Question

For purification of copper, a thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from the impure rod is sought to be transferred to the thin copper plate. Which electrode should be attached to the positive terminal of the battery?
(A) Pure copper
(B) Copper
(C) Impure copper
(D) Aluminum

Answer 1

Recall the reaction takes place in the process of purification of copper by electrolysis. The positive copper ions produced in the solution after electrolysis gets transferred to the electrode which is connected to the negative terminal of the battery. The other terminal should be connected to the positive terminal of the battery.

Complete step by step solution:
In the process of purification of copper, two electrodes of copper are placed in the solution of copper sulphate. The two electrodes are: impure copper rod and thin copper plate. When these rods are connected to the battery, the electricity passes through the solution and the solution undergoes electrolysis. So, there is a formation of $C{u^{2 + }}$ and ${\text{SO}}_4^{2 - }$.
We know that the copper ions ($C{u^{2 + }}$) are positively charged. Therefore, the copper ions will attract towards the plate which is connected to the negative terminal of the battery. Since we have given that the copper ions are transferred to the thin copper plate, then the thin pure copper plate must be connected to the negative terminal of the battery. In this case the other electrode that is impure copper rod must be connected to the positive terminal of the battery.
Reactions that take place in this process are:
At cathode (pure copper rod): $C{u^{2 + }} + 2e \to Cu$ (Reduction of $C{u^{2 + }}$)
The $Cu$ atoms produced get deposited on the cathode.
At anode (impure copper plate): $Cu \to C{u^{2 + }} + 2e$ (oxidation of $Cu$)
Copper ions from the solution get deposited on pure copper rod when electric current is passed in this arrangement and the copper ions from impure copper rod are liberated into the solution.

So, the correct answer is option (C).

Note: In this process, the negative ions ${\text{SO}}_4^{2 - }$ are attracted towards impure copper rod but do not change unlike copper ions deposit at the cathode by reduction reaction. During electrolysis, the mass of the impure copper rod decreases as copper dissolves and the mass of pure copper rod increases due to deposition of copper over it.