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In refining copper, an aqueous solution of copper(II) sulphate is electrolyzed, using impure copper as the anode and pure copper as the cathode.

At the anode, metal impurities higher than copper in the electrochemical series such as zinc, iron and lead are oxidized first, then copper loses its electrons to form copper(II) ions. Metal impurities lower than copper in the electrochemical series are hardly oxidized and deposit at the bottom of the tank as sludge. So, I have two questions:

  1. If ions from metals more reactive than copper exist in the solution, why aren't they preferentially discharged first to form zinc or iron metal on the other copper electrode first rather than copper?

  2. Why can the pure copper cathode be thickened with pure copper only?

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    $\begingroup$ More reactive metals aren't preferentially discharged precisely because they are more reactive. As for copper cathode, well, what choice does it have? There is current running through it and there is copper which is going to be deposited, no matter what. $\endgroup$ – Ivan Neretin Dec 25 '17 at 7:14
  • $\begingroup$ What do you mean by the last question, "Why can pure copper cathode be thickened with pure copper?". It doesn't make any sense. How can you thicken a pure copper cathode by pure copper? I think you should explain that question by a few more words. $\endgroup$ – Shoubhik Raj Maiti Dec 25 '17 at 12:29
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To your first question: Consider what happens if you put an iron nail or a zinc pellet into a copper(II) solution: the less noble metals will oxidize and reduce the copper ions to copper(0). So even if some of the less noble metal ions are reduced and deposited at the cathode, they would react with the copper ions (that are the most numerous in this scenario).

To your second question: if ions of more noble metals than copper were in the solution, they would be deposited on the cathode. However, by carefully tuning the voltage and current, no more noble metal ions are produced at the anode. As stated by Ivan Neretin in the comments, the cathode will reduce what is available to be reduced: copper ions (which are most numerous and the potential fits).

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