Important Questions for CBSE Class 12 Chemistry Chapter 6 - General Principles and Processes of Isolation of Elements 2024-25

Very Short Answer Questions.                                                                       1 Mark

1. Name three metals that occur in native state in nature.

Ans: The metals that occur in native state in nature are ${\text{Au,Ag}}$and ${\text{Pt}}$

2. What are collectors in froth flotation process? Give one example.

Ans: Collectors enhance the non-wettability of ore particles. One such example is pine oil.

3.Give the names and formulae of three ores which are concentrated by froth floatation process.

Ans: The names and the formula of ores concentrated by froth floatation method is as follows

(i) Galena-${\text{PbS}}$

(ii) Zinc blende-${\text{ZnS}}$

(iii) Cinnabar-${\text{HgS}}$

4. Among ${\text{Fe,Cu,Al}}$ and ${\text{Pb}}$, which metal (s) cannot be obtained by smelting.

Ans: Aluminum metal can not be obtained by smelting.

5. What is the thermodynamic criteria for the feasibility of a reaction? 

Ans: The thermodynamic criteria for the feasibility of a reaction is that Gibbs free energy should be negative. i.e. $\Delta G =  - ve$

6. Why can’t aluminum be reduced by carbon?

Ans: Aluminum can’t be reduced by carbon because aluminum is a stronger reducing agent than carbon.

7. Name the most important form of iron. Mention its one use. 

Ans: Cast iron is the most important form of iron and is used in making castings,  sleepers, toys etc.

8. Name the impurities present in bauxite ore.

Ans: The impurities present in bauxite ore are ${\text{Si}}{{\text{O}}_{\text{2}}}{\text{,F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}$and ${\text{Ti}}{{\text{O}}_{\text{2}}}$

9. What is the composition of copper matte.

Ans: Copper matte is composed of ${\text{C}}{{\text{u}}_{\text{2}}}{\text{S}}$ and ${\text{FeS}}$

10. Which form of copper is called blister copper.

Ans: The solidified copper obtained has blistered appearance due to evolution of sulfur dioxide so it is called blister copper. 

11. What are froth stabilisers. Give two examples

Ans: Froth stabilizers are the substances which help in stabilizing froth which is formed in froth floatation process. Two such examples are cresol and aniline.

12. A sample of galena is contaminated with zinc blend. Name one chemical which can be used to concentrate galena selectively by froth floatation method

Ans: The chemical used is sodium cyanide (${\text{NaCN}}$) which can be used to concentrated galena by froth floatation method.

13. What are the constituents of German silver.

Ans: The constituents of German silver are ${\text{Cu}} \to 25 - 30\% $,${\text{Zn}} \to 25 - 30\% $ and ${\text{Ni}} \to 40 - 50\% $

14. Why is froth floatation process selected for concentration of the sulphide ore?

Ans: Froth floatation process is selected for the concentration of sulphide ores as sulfide ore particles are wetted by oil but gangue particles are wetted by water.

15. Write the reaction involved in the extraction of copper from low grade ores.

Ans: The reaction involved in the extraction of copper from low grade ores is as follows

The process involved is leaching of ore with acid or bacteria.

$C{u^{2 + }}(aq) + {H_2}(g) \to Cu(pure) + 2{H^ + }$

16. Although aluminium is above hydrogen in the electrochemical series, it is stable in air and water. Why? 

Ans: Aluminum is above hydrogen in the electrochemical series yet it is stable in air and water due to formation of inert oxide ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}$

17. Which method of purification is represented by the following reaction 

$Ti + 2{I_2}(g)\xrightarrow{{523K}}Ti{I_4}(g)\xrightarrow{{1700K}}Ti(s) + 2{I_2}(g)$

Ans: The following method represents Van Arkel process which is used for refining zirconium or titanium.

18. Zinc is used but not copper for the recovery of metallic silver from the complex ${{\text{[Ag(CN}}{{\text{)}}_{\text{2}}}{\text{]}}^{\text{ - }}}$, although electrode potentials of both zinc and copper are less than that of ${\text{Ag}}$. Explain why?

Ans: Zinc has a faster reaction rate than that of copper and zinc is more cheaper than copper so zinc is used and copper is not used.

