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Topic: electrolytic cell  (Read 4447 times)

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lmonte

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electrolytic cell
« on: June 09, 2005, 11:30:03 PM »
In an electrolytic cell with a zinc chloride solution, an anode made of copper and cathode made of chromium. What would be produced at the cathode and what would be produced at the anode?  From the reduction potential chart, you would expect the Cr to undergo oxidation to form Cr3+.  But can this be produced at the anode if the Cr is the cathode?  Or would it be the copper that oxidizes?

Offline xiankai

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Re:electrolytic cell
« Reply #1 on: June 10, 2005, 04:18:00 AM »
u cant hope to produce Cr3+ ions at the cathode because reduction not oxidation takes place there. instead, free zinc cations in the solution are oxidised to zinc metal, effectively plating the chromium cathode. (zinc is more reactive than chromium and will be oxidised even if chromium was present in solution anyway)

since chromium is more easily oxidised then copper, Cr3+ ions will be formed at the anode, except for the fact there's no chromium to begin with. since most metals are prefferably oxidised in place of the halogen ions, and copper is not an exception, copper will be oxidised instead of chloride. copper ions would be produced at the anode then.
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Offline woelen

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Re:electrolytic cell
« Reply #2 on: June 10, 2005, 05:03:14 AM »
In an electrolytic cell with a zinc chloride solution, an anode made of copper and cathode made of chromium. What would be produced at the cathode and what would be produced at the anode?  From the reduction potential chart, you would expect the Cr to undergo oxidation to form Cr3+.  But can this be produced at the anode if the Cr is the cathode?  Or would it be the copper that oxidizes?
At the anode, you will have oxidation and at the cathode you will have reduction.

At the anode, the following species are present:

Cu
Cl(-)
H2O
Zn(2+)

Oxidizing Zn(2+) to an higher oxidation state is not possible (at least not in the aqueous state, probably not at all). Of all other species, present at the anode, the copper is easiest oxidized, so copper will go into solution as Cu(2+).
In practice, things are more complicated with copper anodes, combined with chloride, but for the time being, forget about that.


At the cathode, the following species are present:

Zn(2+)
H2O
Cr
Cl(-)

Reducing Cr to a neagative ion and reducing Cl(-) to a more negative ion, forget about that, it is not possible. What remain as candidates for reduction are water and Zn(2+). I guess that you'll get some Zn metal, plated at the cathode, but you'll certainly also get H2 and OH(-) from the water, the latter forming a spongy precipitate of Zn(OH)2 on/around the cathode. Zinc metal is not very noble and so, I'm not really sure what precisely happens at the cathode. Most likely you'll get an impure mix of Zn and Zn(OH)2 around the cathode.
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