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:
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?
Why can the pure copper cathode be thickened with pure copper only?