19. Write the composition of molten mixture which is electrolyzed to extract aluminum

Ans: The composition of the molten mixture which is electrolyzed to extract aluminum is molten ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + N}}{{\text{a}}_{\text{3}}}{\text{Al}}{{\text{F}}_{\text{6}}}$ or ${\text{Ca}}{{\text{F}}_{\text{2}}}$

Short Answer Questions.                                                                                 2 Mark

20. What is hydrometallurgy? Give one example where it is used for metal extraction.

Ans: Leaching followed by reduction is called hydrometallurgy. It is used in copper extraction.

21. Name the process for the benefaction/concentration of (i) an ore having lighter impurities (ii) sulphide ore.

Ans: 

(i) The process of concentration of an ore having lighter impurities is termed as gravity separation.

(ii) The process of concentration of a sulphide ore is termed as froth floatation.

22. Mention the role of cryolite in the extraction of aluminum. 

Ans: Cryolite has a property of high melting point and low conductivity. So ,it is used in reducing melting point and increases conductivity.

23. Mention the role of following: 

(a) ${\text{Si}}{{\text{O}}_{\text{2}}}$ in the metallurgy of ${\text{Cu}}$.

(b) $CaC{O_3}$in the metallurgy of $Fe$. 

(c) ${\text{CO}}$in the metallurgy of iron 

(d) ${{\text{I}}_{\text{2}}}$in the purification of zirconium

Ans:

(a) ${\text{Si}}{{\text{O}}_{\text{2}}}$ functions as a flux in the metallurgy of ${\text{Cu}}$

(b) $CaC{O_3}$ is decomposed to ${\text{CaO}}$ which acts as a flux.

(c) ${\text{CO}}$ acts as a reducing agent in the metallurgy of iron.

(d) ${{\text{I}}_{\text{2}}}$ forms a volatile complex with ${\text{Zr}}$

24. Extraction of copper directly from sulphide ore is less favourable than from its oxide through reduction. Explain.

Ans: The reactions are as follows

$2Cu(s) + C(s) \to C{S_2}(s) + 2Cu(s)$

$CuO(s) + C(s) \to CO(g) + Cu(g)$

In the second reaction Gibbs free energy is more negative than first so the process is more spontaneous and occurs readily.

25. The graphite electrodes in the extraction of ‘alluminium’ by Hall-Heroult process need to be changed frequently. Why?

Ans: During the electrolysis, aluminium is deposited at the cathode and oxygen is liberated at the anode and sometimes its react with the carbon in the graphite to form carbon-dioxide, so anode burnt away slowly.  Thus, the anodes have to be replaced periodically.

26. Write the chemical formulae of the following ores (a) Haematite (b) Magnetite (c) Limonite (d) Siderite.

Ans: The chemical formulae of the ores are as follows

(a) Haematite:${\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}$

(b) Magnetite:${\text{F}}{{\text{e}}_{\text{3}}}{{\text{O}}_{\text{4}}}$

(c)Limonite:${\text{F}}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}}$

(d) Siderite:${\text{FeC}}{{\text{O}}_{\text{3}}}$

27. Give equations for the industrial extraction of zinc from calamine. 

Ans: The equation for the industrial extraction of zinc from calamine is as follows

$ZnC{O_3} \to ZnO + C{O_2}$(calcination)

$ZnO + C \to Zn + CO$(reduction)

28. Name the elements present in anode mud during refining of copper. Why does it contain such elements? 

Ans: The elements present in anode mud during refining of copper is ${\text{Au,Ag}}$.These elements are not oxidized at anode and are less electropositive than copper.

29. Write the Chemical reactions taking place in different zones in the blast furnace for the extraction of iron from its ore.

Ans: The chemical reactions taking place in different zones in the blast furnace for the extraction of iron from its ore is as follows:

$3F{e_2}{O_3} + CO \to 2F{e_3}{O_4} + C{O_2}$

$F{e_3}{O_4} + 4CO \to 3Fe + 4C{O_2}$ (in the temperature range of ${\text{500 - 800}}\;{\text{K}}$)

$F{e_2}{O_3} + CO \to 2Fe + C{O_2}$

$C + C{O_2} \to 2CO$

$FeO + CO \to Fe + C{O_2}$ (in the temperature range of ${\text{900 - 1500}}\;{\text{K}}$)

30. How are impurities separated from bauxite ore to get pure alumina? 

Ans: The impurities are separated from bauxite ore to get alumina by leaching process. The reaction is as follows

$A{l_2}{O_3}(s) + 2NaOH(aq) + 3{H_2}O(l) \to 2Na[Al{(OH)_4}](aq)$

$2Na[Al{(OH)_4}](aq) + C{O_2}(aq) \to A{l_2}{O_3}.x{H_2}O(s) + 2NaHC{O_3}(aq)$

$A{l_2}{O_3}.x{H_2}O \to A{l_2}{O_3}(s) + x{H_2}O(g)$ (alumina)

31. Why is the reduction of a metal oxide easier if metal formed is in liquid state at the temperature of reduction?

Ans: In the liquid state of a metal, the entropy is more positive as compared to the solid state so that the Gibbs free energy becomes more negative and the reduction process occurs readily.

