General Principles and Processes of Isolation of Elements Class 12 Chemistry Exam Questions

Exam Questions Class 12

Please refer to the below General Principles and Processes of Isolation of Elements important questions for Class 12 Chemistry. These questions and answers have been prepared as per the latest NCERT Book for Class 12 Chemistry. Students should go through chapter wise Class 12 Chemistry Important Questions designed as per the latest examination pattern issued by CBSE.

Very Short Answer Questions :

Question. What is the role of limestone in the extraction of iron from its oxides? 
Answer : Limestone decomposes to form CaO and CO2.
CaO thus formed acts as a flux and combines with silica (present as impurity) to form fusible calcium silicate slag.
    CaCO3 → CaO + CO2
    CaO + SiO2 → CaSiO3
        (Silica)        (slag)

Question. What type of ores can be concentrated by magnetic separation method? 
Answer : If either the ore or the gangue particles are capable of being attracted by magnetic field, then such ore is concentrated by magnetic separation method.

Question. Which reducing agent is employed to get copper from the leached low grade copper ore?
Answer : Scrap iron or H2.

Question. Although thermodynamically feasible, in practice, magnesium metal is not used for reduction of alumina in the metallurgy of aluminium. Why?
Answer :  Below the temperature (1623K) corresponding to the point of intersection of Al2O3 and MgO curves in ellingham diagram magnesium can reduce alumina. But magnesium is a much costlier metal than aluminium and hence the process will be uneconomical.

Question. Write the principle of the following method. Froth floatation method 
Answer : The principle behind this method is that the sulphide ore is preferentially wetted by oils while gangue is preferentially wetted by water.
Then on agitation froth is formed that carries mineral particles and can be skimmed off.

Question. Write the role of graphite rod in the electrometallurgy of aluminium.
Answer : Graphite anode is used in the electrometallurgy of aluminium from alumina, Al2O3.
Oxygen liberated at high temperature reacts with graphite to form both CO2 and CO gases and prevent the liberation of O2 gas at the anode which may react with Al metal to give Al2O3 again.
        At anode : C(s) + O2– (melt) → CO(g) + 2e
        C(s) + 2O2– (melt) → CO2(g) + 4e

Question. What is ‘copper matte’? 
Answer : Copper matte is mixture of Cu2S and FeS.

Question. Why does copper obtained in the extraction from copper pyrites have a blistered appearance?
Answer : Copper obtained in the extraction from copper pyrites has a blistered appearance due to the evolution of SO2.
2Cu2S + 3O2 → 2Cu2O + 2SO2
2Cu2O + Cu2S → 6Cu + SO2

Question. Name the chief ores of aluminium and zinc.
Answer : The chief ore of aluminium is bauxite and that of zinc is zinc blende or sphalerite.

Question. What is the function of SiO2 in the metallurgy of copper? 
Answer : During the metallurgy of copper from copper pyrites, CuFeS2, its roasting gives FeO besides Cu2O and SO2.

Question. What types of metals are usually purified by the method of zone refining? Give an example.
Answer : The metals such as germanium, silicon, gallium, etc. which are used as semiconductors are purified by zone refining which is based upon the principle that impurities are more soluble in the melt than in the solid state of metals.

Question. Give reason for the following :
Zinc oxide can be reduced to metal by heating with carbon but Cr2O3 cannot be reduced by heating with carbon.
Answer :  Carbon is suitable reducing agent for reduction of zinc oxide. Reduction of Cr2O3 by carbon is not thermodynamically favourable.

Question. Describe the role of the following :
NaCN in froth floatation process
Answer : NaCN is used as a depressant in froth floatation process which selectively prevents ZnS from coming to the froth but allows PbS to come with the froth.

Question. Describe the principle controlling the following process :
Vapour phase refining of titanium metal.
Answer : In this method, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal.
Ti + 2I2523 K  TiI4
TiI4 →1700 K Ti + 2I2

Question. Indicate the principle behind the method used for the refining of zinc. 
Answer : Zinc is refined by electrolytic refining.
In this method, the impure metal is made to act as anode. A strip of the same metal in pure form is used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the same metal. The more basic metal remains in the solution and the less basic ones go to the anode mud.
At anode : Zn → Zn2+ + 2e
At cathode : Zn2+ + 2e → Zn

Question. Write the principle of the following :
Froth floatation process
Answer : Froth floatation process : This method is based on the principle that the ore particles are preferentially wetted by oil while gangue particles are wetted by water. This principle is applied mainly to concentrate sulphide ores.

Question. Describe the principle controlling the following process :
Preparation of pure alumina (Al2O3) from bauxite ore. 
Answer : Al2O3 present in bauxite is soluble in concentrated NaOH solution whereas impurities are not, Al2O3 is reprecipitated from the solution.

Question. Name the principal ore of aluminium.
Answer : The chief ore of aluminium is bauxite and that of zinc is zinc blende or sphalerite.

