Contents
Important Questions for CBSE Class 12 Chemistry – General Principles And Processes Of Isolation Of Elements
Previous Years Questions
2015
Short Answer Type Questions [II] [3 Marks]
Question 1. (i) Name the method used for the refining of titanium.
(ii) What is the role of Zn in the extraction of silver?
(iii) Reduction of metal oxide to metal becomes easier if the metal obtained is in liquid state. Why?
Answer: (i) van Arkel method is used.
(ii) Zn is used as a reducing agent.
(iii) It is because AS is more positive, AG is more negative, therefore, reduction is easier.
Question 2. (i) Name the method of refining to obtain low boiling metal like mercury,
(ii) What is the role of pine oil in froth floatation process?
(iii) Reduction of metal oxide to metal becomes easier if the metal obtained is in liquid state. Why?
Answer: (i) Distillation.
(ii) It acts as collector, which enhances the non-wettability of mineral particles.
(iii) Refer Ans. to Q.l(iii).
Question 3. (i) Indicate the principle behind the method used for the refining of zinc,
(ii) What is the role of silica in the extraction of copper?
(iii) Which form of the iron is the purest form of commercial iron?
Answer: (i) Impure zinc will change into Zn+2 ions which are attracted towards cathode and change into pure zinc. Impurities are left behind.
(ii) FeO is gangue. Si02 acts as flux to remove FeO by chemical reaction in the form of slag.
FeO + Si02 ——-> FeSi03.
(iii) Wrought iron.
Question 4. (i) Name the method of refining to obtain silicon of high purity.
(ii) What is the role of SiOz in the extraction of copper?
(iii) What is the role of depressants in froth floatation process?
Answer: (i) Zone refining method.
(ii) Si02 acts as a flux. It reacts with gangue (FeO) to form slag (iron silicate)
FeO + Si02 ———> FeSiO3
(iii) Depressants help to separate two sulphide ores by froth floatation process, e.g. NaCN does not allow ZnS to enter froth but allows PbS.
Question 5. (i) Indicate the principle behind the method used for the refining of Nickel.
(ii) What is the role of dilute NaCN in the extraction of gold?
(iii) What is ‘copper matte’?
Answer: (i) Impure Ni reacts tvith CO (carbon monoxide) to form volatile Ni(CO)4 which decomposes at higher temperature to form pure Ni.
(ii) NaCN forms a complex with gold, Na[Au(CN)2], which on reduction with Zn gives gold metal.
(iii) Copper matte consists of Cu2S and FeS.
2014
Very Short Answer Type Questions [1 Mark]
Question 6. Which reducing agent is employed to get copper from the leached low grade copper ore?
Answer: Iron
Fe(s) + Cu+2(aq) ——–> Fe+2 + Cu(s)
Question 7. What is the role of zinc metal in the extraction of silver?
Answer: Refer Ans. to Q. 1 (ii)
Question 8. Name the method that is used for refining of nickel.
Answer: Mond’s process (vapour phase refining).
Question 9. Name the method used for refining of copper metal.
Answer: Electrolytic refining.
Question 10. What is the function of collectors in the froth floatation process for the concentration of ores?
Answer: Collectors enhance non-wettability of sulphide minerals, e.g. fatty acids, pine oil, xanthates.
Question 11. On what principle is the method of zone refining of metals based?
Answer: It is based on the principle that impurities are more soluble in melt than pure metal.
Question 12. Name the depressant which is used to separate ZnS and PbS ores in froth floatation process.
Answer: Sodium cyanide (NaCN).
Short Answer Type Questions [I] [2 Marks]
Question 13. Outline the principles behind the refining of metals by the following methods:
(i) Zone refining method
(ii) Chromatographic method.
Answer: (i) Zone refining method: It is based on the principle that impurities are more soluble in melt (molten state) than in pure metal.
(ii) Chromatographic method: It is based on the principle of differential adsorption due to which components get separated.
Question 14. Write the principle behind the following methods of refining:
(i) Hydraulic washing
(ii) Vapour phase refining
Answer: (i) Hydraulic washing: It is based on difference in the density of mineral and gangue in an ore.
