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Old 02-11-2020, 11:50 AM
KidCharlemagne is offline
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Question about zinc-copper voltaic cell.


Learning about electrochemistry basics and just read up on the copper-zinc voltaic cell. What I donít understand is why zinc sulfate and copper sulfate is used. I understand why you need an electrolyte but I donít understand why those are used specifically. The zinc cations donít appears to be participating at all. I could maybe see why copper sulfate is used but even that is just a guess. Why not use NaCl for the half cells (and the bridge)? Thanks in advance.
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Old 02-12-2020, 09:24 PM
am77494 is offline
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Originally Posted by KidCharlemagne View Post
Learning about electrochemistry basics and just read up on the copper-zinc voltaic cell. What I donít understand is why zinc sulfate and copper sulfate is used. I understand why you need an electrolyte but I donít understand why those are used specifically. The zinc cations donít appears to be participating at all.
Think about this : do you want reactions to go on when the cell is not being used for power generation (i.e. the circuit is not completed )?

If your answer is NO, then you need to have electrolytes that do not react with the copper/zinc unless the circuit is complete. So for example, you can use H2SO4 on the zinc side but it will react continuously to make Hydrogen. Hydrogen also ďsticksĒ to the metal reducing the surface area and the effect is called polarization.

Next you may ask why sulfate and not chloride. And here Iíd ask you to think what kind of gases maybe evolved if the electrolyte is near exhausted.

Also - think of the typical impurities in the metal electrodes and their interaction with the electrolytes. Do you want them in solution or precipitated ?

Also - look at ionic mobility of the sulfate ion versus other ions.


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Why not use NaCl for the half cells
This one is obvious. Zinc is going from metal to ion and copper is going from ion to metal. You need copper ions for that and NaCl has no copper ions.


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....(and the bridge)? .
Read ď Liquid junction potentialĒ here https://en.m.wikipedia.org/wiki/Liqu...tion_potential

Where the salt bridge makes ionic contact with either electrolyte, there exists a junction potential (just like when two different metal wires are contacted). You want to minimize this potential and hence the use of KNO3.
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