

Anode kind of loses its meaning in an electrolytic cell - everything is reversed.

So the ions just stay in the part of the agar that is immersed in the electrolyte? Because I don't know how else the salt bridge can balance the charge 

That's my understanding, yes.


And from your original post, I got that quote from Analytical Chemistry, An Introduction, by Skoog, West, Holler, and Crouch; 7th Ed.

[Borek]

But what are the ions? Are they the ions of the electrolyte in the salt bridge?

Yes.

So the ions just stay in the part of the agar that is immersed in the electrolyte? Because I don't know how else the salt bridge can balance the charge 

In reality they slowly contaminate the solution. However, if they were not kept in agar, they will diffuse into solution quite fast. As they are 'bounded' to the salt bridge, they leave it mostly when additional external force (from the electric field) is applied.

[o1ocups]

OK thanks guys. So basically the ions don't leave the salt bridge and go into either solution--they just move around inside the salt bridge to balance the charge on both sides.

[Borek]

Please reread my previous post, I have edited it in the meantime.