Class 12 Chemistry Isolation of Elements Electrochemical Principles of Metallurgy

Electrochemical Principles of Metallurgy

  • In the reduction of a molten metal salt, electrolysis is done.
  • Consider the equation:-
    • ΔG(0) = – nE(-)F equation (1)
    • n= number of electrons and E(-) = electrode potential of redox couple formed in the system.
    • More reactive metals have large negative values of the electrode potential.
    • This implies their reduction is very difficult.
  • If the difference of two E(-) values corresponds to a positive E(-)  and consequently negative ΔG(0)  in equation (1) .
  • As a result then the less reactive metal will come out of the solution and the more reactive metal will go to the solution.
    • For example:- Cu2+ (aq) + Fe(s) → Cu(s) + Fe2+ (aq)
    • In simple electrolysis, the Mn+ ions are discharged at negative electrodes (cathodes) and deposited there. Sometimes a flux is added for making the molten mass more conducting.

Aluminium

  • In the metallurgy of aluminium, purified Al2O3 is mixed with Na3AlF6 or CaF2 which lowers the melting point of the mix and brings conductivity.
  • The fused matrix is electrolysed.
  • Steel cathode and graphite anode are used. The graphite anode is useful here for reduction to the metal.
  • The overall reaction may be taken as:
    • 2Al2O3 + 3C → 4Al + 3CO2
  • This process of electrolysis is widely known as Hall-Heroult process.
  • The electrolysis of the molten mass is carried out in an electrolytic cell using carbon electrodes. The oxygen liberated at anode reacts with the carbon of anode producing CO and CO2.
  • This way for each kg of aluminium produced, about 0.5 kg of carbon anode is burnt away.
  • The electrolytic reactions are:
  • Cathode: Al3+ (melt) + 3e– → Al(l)
  • Anode: C(s) + O2– (melt) → CO(g) + 2e
  • C(s) + 2O2– (melt) → CO2 (g) + 4e-

Class_12_Chemistry_Isolation_Current_Metals_Extracion_Of_Aluminium

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