Question. What is the role of coke in the extraction of iron from its oxides? 
Answer : Coke reduces iron oxide to iron.
FeO(s) + C(s) → Fe(s/l) + CO(g)

Question. What is the function of collectors in the froth floatation process for the concentration of ores.
Answer : Collectors enhance non-wettability of the mineral particles. Pine oil can be used as collectors.

Question. Write the reaction involved in the following process :
Recovery of gold after gold ore has been leached with NaCN solution. 
Answer : Gold is recovered from [Au(CN)2]  complex by displacement method using a more electropositive metal zinc.
2[Au(CN)2](aq) + Zn(s) → 2Au(s) + [Zn(CN)4]2–(aq) 

Question. Name the methods used for refining of following metals.
(i) Nickel (ii) Titanium
Answer : The role of CO is to convert impure nickel to volatile nickel tetracarbonyl whereas the impurities are left behind in the solid state. The vapour of nickel tetracarbonyl on heating at 450-470 K decomposes to give pure nickel metal and carbon monoxide.

Question. Name the depressant which is used to separate ZnS and PbS ores in froth floatation process.
Answer : NaCN is used to separate ZnS and PbS ores in froth floatation process.

Question. Why is it that only sulphide ores are concentrated by froth floatation process?
Answer : This is because the sulphide ore particles becomes lighter when preferentially wetted by oil and rise to surface along with the froth, whereas gangue particles are preferentially wetted by water become heavier and settle down.

Question. How is copper extracted from a low grade ore of it? 
Answer : Copper is extracted by hydrometallurgy from low grade ores. It is leached out using acid or bacteria. The solution containing Cu2+ is treated with scrap iron or H2.
          Cu2+(aq) + H2(g) → Cu(s) + 2H+(aq)

Question. 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 : Ellingham diagram for oxides of metals and carbon shows
    C(s) + O2(g) → CO2(g)                                                   (i)
    2C(s) + O2(g)  → 2CO(g)                                                (ii)
    2CO(g) + O2(g)  → 2CO2(g)                                            (iii)
that at 673 K, ΔfG°(CO2) < ΔfG°(CO) hence
formation of CO2 gas is more favourable than CO gas. Hence, out of C and CO, CO is a better reducing agent.

Question. What role is played by CO2 in getting pure alumina (Al2 O3) in the extraction of aluminium?
Answer : The aluminate in solution is neutralised by passing CO2 gas and hydrated Al2O3 is precipitated.
2Na[Al(OH)4](aq) + CO2(g) → Al2O3.xH2O(s) + 2NaHCO3(aq).

Question. Describe the role of NaCN in the extraction of gold from gold ore. 
Answer : NaCN is used for leaching of gold ore in the presence of air to form soluble gold complex from which metal is displaced by adding more reactive metal.
4Au(s) + 8CN(aq) + 2H2O(l) + O2(g) → 4[Au(CN)2](aq) + 4OH(aq)

Question. Name the method of refining of nickel.
Answer : Mond’s process : Impure nickel is treated with carbon monoxide at 60-80°C to form volatile nickel carbonyl which decomposes to give pure nickel at 180°C.
      Ni + 4CO → Ni(CO)4 → Ni + 4CO
     Impure                         Pure

Short Answer Questions :

Question. What is meant by the term, ‘Chromatography’ What criterion is followed for the selection of the stationary phase in chromatography?
Answer : The term chromatography was derived from the Greek word “chroma”, meaning colour and “graphy”, for writing. It was used for the first time to separate the colour pigments of plants. Chromatography is a technique for analysing or separating mixtures of gases, liquids or dissolved substances.
The criterion of selecting, the stationary phase depends on the different adsorbing powers of the components of a mixture to be separated.
Generally used adsorbent material is silica gel or alumina. The adsorbent can be spread over a flat glass plate (thin – layer chromatography) or taken in a glass tube (column chromatography).
In paper chromatography, the stationary phase is water held in the pores of the paper. Here, the separation depends upon the different distribution of the components between water in stationary phase and the element.

Question. Describe how the following changes are brought about :
(i) Pig iron into steel
(ii) Zinc oxide into metallic zinc.
Answer : (i) Pig iron is mixed with scrap iron and heated in a furnace. After removal of impurities required quantity of spiegel is added to make steel.
(ii) Zinc oxide is mixed with coke and clay and brickettes are made. The brickettes are heated at 1673 K to give metallic zinc.
          ZnO(s) + C(s)1673 K Zn(s) + CO(g)
(ii) Free energy change for the reduction of copper sulphide to copper by carbon is positive.
whereas, ΔrG° for the reduction of copper oxide to copper by carbon is negative and hence feasible.

Question. (a) Out of PbS and PbCO3 (ores of lead), which one is concentrated by froth floatation process preferably ?
(b) What is the significance of leaching in the extraction of aluminium?
Answer : (a) Sulphide ores are concentrated by froth floatation process preferebly, hence, PbS is concentrated more preferebly than PbCO3 by this method.
(b) The significance of leaching in the extraction of aluminium is to concentrate pure alumina (Al2O3) from bauxite ore.

