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Author: Subject: Potential in between Cu and Zn electrode in saturated NaCl solution
msp2
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[*] posted on 7-8-2012 at 10:13
Potential in between Cu and Zn electrode in saturated NaCl solution


No salt bridges or anything. How is this calculated and what reactions are involved?

I know this is probably a very basic question (or is it?), unfortunately I have been away from chemistry a long time.

Regards, MSP

[Edited on 7-8-2012 by msp2]
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weiming1998
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[*] posted on 9-8-2012 at 01:07


Quote: Originally posted by msp2  
No salt bridges or anything. How is this calculated and what reactions are involved?

I know this is probably a very basic question (or is it?), unfortunately I have been away from chemistry a long time.

Regards, MSP

[Edited on 7-8-2012 by msp2]


I don't understand what specific potential that you are talking about here, but here are the reactions that may occur:

Sodium chloride electrolysis produces chlorine gas and NaOH.

At the cathode: 2H+ +2e- --->H2.
At the anode: 2Cl- --->Cl2+2e- .

When copper acts as the anode, the chlorine oxidizes the copper, to form CuCl2, which some of it reacts with the zinc via a single displacement reaction to form ZnCl2 (hydrated) and Cu metal, which might plate the zinc cathode with a layer of copper. Others react with the OH- in the water to form Cu(OH)2. The ZnCl2 formed reacts with the OH- as well, forming hydrated ZnO. The end products are NaCl, NaClO and assorted metal salts (reaction of chlorine with metal electrode isn't 100% efficient), Cu(OH)2 and Zn(OH)2. An extremely small amount of Cu2+, Zn2+ and OH- ions will also exist in solution

When zinc acts as the anode, the zinc oxidizes to form ZnCl2, which again reacts with the OH- to form hydrated ZnO. The end products are again Zn(OH)2, NaCl, NaClO, and small amounts of Zn2+ and OH-.
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bquirky
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[*] posted on 9-8-2012 at 10:07


I think he might mean what the potential of the cell is if it where used as a battery rather than as an electrolysis device

A guess straight off the top of my head without looking up the potential tables. if you had copper and zinc electrodes in sodium chloride you should see about 1 volt if it is open circuit. if you where to oxidize the copper electrode then you might see about 1.2- 1.4 volts (depending on the average oxidization state of the copper).

But.. you wont be able to draw much current at all from such a cell for several reasons. the two main two being.

Sodium chloride solution isn't the most conductive electrolyte.

and a phenominum called "electrode polirisation" occurs basically you get gas buildup on the electrodes forming an insulating layer between the electrode and the electrolyte.

this is one of the classical problems that plagued natural philosophers hundreds of years ago. the solution is to design the cell such as each half reaction on each electrode dose not involve a gas phase. that is why alakaline batterys today use a carbon/managanise oxide paste as one of its electrodes the manganise changes its oxidization state instead of forming insulating gas bubbles.




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