(ii) Vapour phase refining: In this method, metal is converted into volatile compound which on heating gives pure metal, whereas impurities do not react.
Question 15. Write the principles of the following methods:
(i) Froth floatation method
(ii) Electrolytic refining
Answer: (i) Sulphide minerals become wet by oils, whereas gangue particles by water. (ii) Impure metal changes into ions which gain electrons at cathode to form pure metal, whereas impurities are left as anode mud.
Question 16. Explain the principle of the method of electrolytic refining of metals. Give one example.
Answer: Electrolytic refining: Impure metal changes into ions in aqueous solution of its salt which are attracted towards pure metal (cathode) and get deposited. Example: Impure copper is taken as anode. Pure copper is taken as cathode. Acidified CuS04 solution is taken as electrolyte.
Question 17. Describe the role of the following:
(i) Si02 in the extraction of copper from copper matte.
(ii) NaCN in froth floatation process
Answer: (i) Refer Ans. to Q.4(ii).
(ii) It acts as ‘depressant’ and separates ZnS and PbS. It allows PbS to form froth but prevents ZnS.
Question 18. Write the principle behind the froth floatation process. What is the role of collectors in this process?
Answer: It is based on the principle that mineral particles become wet by oil, whereas gangue particles by water.
Collectors, such as pine oil, fatty acid, xanthates enhance the non-wettability of the mineral particles.
Question 19. Write the chemical reactions involved in the extraction of silver from silver ore.
Answer:
Question 20. Write the role of the following:
(i) CO in the purification of nickel
(ii) Graphite rod in the electrometallurgy of aluminium.
Answer: (i) CO reacts with impure Ni to form Ni(CO)4 which decomposes to form pure Ni on heating.
(ii) Graphite rod is a conductor, therefore, it is used as electrodes in the metallurgy of aluminium.
Question 21. Write the role of the following:
(i) Iodine in the refining of zirconium
(ii) Silica in the extraction of copper from copper matte.
Answer: (i) I2 reacts with impure ‘Zr’ to form Zrl4 which on heating decomposes to form pure ‘Zr’.
(ii) Refer Ans. to Q.4(ii).
2013
Very Short Answer Type Questions [1 Mark]
Question 22. What is the composition of ‘Copper matte’?
Answer: It contains Cu2S and FeS.
Question 23. Name the method used for refining of germanium metal.
Answer: Zone refining is used for the refining of germanium metal.
Question 24. On what principle is chromatography based?
Answer: Chromatography is based on differential adsorption.
Question 25. Name the method used for the refining of titanium metal.
Answer: Titanium is purified by van Arkel Method.
Question 26. Name the method that is used for refining of nickel.
Answer: Refer Ans. to Q.8.
Question 27. Name the method used for refining of copper metal.
Answer: Refer Ans. to Q.9.
Short Answer Type Questions [I] [2 Marks]
Question 28. (a) Which solution is used for the leaching of silver metal in the presence of air in the metallurgy of silver?
(b) Out of C and CO, which is a better reducing agent at the lower temperature range in the blast furnace to extract iron from the oxide ore?
Answer: (a) Potassium cyanide solution is used for leaching of silver metal in presence of air in the metallurgy of silver.
(b) Carbon monoxide(CO) is better reducing agent at lower temperature range
Fe2O3+ CO —-> 2FeO + C02
Question 29. (a) Which of the following ores can be concentrated by froth floatation method and why?
Fe2O3, ZnS, Al2O3
(b) What is the role of silica in the metallurgy of Copper?
Answer: (a) ZnS can be concentrated by froth floatation process because sulphide ores are preferentially wetted by pine oil, whereas gangue particles are wetted by water.
(b) Refer Ans. to Q.4(ii).
Question 30. (а) Give an example of zone refining of metals.
(b) What is the role of cryolite in the metallurgy of aluminium?
Answer: (a) The impure germanium is heated with the help of circular heater at the one end of the rod. The molten zone is moved forward along with heater and reaches the other end which is discarded. Pure metal rod crystallizes out of the melt.
