Metals and Non-Metals — Science STD 10 — Question

For the refining of an impure metal by the process of electrolysis, a thick block of impure metal is made anode (connected to +ve terminal of the battery) and a thin strip of the pure metal is made cathode (connected to -ve terminal of battery). A water soluble salt (of the metal to be refined) is taken as electrolyte. On passing current, impure metal dissolves from the anode and goes into the electrolyte solution. And pure metal from the electrolyte deposits on the cathode. Electrolytic refining of copper: In an electrolytic tank, acidified copper sulphate (CuSO₄⁺ dilute H₂O₄) solution forms the electrolyte. A block of impure copper is made into an anode by connectig the positive terminal of a power supply (battery). A thin strip of highly pure copper metal is the cathode of the cell. The negative terminal of the power supply is connected to it.

A small electric curr ent is passed through the cell. Atoms from the anode enter the electrolyte. The copper from the anode gets converted into copper sulphide. An equal number of copper atoms from the solution get deposited on the cathode. This is to keep the concentration of the solution constant. Impurities from the anode block either remain in solution or collect below the anode, as they are unable to displace copper form the sulphate solution. The insoluble impurities remain in the electrolyte and are called anode mud. Copper sulphate solution contains ions of Cu²⁺ and SO₄^2- . The following reactions take
place at the anode and cathode when an electric current is passed.

At cathode : $\underset{\substack{\text { Copper ion } \\ \text {  (From electrolyte)   }}}{ Cu ^{2+} } \xrightarrow[\text { (Reduction) }]{+ \text { 2electrons }} \underset{\substack{\text { Copper atom } \\ \text {  (Deposits on cathode)}}}{ Cu }$
At anode : $\underset{\substack{\text { Copper atom } \\ \text {(From impure anode) }}}{ Cu } \xrightarrow[\text { (oxidation) }]{- \text { 2electrons }} \underset{\substack{\text { Copper ion } \\ \text {(Goes into electrolyte)}}}{ Cu^{+2} }$

Pure copper is scraped or removed from the cathode. Anode becomes thinner as the electrolysis process proceeds. Some important metals like gold and silver are present in the anode mud. These can be recovered separately.