If a copper sample containing some zinc impurity is to be purified by electrolysis, the anode and the cathode must be which of the following?

a. anode: pure copper cathode: pure zinc
b. anode: pure zinc cathode: pure copper
c. anode: pure copper cathode: impure copper
d. anode: impure copper cathode: pure copper
e. anode: impure copper cathode: pure zinc

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asked by John
  1. What do you think and why? Remember that the anode is where oxidation takes place and will be negative.

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  2. I think c and d can be eliminated because they only have all coppers. Zinc needs to be an anode or cathode. I think that e is correct, because there needs to be an impurity somewhere, but I am not sure.

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    posted by John
  3. I want to correct something I wrote. The first part is ok. The second part is not ok. In a voltaic cell, oxidation occurs at the anode and it is negative. But when I just re-read the question, it says that the impure copper is to be purified by electrolysis. In an electrolytic cell, the anode is + and the cathode is -.

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  4. You have a piece of copper metal, mostly copper but with a small amount of Zn in it, and you want to purify it so the copper is by itself and the Zn is not in it. The problem calls that the impure copper. Here is what we want to happen. We want the Cu/Zn in the impure copper/zinc rod or bar to go into solution as this.
    Cu ==> Cu+2 + 2 electrons
    Zn ==> Zn^2 + 2 electrons.
    So one electrode will be the impure piece of Cu/Zn we have been given.

    At the other electrode, we want the Cu^+2, now in solution form, to plate out. What will we plate it out on. A thin strip of copper metal (in the problem called pure copper), of course. And the electrolysis cell will have insufficient voltage to plate out the Zn^+2, now in solution, so it will stay in solution.
    That eliminates answers a, b, and e since we aren't trying to purify Zn. We want impure Cu/Zn to go into solution at one electrode and for the copper ions then in solutiion to plate out at the other electrode. If we want + charged Cu^+2 ions to plate out, they will be attracted to the negatively charged electrode of the cell, the cathode in an electrolytic cell; therefore, we place a thin strip of pure copper for the cathode. That means we place our impure Cu/Zn metal bar (or whatever shape it has) as the anode at the positvely charged electrode. That makes d the correct choice.

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