(b) Cryolite reduces the melting point of purified bauxite and increases electrical conductivity. Molten cryolite also acts as a solvent.
Question 31. (a) Name the method used for removing gangue from sulphide ores.
(b) How is wrought iron different from steel?
Answer: (a) Froth floatation process
(b) Wrought iron is the purest form of iron. Steel is an alloy of iron and 0.5
to 1.5% C (carbon).
Question 32. Outline the principles of refining of metals by the following methods:
(i) Zone refining
(ii) Vapour phase refining
Answer: (i) Refer Ans. to Q.13(i).
(ii) Refer Ans. to Q. 14(ii).
Question 33. Name the principal ore of aluminium. Explain the significance of leaching in the extraction of aluminium.
Answer: Bauxite is the principal ore of aluminium.
Leaching:
Question 34. How can you separate alumina (Al2O3) from silica present in bauxite ore? Write the chemical equations for the reactions involved.
Answer: The ore is concentrated by treating powdered ore with sodium hydroxide. Al203 goes into solution forming sodium meta-aluminate. Si02 also dissolved to form sodium silicate.
The solution is filtered to remove unsoluble impurities. Al(OH)3 is precipitated by passing C02 which lowers the pH. The solution can be seeded with the help of freshly prepared Al(OH)3, i.e. by adding a small amount of Al(OH)3, complete precipitation of Al(OH)3 can be done. The sodium silicate remains in the solution and gets separated.
Question 35. (a) What are the constituents of ‘copper matte’?
(b) What is the role of depressant in froth floatation process?
Answer: (a) Cu2S and FeS are the constituents of copper matte.
(b) Depressants are used to separate two sulphide ores. NaCN is used as depressant in separation of ZnS and PbS. It selectively prevents ZnS from coming to froth but allows PbS to come with froth.
2012
Very Short Answer Type Questions [1 mark]
Question 36. What is the role of graphite in the electrometallurgy of aluminium?
Answer: Graphite rod is a conductor, therefore, it is used as electrodes in the metallurgy of aluminium.
Question 37. How is copper extracted from a low grade ore of it?
Answer: Extraction of Copper from low grade ores and scraps: It is extracted by hydrometallurgy. It is carried out in two steps:
Leaching: Low grade copper ores and scraps are leached by using acid or bacteria.
Reduction: The solution containing copper ions is treated with H2.
Cu+2(aq) + H2(g) ———–> Cu(s) + 2H+(aq)
Question 38. What is the role of collectors in Froth Floatation process?
Answer: Refer Ans. to Q. 18.
Question 39. What is the role of a depressant in Froth Floatation process for the concentration of a sulphide ore?
Answer: Depressants are used to separate two sulphide ores. NaCN is used as depressant in separation of ZnS and PbS. It selectively prevents ZnS from coming to froth but allows PbS to come with froth.
Question 40. Write the reaction involved in the extraction of silver after the silver ore has been leached with NaCN.
Answer:
Short Answer Type Questions [I] [2 Marks]
Question 41. Describe the principle involved in each of the following processes:
(i) Mond process for refining of Nickel.
(ii) Column chromatography for purification of rare elements.
Answer: (i) Mond Process: Nickel is purified by Mond’s process. Nickel, when heated in stream of carbon monoxide, forms volatile Ni(CO)4 which decomposes to pure nickel.
(ii) Column Chromatographic Method: It is based on the principle of separation or purification by chromatography which is based on differential adsorption on an adsorbent. In column chromatography, Al203 is used as an adsorbent.
The mixture to be separated is taken in a suitable solvent and applied on the column. They are then eluted out with suitable solvent (eluent). The weakly adsorbed component is eluted first, then the more strongly adsorbed and so on.
This method is suitable for those elements which are available only in minute quantities, i.e. rare elements and the impurities are not very much different in their chemical behaviour from the element to be purified.
Question 42. Describe the following:
(i) The role of cryolite in electro metallurgy of aluminium.
(ii) The role of carbon monoxide in the refining of crude nickel.
