The highly reactive metals are extracted by the electrolysis of their molten chlorides or oxides. During electrolysis of molten salts, metals are always produced at the cathode because metal ions are always positively charged and get attracted to the cathode during electrolysis. The metals extracted are very pure.
The highly reactive metals are extracted by electrolysis of their molten chlorides.
2NaCl → 2 Na + Cl2
2 Na+ + 2e- → 2Na ( Na is produced at cathode)
2Cl- - 2e- → Cl2 (Cl is produced at anode)
Aluminium metal is extracted by electrolysis of metal oxide.
2Al2O3 → 4Al + 3O2
The moderately reactive metals are extracted by the reduction of their oxides with carbon, aluminium, sodium or calcium. The concentrated ores can be converted into metal oxide by the process of calcination or roasting.
i) Calcination – A carbonate ore is heated strongly in the absence of air to convert into metal oxide.
ZnCo3 → ZnO + CO2
ii) Roasting - Sulphide ore is strongly heated in the presence of air to convert it into metal oxide.
2ZnS + 3O2 → 2ZnO + 2SO2
The metal oxides obtained by calcination or roasting are converted into the free metal by using reducing agents like carbon, aluminium, sodium or calcium which depend upon the reactivity of the metal.
a) Reduction of metal oxide with Carbon - Carbon reduces metal oxide to free metal.
ZnO + C → Zn + CO
b) Reduction of metal oxide with Aluminium - Manganese and Chromium metals are extracted by the reduction of their oxides with aluminium powder which reduces the metal oxide to metal and is itself oxidized to aluminium oxide.
3MnO2 + 4 Al → 3Mn + 2Al2O3 + Heat
Thermite Reaction :-
The reduction of a metal oxide to form metal by using aluminium powder as a reducing agent. The reactions are highly exothermic so the metals produced are in molten state.
Fe2O3 + 2Al → 2Fe + Al2O3 + Heat
Iron metal is produced in molten state and then poured between the broken iron pieces to weld them .This process is called aluminothermy or thermite welding.
The Less reactive metals are extracted by the reduction of their oxides by heat alone.
i) Mercury can be extracted by heating it sulphide ore called cinnabar ( HgS) in air.
2HgS + 3O2 → 2HgO + 2SO2
2HgO → 2Hg + O2
ii) 2Cu2S +3O2 → 2Cu2O + 2SO2
2Cu2O + Cu2S → 6Cu + SO2
3. Refining of metals :-
The process of purifying impure metals which depends on the nature of metal and its impurities.
Electrolytic Refining :- It means refining by electrolysis.eg. Cu, Zn, Sn, Pb, Ch, Ni, Ag, Au.
a) A thick block of impure metal – anode ( connected to +ve)
b) A thin strip of pure metal – cathode ( connected to – ve)
c) A water soluble salt of the metal to be refined – electrolyte.
On passing electric current , impure metal dissolves from the anode and goes into electrolyte solution and pure metal from the electrolyte solution deposits on the cathode. The soluble impurities go into the solution whereas insoluble impurities settle down at the bottom of the anode as ‘anode mud’.
Electrolytic Refining of copper :-
Thick block of impure copper – anode
Thin strip of pure copper – cathode
Acidified copper sulphate solution – electrolyte
Pure metal is produced on the cathode.At Cathode : Cu2+ + 2e- → Cu
At anode : Cu - 2e- → Cu2+
