Oxidation states and redox reactions
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Copper is deposited over zinc sheet, when zinc is placed in copper sulphate, since zinc is more electropositive than copper. Instead of getting brown colour of copper on the zinc, a black colour was observed. Please explain this.
Similarly, a black colour was obtained on iron during the electrolysis of copper sulphate with iron as negative electrode. Here also we expected brown colour of copper on iron.
Corrie writes ....
It does not surprise me that you see a black, or very dark brown, colour when copper forms on zinc as I have seen this myself. Certainly when the copper deposits initially and is very finely divided (powdery if it were dried), then the colour often seems to be dark, but as the reaction slows down the deposit usually starts to take on a more 'coppery' colour.
The colour seems to be related to the speed of the deposition, and hence the degree of crystallinity of the copper. The more finely divided the copper is, the darker it will appear. Try using different concentrations of copper ions in solution.
In another well-known system, when copper metal is dipped into silver nitrate solution, the initial deposit of silver metal is a dull, powdery, grey solid but changes to beautiful long, shiny crystals of silver as the reaction slows down!
What you observe in electrolysis is probably due to the same factor, i.e. speed of deposition. It is well known in commercial electroplating that the appearance of the metal plating being produced is very much dependent on the current (i.e. the speed of the deposition), as well on several other factors.
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updated: 17 August 2007