Answer: (i) Refer Ans. to Q.30 (b).
(ii) Refer Ans. to Q.20(i).
Question 43. Which methods are usually employed for purifying the following metals:
(i) Nickel (ii) Germanium
Mention the principle behind each one of them.
Answer: (i) Refer Ans. to Q.41 (i).
(ii) Zone refining: It is based on the principle that impurities are more soluble in the melt (molten state) than in the solid state of the metal. Germanium is purified by Zone refining.
Question 44. Explain the role of each of the following:
(i) NaCN in the extraction of silver.
(ii) Si02 in the extraction of copper.
Answer: (i) Ag2S + 4NaCN ——> 2Na[Ag(CN)2] + Na2S
NaCN forms a soluble complex with argentite, ore of silver.
(ii) Refer Ans. to Q.4(ii).
Question 45. Name one chief ore each of copper and aluminium. Name the methpd used for the concentration of these two ores.
Answer:
2011
Very Short Answer Type Questions [1 Mark]
Question 46. Differentiate between a mineral and an ore.
Answer: Mineral is a naturally occuring substance from which metal may or may not be extracted profitably.
Ore is a naturally occuring rocky material which contains sufficient quantity of mineral from which metal can be extracted profitably.
Question 47. Why is it that only sulphide ores are concentrated by ‘froth floatation’ process?
Answer: It is because sulphide ores are preferentially wetted oil whereas gangue by water.
Question 48. What type of ores can be concentrated by magnetic separation method?
Answer: Those ores which are magnetic in nature, i.e. attracted by magnet, whereas impurities are non-magnetic in nature or vice versa are concentrated by magnetic separation-.
Short Answer Type Questions [i] [2 Marks]
Question 49. Describe the principle controlling each of the following processes:
(i) Preparation of cast iron from pig iron.
(ii) Preparation of pure alumina (Al203) from bauxite ore.
Answer: (i) Cast iron is made by melting pig iron with scrap iron and coke using hot air blast. It has slightly lower carbon content (about 3%) and is extremely hard and brittle.
(ii) Bauxite contains Si02 and Ti02 as impurities. The powdered ore is heated with caustic soda (NaOH) at 473-523 K at 35-36 bar pressure. Sodium hydroxide reacts with bauxite ore to form sodium tetrahydroxoaluminate. Silica forms sodium silicate, whereas other insoluble impurities can be filtered off.
Al203(S) + 2NaOH(aq) + 3H20(l) ———> 2Na[Al(OH)4](aq)
Question 50. Describe the principle controlling each of the following processes:
(i) Vapour phase refining of titanium metal.
(ii) Froth floatation method of concentration of a sulphide ore.
Answer:
Question 51. Describe the principle controlling each of the following processes:
(i) Zone refining of metals
(ii) Electrolytic refining of metals.
Answer: (i) Refer Ans. to Q.13 (i).
(ii) Refer Ans. to Q.15 (ii).
Short Answer Type Questions [II] [3 Marks]
Question 52. State the principle on which each of the following processes operates:
(i) Recovery of silver after the silver ore has been leached with NaCN.
(ii) Electrolytic refining of a metal.
(iii) Vapour phase refining of metal.
Answer: (i) Refer Ans. to Q.40.
(ii) Refer Ans. to Q.15 (ii).
(iii) Refer Ans. to Q.14 (ii).
Question 53. Describe the principle behind each of the following processes:
(i) Vapour phase refining of a metal.
(ii) Electrolytic refining of a metal.
(iii) Recovery of silver after silver ore was leached with NaCN.
Answer: (i) Refer Ans. to Q. 14(iii).
(ii) Refer Ans. to Q.15 (ii).
(iii) Leaching (Chemical separation): It is a process in which an ore is treated with a suitable reagent w’hich dissolves ore but not the impurities.
Recovery of silver after silver ore was leached with NaCN:
Ag2S + 4NaCN ——-> 2Na[Ag(CN)2] + Na2S
2Na[Ag(CN)2] + Zn ——-> Na2[Zn (CN)4] + 2Ag
Question 54. Explain the role of each of the following in the extraction of metals from their ores:
(i) CO in the extraction of nickel.
