Question

Describe the principle involved in the electrolytic refining of metals?

Answer 1

The oxides of the highly electropositive metals like Na, K, Mg, Ca, Al, etc., cannot be reduced easily with carbon at moderate temperatures. For reduction, a very high temperature is required at which the metal may combine with carbon to form a carbide. These metals are thus extracted by the electrolysis of their oxides, hydroxides or chlorides in fused state. Sometimes, a small amount of some other salt is added as to lower the fusion temperature or to increase the conductivity or both. The metal is liberated at the cathode. Sodium is obtained by the electrolysis of fused mixture of NaCl and CaCl₂ (Down's process) or by electrolysis of fused sodium hydroxide (Castner's process).
The anode reaction is: 2Cl^– → Cl₂ + 2e^–
The cathode reaction is: 2Na⁺ + 2e^– → 2Na
for an overall reaction of 2Na⁺ + 2Cl^– → 2Na + Cl₂