CLASS 12 Q and A OF CHAPTER GENERAL PRINCIPLES AND PROCESS OF ISOLATION OF METALS

1. Give an example of extraction based on oxidation reduction.

Ans 1. An example based on extraction by oxidation is extraction of chlorine from brine.

2. How are gold and silver extracted?

Ans 2. Gold and silver are extracted by leaching the metal with CN- . The metal is later recovered by displacement method in which zinc acts as reducing agent.

3Give two examples of metal refined by  a) Distillation b) Liquation c) Electrolytic refining

Ans 3. a) Distillation – Zinc and Mercury b) Liquation – Tin and Antimony c) Electrolytic refining – Copper and Zinc.

4.Which method is used for refining of silicon or gallium?

Ans 4. The refining of Silicon or gallium is done by Zone refining

5.What is the principle behind zone refining?

           Ans 5. The principle of zone refining is that impurities are more soluble in the melt then in the        solid state of the metal.

 6.Give the requirements for vapour phase refining?

Ans.6. The two requirements of vapour phase refining are :- 1) The metal should from a volatile compound with an available reagent, 2) The volatile compound should be easily decomposable so that recovery is easy.

7. Which method is used for refining of zirconium? Explain.

Ans.7. Zirconium and Titanium are refined by van Ankle process, Here the crude metal is heated in an evacuated vessel with iodine. Zr+  2 I2 + →  ZrI4 The metal iodide is decomposed on a tungsten filament, electrically heated to about 1800K and pure metal is deposited on the filament. 

ZrI4 → Zr +2I2.

8.What is the principle behind chromatography? Name some types of chromatographic techniques.

Ans 8. The principle behind chromatography is that different components of a mixture are differently adsorbed on an adsorbent. Some of the chromatographic techniques are paper chromatography, column chromatography, gas chromatography etc.

9.  What is pig iron?

 Ans 9. The iron which is obtained from blast furnace and contains about 4% carbon and many other impurities in smaller amounts like S, P, Si, Mn etc, is called pig iron.

10  What is cast iron?

Ans 10. Iron obtained by melting pig iron with scrap iron and coke using hot air blast is cast iron.

 11   What is wrought iron?

Ans 11. Wrought iron and malleable iron is the purest form of commercial iron which is prepared from cast iron by oxidizing impurities in a reverberatory furnace lined with heamatite.

12 What is added as flux in extraction of iron?

Ans 12. Limestone is used as flux in extraction of iron.

13 Explain the extraction of copper?

Ans 13. The sulphide ores of copper are roasted to give oxides: 2Cu2S + 3O2→2Cu2O + 2SO2.

 The oxide can then be easily reduced to metallic copper using coke. 

2 Cu O +C → 2Cu+CO 

The impurities like iron oxide are removed as slag by reacting with SiO2, added as flux. 

FeO + SiO2 + → FeSiO3.

14. What is Blister copper?

Ans 14. The solidified copper obtained after extraction has blistered appearance due to evolution of SO2 is called blister copper.

15What is the basis of reduction of a molten metal salt? Explain ?

Ans.15 In the reduction of molten metal salt, electrolysis is done which is based on electrochemical principles following equation∆G 0  = −  nFE Here n is the number of electrons and E0 is the electrode potential of redox couple. More reactive metals have large negative values of the electrode potential and are difficult to reduce.

16. Why is cryolite used during extraction of Aluminum?

Ans.16Cryolite is used to lower the melting point of alumina and increase conductivity.

17. How is copper extracted from low grade ores?

Ans17. Copper is extracted by hydrometallurgy from low grade ores. It is leached out using acid or bacteria.

18.Define the term - metallurgy?

Ans. The scientific and technological process used for isolation of the metal from its ores is known as metallurgy.

19. What is gangue?

Ans. The earthly or undesired materials which contaminate ore are called gangue.

20.Give some important ores of Aluminum, Iron, Copper and Zinc?