(ii) Zinc in the extraction of silver.
(iii) Silica in the extraction of copper.
Answer: (i) Impure nickel reacts with CO to form tetracarbonyl nickel Ni(CO)4which on heating forms pure nickel.
(ii) Zn acts as a reducing agent in the extraction of Ag.
(iii) Refer Ans. to Q.3(ii).
Question 55. Write the reactions involved in the following processes:
(i) Leaching of bauxite ore to prepare pure alumina.
(ii) Refining of zirconium by van Arkel method.
(iii) Recovery of gold after gold ore has been leached with NaCN solution.
Answer: (i) Refer Ans. to Q.33.
(ii) van Arkel Method: It is used to get ultra pure metals. Zirconium (Zr) is purified by this process. Zr is heated in iodine vapours at about 870 K to form volatile Zrl4 which is heated over tungsten filament at 2075 K to give pure Zr.
(iii) Extraction of gold.
In this reaction, zinc acts as a reducing agent.
Question 56. State the principles of the following methods of refining crude metals:
(i) Zone refining
(ii) Liquation method
(iii) Chromatographic method
Answer: (i) Zone refining: It is based on the principle that impurities are more soluble in the melt (molten state) than in the solid state of the metal.
The impure metal is heated with the help of circular heaters at one end of the rod of impure metal. The molten zone is moved forward along with impurities and reaches the other end and is discarded. Pure metal crystallises out of the melt.
The process is repeated several times and heater is moved in the same direction. It is used for purifying semiconductors like B, Ge, Si, Ga and In.
(ii) Liquation Method: Those metals which have impurities, whose melting points are higher than metal, can be purified by this method. For example, in this method, Tin (Sn) metal can be purified. Tin containing iron as impurity is heated on the top of sloping furnace. Tin melts and flows down the sloping surface where iron is left behind and pure tin is obtained.
(iii) Chromatographic Method: It is based on the principle of separation or purification by chromatography which is based on differential adsorption on an adsorbent. In column chromatography, Al2Os is used as an adsorbent.
2010
Very Short Answer Type Question [1 Mark]
Question 57. Copper matte is charged into a silica lined converter in extraction of copper. What is the role of silica lining here?
Answer: Silica acts as a flux. It reacts with gangue (FeO) to from FeSi03 (slag) which is easily removed.
Question 58. How is chemical reduction different from electrolytic reduction? Name a metal each which is obtained by
(i) electrolytic reduction, (ii) chemical reduction.
Answer: In chemical reduction, metal is obtained by the reduction of its ore by a suitable reducing agent, whereas in electrolytic reduction, reduction is carried by passing electric current through molten ore.
(i) Aluminium is obtained by electrolytic reduction.
(ii) Iron is obtained by chemical reduction.
Short Answer Type Questions [II] [3 Marks]
Question 59. Describe how the following changes are brought about:
(i) Pig iron into steel.
(ii) Zinc oxide into metallic zinc.
(iii) Impure titanium into pure titanium.
Answer: (i) Pig iron contains 4% C and small amount of S, P, Si and Mn. It is heated strongly in Bassemer converter in presence of oxygen so that C gets oxidised to C02, P to P2O3, S to S02 which are removed as gases. Si changes to Si02 and Mn changes to MnO. Both react to form MnSiO3 which is removed as slag. Thus, we get wrought iron. Further, we add 0.5% of carbon and heat to get steel.
(ii) Extraction of zinc from zinc oxide: The reduction of zinc oxide is carried ‘ out by coke at 673 K.
The oxide is heated with coke.
Carbon monoxide formed can also reduce ZnO further to zinc.
(iii) Ti is purified by van Arkel method. Ti is heated with iodine vapours at about 870 K to form volatile Til4 which is heated over tungsten filament at 2075 K to give pure Ti.
Question 60. Describe the role of
(i) NaCN in the extraction of gold from gold ore.
(ii) Si02 in the extraction of copper from copper matte.
