salt bridge

[merlin]

I'm wondering, a common salt bridge used such as KNO3 provides ions to the electrolyte inorder to balance the ions consumed or produced in each cell, however since KNO3 always gives out it's ions then won't it eventually run out very quickly?

[cara.mel]

Yes, it will eventually dry up and run out.

[merlin]

ah ok, but does the electrons only flow from one cell to the other through the wire or does it actually continue to flow through the salt bridge and into the other cell? 

[cara.mel]

Electrons don't go through the salt bridge. At the cathode where they go M+ + e- => M (where M = metal), that is the end of the electron's run.

See this thread for more about salt bridge: http://vcenotes.com/forum/index.php/topic,5110.0.html

[merlin]

CHeers

[bec]

Have other people learnt about Daniell Cells which don't have a salt bridge? We went over it in class quickly but it's not in the text book and I don't know if I should bother studying it in more detail - how likely is it that they would put them on the exam?

[Pandemonium]

The Daniell cell only refers to really, a cell that has the formula:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s).

When you have the electrolyte/electrode mixtures (edit: HALF CELLS) in separate containers, then you require a salt-bridge.

When you don't, the polarity expected to be produced by the Zn2+ in a separated system is nullified when we consider the fact that the reaction itself consumes Cu2+ from the same mixture.

eg. take some Cu2+, make some Zn2+. Keep doing this over and over again. Since Cu2+ is being consumed at the same rate as Zn2+, there is no net charge produced. The purpose of the salt bridge is to nullify this charge, but it doesn't exist, thus no requirement for it.

[dusty_girl1144]

Have other people learnt about Daniell Cells which don't have a salt bridge? We went over it in class quickly but it's not in the text book and I don't know if I should bother studying it in more detail - how likely is it that they would put them on the exam?

well .... i havent even learnt about it without salt bridges :S

is that anything to do with primary and secondary cells?

[Glockmeister]

The Daniell cell does have a salt bridge...

[Mao]

she might have meant electrochemical cells where the compartments are separated by a porous materials or a membrane, which don't require a salt bridge

[bec]

Yup, that's what i meant.
Doesn't sound like it's too important..Besides, I read over my notes on it again, and it doesn't seem too hard either

[dusty_girl1144]

how i wish i knew what you guys were on about

by any chance is it the cell where there is a solution and it flows to the cathode and anode and needs a source of electricity to get the reaction started?

[Collin Li]

No, by definition, the anode is where electrons leave. Electrons can never be "forced down" an anode, because then it automatically makes that the cathode (by definition!)

But yes, you're getting somewhere. You can force non-spontaneous reactions to occur with electricity. That is electrolysis.

[dusty_girl1144]

ahhhh ok ....

so what is the point of electrolysis then?

[Collin Li]

You sort of need to see a diagram to really see how it works.

When a battery recharges, its cathode becomes the anode and vice versa.

This makes sense because recharging requires taking the backward reaction (which would be endothermic and energy-requiring, as opposed to exothermic energy-releasing). Everything goes in reverse!

So that means the place where reduction was happening (cathode) is now oxidising (anode), and vice versa.

See how the definitions of the cathode and the anode don't change, but the matter of which electrode is which, does?