Ans. Metal Ore

 Aluminum -Bauxite

Iron- Hematite, Magnetite

Copper -Copper pyrite, Copper glance

Zinc -Zinc blende, Calamine

21.What is the basis of hydraulic washing? For which type of ores is it used?

Ans. Hydraulic washing is based on the differences in the gravities of the ore and the gangue particles. It is used when ore is heavy and impurities are lighter.

22.. Explain froth floatation method.

Ans.  Froth floatation is used for removing gangue from sulphide ores. Here a suspension of the powdered ore is mixed with water. To it collectors and froth stabilizer are added. A rotating paddle agitates the mixture and draws air in it. As a result froth is formed. The mineral particles become wet by froth (oil) and gangue particles by water. The froth is skimmed off and dried for the recovery of ore.

23. What is the role of a depressant in the floatation process? Give an example?

Ans. Depressant adjusts the proportion of oil and water so as to separate two sulphide ores. e.g. in case of an ore containing ZnS and PbS, NaCN is used an depressant which selectively prevents ZnS from coming to the froth.

24. What is used as collectors and froth stabilizers in froth floatation process? What is their role in the process?

Ans . Pine oil, fatty acids or xanthate can be used as collectors to enhance the non- wettability of mineral particles whereas cresols or aniline can be used as froth stabilizer to stabilize the froth in froth floatation process.

25. . What is leaching? Explain with an example.

Ans . Leaching is used if the ore is soluble in suitable solvent. e.g. Aluminum is leached from bauxite by using a conc. solution of NaOH at 473-523 K and 35-36 bar pressure. Al2O3 is leached out an sodium aluminates leaving behind impurities-

 Al2O3 (S) + 2NaOH(aq)+3H2O  → 2Na[Al(OH)4 ] 

The aluminate solution is centralised by passing CO2 gas & hydrated Al2O3 is precipitated which is filtered, dried and heated to give pure Al2O3. 

2Na[Al(OH)4 ] + CO2→ + 2NaHCO3(aq)

Al2O3.xH2O(s) → Al2O3(s) + xH2O(g).

26. Which reagent is used for leaching of Gold or silver? Write the equations involved.

Ans.A dil. Solution of NaCN or KCN in the presence of air is used as reagent for leaching of silver and gold. The equations are

4M(s) + 8CN-(aq) +2H2O(aq) + O2(g) -------------- 4[Mn(CN)2]-(aq) + 4OH-(aq)

2[M(CN)2]-(aq) + Zn(s) -------------- [Zn(CN)4]-2(aq) + 2M(s).

27. What are Ellingham diagrams?State one limitation of Ellingham diagrams.

Ans. Ellingham diagrams are graphical representation of variation of ∆ G vs T for the formation of oxides of elements i.e., for the reaction 2xM(s) +O2(g) →2MxO(g). Ellingham diagrams only tell us about the feasibility of a reaction. They do not tell anything about the reaction kinetics.

28. Write the relationship between Gibbs free energy, enthalpy change and change in entropy?

Ans. When ∆ S is entropy change , ∆H is enthalpy change , than at temperature T, the change is Gibbs free energy is given by ∆ G = ∆H – T∆ S.

29.What is the condition for a reduction reaction to occur in terms of free energy change? How can it be achieved?

Ans. When the value of ∆ G is negative , the reduction reaction is said to be spontaneous 1) It ∆ S is positive , on increasing the temperature, the value of T∆ S would increase & ∆ G will become negative. 2) For a process which is otherwise having ∆ G positive & is non- spontaneous, , it can be coupled with a reaction having highly negative ∆ G value so that the overall ∆ G is negative and the process can take place.

30. Sulphide and carbonate ores are converted to oxide before reduction. Why?

Ans. Since the reduction of oxide ores involves a decrease in Gibb’s free energy making ∆ G value more negative, it is easier to reduce oxides therefore suphide and carbonate ores are converted to oxides before reduction.