Quick Salt Bridge Question

[cwb]

Hey everyone. Just had a quick question on salt bridges... I was under the impression that they could be used up, exact words in my Kaplan book "During the course of the reaction electrons flow from the anode through the wire and voltmeter, toward the cathode. The anions flow externally (via the bridge) into the ZnSO4, and the cations flow into the CuSO4. This flow depletes the salt bridge, and, along with the finite quantity of Cu2+ in the solution, accounts for the relatively short lifetime of the cell."

However in one of the Kaplan practice passages it states that a salt bridge always stays intact... Is this just a wording issue that I am not seeing or are they saying different things (meaning could is possibly be that they mean a salt bridge is always physically present or something and its just the cations and anions that are used up?). Any help here would be great because I really suck at Electrochemistry.

Also if any of you have any tips for Electrochemistry problems that would be great as well. Thanks in advance.

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[Fups]

I hated Electrochemistry also, so... I studied it! I don't really know enough to answer your specific question, but here are some things I learned which has helped me TREMENDOUSLY...

- Red Cat (reduction at cathode), An Ox (oxidation at anode)
- cell voltage is spontaneous when it is POSITIVE. (unlike Gibbs free energy being negative when it is spontaneous)
- Galvanic cells are create an electric current sponteneously, where an electrolytic cells are powered by an external voltage source, making them non-spontaneous.
- NEVER multiply the reduction potentials by a coefficient b/c the potentials are "intrinisic and do not depend on the # of moles of a species." (TPR)

these little MUST knows, make answering Electrochem questions much more managable.

Cheers!

 

[sahil22]

Hey everyone. Just had a quick question on salt bridges... I was under the impression that they could be used up, exact words in my Kaplan book "During the course of the reaction electrons flow from the anode through the wire and voltmeter, toward the cathode. The anions flow externally (via the bridge) into the ZnSO4, and the cations flow into the CuSO4. This flow depletes the salt bridge, and, along with the finite quantity of Cu2+ in the solution, accounts for the relatively short lifetime of the cell."

However in one of the Kaplan practice passages it states that a salt bridge always stays intact... Is this just a wording issue that I am not seeing or are they saying different things (meaning could is possibly be that they mean a salt bridge is always physically present or something and its just the cations and anions that are used up?). Any help here would be great because I really suck at Electrochemistry.

Also if any of you have any tips for Electrochemistry problems that would be great as well. Thanks in advance.

The use of the salt bridge is to keep the solutions neutral.
For example::
You have Zn rods in ZnSO4 as anode and Cu in CuSO4 as cathode.

Then as time passes Zn rods will oxidize to Zn2+ giving up 2 electrons which goto the copper electrode and reduce a Cu2+ to Cu.This is basically how a cell works.

But you should realize, that Zn->Zn2+ will increase the positive ion charge (adding 2 positive charges for evvery Zinc broken down to Zinc ion) and similarly the Cu side will generate -ve charge overall. These will make the cell harder to work coz after a while you will need to move electrons from a side that has +ve charge to a side that has -ve charge developed. So to neutralize these charges, a salt bridge (which is usally a salt of a strong acid and base) is setup which moves the +ve ions to the side that develops the -ve charge and vice versa. As time progresses salt brighe is thus used up to balance the developed charges.
I hope that clears your doubts.

You should realize that if we had 2 rods inside a common electrolyte solution, there would be no need of a salt bridge.

 

[medicomel]

I hated Electrochemistry also, so... I studied it! I don't really know enough to answer your specific question, but here are some things I learned which has helped me TREMENDOUSLY...

- Red Cat (reduction at cathode), An Ox (oxidation at anode)
- cell voltage is spontaneous when it is POSITIVE. (unlike Gibbs free energy being negative when it is spontaneous)
- Galvanic cells are create an electric current sponteneously, where an electrolytic cells are powered by an external voltage source, making them non-spontaneous.
- NEVER multiply the reduction potentials by a coefficient b/c the potentials are "intrinisic and do not depend on the # of moles of a species." (TPR)

these little MUST knows, make answering Electrochem questions much more managable.

Cheers!

great review Fups. thanks!