- Gibbs free energy is given by
- If Eo is positive, delta G is negative.
- Therefore the reaction is spontaneous.
- Emf of the redox reaction is made positive.
- More reactive metal is added to the solution containing the relatively less reactive metal ions.
- The more reactive metal will go to into the solution.
- For example copper becomes copper(II) ion on reaction with ag+ ion.
- Copper (reducing agent) undergoes oxidation by removing 2e.
- Silver (oxidising agent) undergoes reduction by gaining 1e.
Similarly copper 2+ ion reacts with zinc as follows.
ELECTROCHEMICAL EXTRACTION OF ALUMINIUM
- Hall – Herold process
- Electrolytic bath – iron tank lined with carbon.
- Electrolyte – 20% solution Alumina, bauxite + molten cryolite(sodium hexafluoroaluminate, Na3AlF6)
- Supporting electrolyte – 10% CaCl2 solution – to lower the melting point of the mixture
- Anode – carbon rods immersed in the electrolyte
- Cathode – carbon lining in the electrolytic tank.
- Bath Temperature : > 1270 K
- At anode oxidation occurs.
- At cathode reduction occurs. Aluminium is deposited at cathode.
Since carbon acts as electrode, it will also participate in the electrochemical reaction.
- Anodes are slowly consumed during electrolysis due to the above two reactions.
- The pure aluminium is reacted at cathode.
- It settles at the bottom.
- The net electrochemical reaction is given below.