32. What is pyrometallurgy? Explain with one example.

Ans: Pyrometallurgy is the process of reduction of a metal oxide by heating with coke or other reducing agent like aluminum, magnesium etc.$ZnO + C\xrightarrow{{975K}}Zn + CO$

33. Write the method to produce Copper matte from copper pyrites

Ans: The method to produce copper matte from copper pyrites is froth floatation process.

34. Copper can be extracted by hydrometallurgy but not zinc. Explain why?

Ans: Copper can be extracted by hydrometallurgy but not zinc because the reduction potential of zinc is lower than that of copper. So, that copper can be reduced from its solution by zinc.     

35. State the principles of refining of metal by the following methods.

(a) Zone refining 

(b) Electrolytic refining

(c) Vapour phase refining

Ans: 

(a) The principle attached with zone refining is that impurities are more soluble in molten state than in the solid state of metal.

(b) The principle attached with electrolytic refining is the tendency of pure metal to deposit on the cathode by passing electricity.

(c) The principle attached with vapour phase refining is that impure metal forms volatile compound which easily decomposes to pure metal on further heating.

36. How is pure copper obtained from its principle ore? Write the chemical reactions occurring during the extraction.

Ans: Pure copper is obtained from its principle ore by following steps

$2CuFe{S_2} + {O_2} \to C{u_2}S + 2FeS + {O_2}$

$C{u_2}S + 3{O_2} \to 2C{u_2}O + 2S{O_2}$

$C{u_2}O + C \to 2Cu + CO$

$2C{u_2}O + C{u_2}S \to 6Cu + S{O_2}$

37. Name the method of refining of the following metals –

(a) Hg 

(b) Sn

(c) Cu 

(d) Ge 

(e) Ni 

(f) Zr

Ans: The method of refining of the metals is as follows:

(a) Distillation

(b) Liquation

(c) Electrolytic refining

(d) Zone refining

(e) Mond process

(f) Van Arkel process

38. The native silver forms a water -soluble compound (B) with dilute aqueous solution of ${\text{NaCN}}$in the presence of a gas (A). The silver metal is obtained by the addition of a metal (C) to (B) and complex (D) is formed as a byproduct. Write the structures of (C) and (D) and identify (A) and (B) in the following sequence –$Ag + AgCN + [A] + {H_2}O \to [B] + O{H^ - } + N{a^ + }$ 

$[C] + [B] \to [D] + Ag$ 

Ans: The structures of A, B, C, D are as follows

${\text{[A] = }}{{\text{O}}_{\text{2}}}$

${\text{[B] = Na[Ag(CN}}{{\text{)}}_{\text{2}}}{\text{]}}$

${\text{[C] = Zn}}$

$[D] = N{a_2}[Zn{(CN)_4}]$

39. In the cynamide extraction process of silver from argentite ore, name the oxidising and reducing agents. Write the chemical equations of the reactions involved.

Ans: In the extraction of silver from argentite ore, the reactions are as follows

$4Ag + 8NaCN + 2{H_2}O + {O_2} \to 4Na[Ag{(CN)_2}] + 4NaOH$

$2{[Ag{(CN)_2}]^ - }(aq) + Zn(s) \to {[Zn{(CN)_4}]^{2 - }}(aq) + 2Ag(s)$

The oxidizing agent is ${{\text{O}}_{\text{2}}}$ and the reducing agent is ${\text{Zn}}$

The important questions and answers discussed above are prepared by our subject matter experts for Chemistry. These questions and solutions are prepared after thorough research on the topics covered in the chapter and in close reference with the prescribed textbooks for Class 12 Chemistry. The required balanced equations for the respective chemical reactions are provided in the answers. So you can refer to these important questions for a quick revision of the key concepts covered in the chapter before the exams. 

Also, you can refer to our NCERT Solutions for Class 12 Chemistry Chapter 6 General Principles and Processes of Isolation and other study resources of the chapter for your exam preparation.  

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