Question. How can you obtain pure alumina (Al2 O3) from a bauxite ore? Give necessary reaction involved.
Answer (i) Leaching of bauxite ore to prepare pure alumina : 

Question. The extraction of gold by leaching with NaCN involves both oxidation and reduction. Justify giving chemical equations. 
Answer : During the leaching process, Au is first oxidised to Au+ by O2 of the air which then combines with CN ions to form the soluble complex, sodium dicyanoaurate (I).
    4Au(s) + 8NaCN(aq) + 2H2O(l) + O2(g)
 Gold (Impure)
                 → 4Na[Au(CN)2](aq) + 4NaOH(aq)
                    Sod. dicyanoaurate (I)
                    (Soluble complex)
Gold is then extracted from this complex, by displacement method using a more electropositive zinc metal. In this reaction,
Zn acts as a reducing agent. It reduces Au+ to Au while itself gets oxidized to Zn2+ which combines with CNions to form soluble complex, sodium tetracyanozincate (II).
            2Na[Au(CN)2](aq)+ Zn(s) → 2Au(s) + Na2[Zn(CN)4](aq)
                                                       Gold                    Sod. tetracyanozincate (II)
Thus, extraction of Au by leaching with NaCN involves both oxidation and reduction.

Question. Give an example of zone refining of metals.
Answer : The metals such as germanium, silicon, gallium, etc. which are used as semiconductors are purified by zone refining which is based upon the principle that impurities are more soluble in the melt than in the solid state of metals.

Question. What are the chief ores of zinc? Write chemical reactions taking place in the extraction of zinc from zinc blende. 
Answer : The chief ores of zinc are
(i) Zinc blende, ZnS
(ii) Calamine, ZnCO3
(iii) Zincite, ZnO.
Extraction of zinc : From zinc blende zinc is extracted by roasting followed by reduction with coke.
(a) Roasting : The concentrated ore is heated with oxygen at 900°C in reverberatory furnace to convert zinc sulphide to zinc oxide.
                    2ZnS + 3O2 → 2ZnO + 2SO2
                  (Zinc blende)        (Zinc oxide)
(b) Reduction : The reduction of zinc oxide is done using coke.
                    ZnO + C →1673 K Zn + CO
The metal is distilled off and collected by rapid chilling.

Question. Write all the reactions involved in the extraction of aluminium from bauxite ore. 
Answer : Aluminium metal is extracted from bauxite (A2O3·2H2O) in two steps.
Stage I (Leaching) : The ore is treated with sodium hydroxide solution. Aluminium oxide and silica dissolve to form sodium aluminate and sodium silicate respectively. Iron oxide and TiO2 is filtered off.

Stage II (Electrolysis) : The alumina is dissolved in molten cryolite Na3[AlF6] and then electrolysed in a large steel tank lined with graphite which acts as cathode. The anodes are made of carbon. On passing current, molten aluminium is produced at cathode and oxygen gas is evolved at the anode which reacts with carbon anode producing CO and CO2. The electrolytic reactions may be written as :
Cathode : 2Al3+ (melt) + 6e → 2Al(l)
Anode : C(s) + O2– → CO(g) + 2e
    C(s) + 2O2– → CO2(g) + 4e
The anode burns away. Therefore the graphite rods must be replaced from time to time.

Question. (a) Write the role of dilute NaCN in the extraction of silver.
(b) What is the role of the collectors in the froth floatation process? Give an example of a collector.
Answer : (a) In the extraction of silver, silver metal is leached with a dilute solution of NaCN in the presence of air from which the metal is obtained.
4Ag + 8NaCN + 2H2O + O2 →4 Na[Ag(CN)2] + 4 NaOH
(b) The role of collectors in the froth floatation process is to enhance non-wettability of the ore particles e.g., pine oil.

Question. Write the principle behind the froth floatation process. What is the role of collectors in this process?
Answer : Froth floatation method : The method has been in use for removing gangue from sulphide ores. In this process, a suspension of the powdered ore is made with water. To it collectors and froth stabilisers are added.
Collectors (e.g., pine oils, fatty acids, xanthates, etc.) enhance non wettability of the mineral particles and froth stabilisers (e.g., cresols, aniline) stabilise the froth.
The mineral particles become wet by oils while the gangue particles by water. A rotating paddle agitates the mixture and draws air in it. As a result, froth is formed which carries the mineral particles. The froth is light and skimmed off.

Question. State the principle on which each of the following process operates :
(i) Electrolytic refining of a metal.
(ii) Vapour phase refining of a metal.
Answer : (i) Zone refining is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal.
(ii) Vapour phase refining : In this method, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal. So, the two requirements are :
(a) The metal should form a volatile compound with an available reagent.
(b) The volatile compound should be easily decomposable, so that the recovery is easy.

Question. Write the principles of the following methods :
(i) Vapour phase refining
(ii) Zone refining
(iii) Chromatography
Answer : (i) Vapour phase refining : In this method, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal.
(ii) Zone refining : This method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal.
(iii) Chromatography : Chromatographic method is based on the principle that different components of a mixture are differently adsorbed on an adsorbent. The adsorbed components are removed using suitable eluent.