Question

The following questions refer to the electrolysis of copper sulphate solution with copper electrodes:
(a) Compare the change in mass of the anode.
(b) What is seen to happen to the colour of the copper sulphate solution if platinum electrodes are used? Explain this observation.
(c) What is the practical application of the electrolysis of copper sulphate solution? Briefly, describe one such application.

Answer 1

(a) During the electrolysis of aqueous copper sulphate using copper electrodes, copper ions are generated at anode, that go into the electrolyte copper sulphate solution. Then, these copper ions are transferred from the electrolyte and deposited at the cathode. Further in this process, impure anode becomes thinner (as mass of copper reduces) and pure cathode becomes thicker (as mass of copper increases).

(b) During the electrolysis of aqueous copper sulphate using platinum electrodes, platinum electrodes do not react in the electrolysis process due to their inert nature. Copper ions from the electrolyte are preferentially discharged at the cathode, leaving behind H⁺ ions in the electrolyte. OH⁻ ions are preferentially discharged at the anode, leaving behind sulphate ions in the electrolyte. Copper ions are then deposited at the cathode and oxygen gas is liberated at the anode. Therefore, the concentration of copper ions in the copper sulphate solution does not remain the same. As the copper ions from the electrolyte get discharged, the blue colour of copper sulphate becomes paler and finally, a colourless sulphuric acid is formed by the combination of H⁺ and SO₄²⁻ ions in the electrolyte.

(c) The practical application of electrolysis of copper sulphate solution is widely used for the electrolytic refining of copper. In this method, impure copper is purified by passing electric current through electrolyte acidified copper sulphate solution, consisting of an impure, thick lump of copper as the anode and a very thin piece of pure copper as the cathode in the electrolytic cell.
When current is passed through the electrolyte, copper ions are generated at the anode, which go into the electrolyte. Then, the copper ions present in the electrolyte are deposited at the cathode. Further in the process, impure anode becomes thin and the cathode becomes thicker by the deposition of reddish brown pure copper metal onto it. The copper deposited would be 99.9% pure. After electrolysis, these pure copper strips are separated and transformed into the shapes required.

In the electrolytic refining of copper, some of the impurities dissolve in the acid, while metals like silver, gold are not soluble in acid settles down under the anode. This is called anode mud. This anode mud is later recovered and the precious metals are separated.

Reaction at cathode: ${\mathrm{Cu}}^{2+}+2{\mathrm{e}}^{-}\stackrel{}{\to }\mathrm{Cu}$
Reaction at anode: $\mathrm{Cu}\stackrel{}{\to }{\mathrm{Cu}}^{2+}+2{\mathrm{e}}^{-}$