(iii) Iodine in the refining of zirconium.
Write chemical equations for the involved reactions.
Answer:
Question 61.Describe the role of the following:
(i) NaCN in the extraction of silver from a silver ore.
(ii) Iodine in the refining of titanium
(iii) Cryolite in the metallurgy of aluminium.
Answer:
Question 62. Describe how the following changes are brought about:
(i) Pig iron into steel
(ii) Bauxite into pure alumina
(iii) Impure copper into pure copper
Answer: (i) Refer Ans. to Q.59 (i).
(ii) Purification of Bauxite by Leaching (Bayer’s process): Bauxite ore is treated with caustic soda. Al2Os dissolves in concentrated solution of caustic soda leaving behind impurities.
Al203(s) + 2OH–(aq) + 3H2O(l) ——-> 2Al(OH)4 (aq)
(iii) Electrolytic refining: In this method, impure copper is taken as anode, pure metal is taken as cathode and acidified CuS04 solution is used as an electrolyte. When electric current is passed, impure copper forms copper ions which are discharged at cathode forming pure copper.
Question 63. Give reasons for the following:
(i) Alumina is dissolved in cryolite for electrolysis instead of being electrolysed directly.
(ii) Zinc oxide can be reduced to the metal by heating with carbon but not Cr2O3.
(iii) Extraction of copper directly from sulphide ores is less favourable than that from its oxide ore through reduction.
Answer: (i) It is done so as to reduce its melting point and increase electrical conductivity.
(ii) It is because ‘Cr’ is stronger reducing agent than ‘Zn’.
(iii) It is because it is easier to reduce oxide to get metal than from sulphide. AG is more negative in the reduction of oxide than of sulphide directly.
Question 64. Describe the principle involved in each of the following processes of metallurgy:
(i) Froth floatation method
(ii) Electrolytic refining of metals
(iii) Zone refining of metals.
Answer: (i) Refer Ans. to Q. 15 (i).
(ii) Refer Ans. to Q. 15 (ii).
(iii) Refer Ans. to Q.13 (i).
2009
Very Short Answer Type Questions [1 Mark]
Question 65. What is meant by the term ‘pyrometallurgy’?
Answer: The process of reducing a metal oxide with coke or any other reducing agent at high temperature is called pyrometallurgy. It is a thermal process of extracting a metal from its ore.
Question 66.Name two metals which occur in nature as oxides.
Answer: (i) Iron occurs as Fe203 (Haematite)
(ii) Aluminium occurs as Al203.2H20 (Bauxite)
Question 67. Why is the froth floatation method selected for the concentration of sulphide ores?
Answer: It is because sulphide ores are preferentially wetted by oil, whereas gangue by water.
Short Answer Type Questions [I] [2 Marks]
Question 68. Explain the role of
(i) Cryolite in the electrolytic reduction of alumina.
(ii) Carbon monoxide in the purification of nickel.
Answer: (i) Refer Ans. to Q.30 (b).
(ii) Refer Ans. to Q.20 (i).
Question 69. Describe the underlying principle of each of the following metal refining methods:
(i) Electrolytic refining of metals
(ii) Vapour phase refining of metals
Answer: (i) Refer Ans. to Q.15 (ii).
(ii) Refer Ans. to Q.14 (ii).
Question 70. Describe the role of the following:
(i) NaCN in the extraction of silver from a silver ore.
(ii) Cryolite in the extraction of aluminium from pure alumina.
Answer: (i) It forms a complex with silver ore which can be easily reduced by zinc to get silver metal.
(ii) Refer Ans. to Q.30 (b).
Question 71. Describe the role of the following:
(i) NaCN in the extraction of silver.
(ii) CO in the purification of nickel.
Answer: (i) Refer Ans. to Q.44 (i).
(ii) Refer Ans. to Q.20 (i).
Question 72. State the principles on which the following operations are based:
(i) Zone refining
(ii) Vapour phase refining
Answer: (i) Refer Ans. to Q.13 (i).
(ii) Refer Ans. to Q.14 (ii).