CBSE Class 12 Chemistry General Principles Notes

General Principles & Process of Isolation of Elements

SUMMARY

1.     The chemical substances in the earth’s crust obtained by mining are called Minerals.

2. Minerals, which act as source for metal, are called Ore. From ore metal can be obtained economically .

3. The unwanted impurities present in ore are called Gangue

4. The entire process of extraction of metal from its ore is called Metallurgy.

5. Removal of gangue from ore is called Concentration, Dressing or Benefaction of ore.

6. Concentration by Hydraulic washing is based on the difference in gravities of ore and gangue particles.

7. Concentration by Magnetic separation is based on differences in magnetic properties of ore components. If either of ore or gangue is capable of attracted by a magnet field, then such separation is carried out.

8. Concentration by Froth Flotation Process is based on the facts that sulphide ore is wetted by oil & gangue particles are wetted by water.

9. Concentration by Leaching is based on the facts that ore is soluble in some suitable reagent & gangue is insoluble in same reagent. e.g. Bauxite ore contains impurities of silica, iron oxide & TiO₂ .The powdered ore is treated with NaOH which dissolve Al & impurities remains insoluble in it.
Al₂O₃ +2N_aOH + 3 H₂O →  2 Na [Al(OH)₄].

10. Calcination involves heating of ore in absence of air below melting point of metal. In this process volatile impurities escapes leaving behind metal oxide.
Fe₂O3.xH₂O →  Fe₂O₃ +xH₂O
ZnCO_{3 →}  ZnO +CO₂

11. Roasting involves heating of ore in presence of air below melting point of metal in reverberatory furnace. In this process volatile impurities escapes leaving behind metal oxide and metal sulphide converts to metal oxide.
2 ZnS + 3 O_2→ 2Z_nO+2SO₂

Slag- It is easily fusible material fusible material, which is formed when gangue still present in roasted ore combines with the flux.

e.g. C_aO (flux) + SiO₂ (gangue) →C_aSiO₃ (slag)

12. Reduction of metal oxide involves heating of metal in presence of suitable reagent coke or CO₂

           CO₂+C →2CO 

13. Pig Iron: - It contains Fe 93-95%, Carbon 2.5-5%, and Impurities 3%.

14. Cast Iron: - It contains Fe 99.5-99.8%, Carbon 0.1-0.2% Impurities 0.3%.

15. Spongy iron: - Iron formed in the zone of reduction of blast furnace is called spongy iron. It contains impurities of C, Mn , Si, etc.

16. A mixture of Cu₂S, FeS & silica is obtained from blast furnace known as Copper matte

17. Bessemerisation of copper matte is done in Bessemer converter in presence of air. Following reactions take place:-
2FeS + 3O₂→2FeO +2 SO₂, FeO + SiO₂ →FeSiO₃ (slag),
2Cu₂S + 3O₂ →2Cu₂O+2SO₂, 2Cu₂O+2Cu2S→ 6Cu + SO₂

18. Melted copper is cooled, and then SO2 is evolved. such copper is known as BLISTER COPPER(98%Cu+2% impurities)

19. Electrolytic reduction of pure alumina takes place in iron box (cathode) with cryolite (Na3AlF₆) & fluorspar CaF_2.Cryolite lowers m.pt. and increases conductivity of alumina.Graphide rods act as anode. Following reactions take place:-

At cathode:- Al³+ + 3e→ Al, At Anode:- 2O^2- →O₂ + 4e

20. Vapour phase refining is used for extraction of Nickel (MOND PROCESS) and Zirconium &Titanium (VAN ARKEL PROCESS). It is based on the principal that metal forms a volatile complex with suitable reagent, which then decomposes at higher temperature to give pure metal .

21. Zone refining is used for extraction of Si, Ge, Ga, etc. It is based on the principal that impurities are more soluble in molten state of metal than solidified state.

Electrolytic-refining It is based on the principal that impure metal is made Anode and pure metal is made the cathode. On passing electricity, pure metal is deposited at the cathode while the impurities dissolve dissolve in solution as anode mud. E.g. copper.

22. Chromatography method is based on selective distribution of various constituents of a mixture between two phases, a stationary phase and a moving phase. The stationary phase can be either solid or liquid on solid support.

23. Column chromatography is based on adsorption phenomenon. This method is useful for those elements, which are available in small amounts and the impurities are not much different in chemical properties from the element to be purified. 

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