Bihar Board - Class 12 Chemistry - Chapter 6: General Principles and Processes Of Isolation Of Elements
Long Question Answer
Q.1. Describe the preparation of Ammonia from Haber’s process ? And also give the condition to produce a maximum amount of Ammonia according to Lechtlier’s principle ?
Ans⇒ On a large scale Ammonia is produced by chemical combination of Nitrogen and hydrogen, according to following reaction :
N2 (g)+3H2(g) 
Hr =-46.1 KJ mole-1
Lechatlier’s principle : In a reversible reaction once equilibrium is established after that any factor is brought to change then equilibrium shifts in that direction so as to destroy the change. There are following condition under which we get maximum amount of ammonia :
(i) Effect of concentration : If we increase the concentration of Nitrogen and hydrogen gas or take out some amount of concentration of ammonia at equilibrium then it favors forward reaction and production of ammonia will increase.
(ii) Effect of temperature : Since the reaction is exothermic so the high production of ammonia will favor at optimum low temperature hence the optimum temperature is 720 k .
(iii) Effect of pressure : Since the production of ammonia decreases with the decrease in the number of moles, so on increasing pressure it favors forward direction and more amount of Ammonia is pressure. The optimum pressure required for this reaction is 200 atmospheric pressure.
(iv) Effect of catalyst: Catalyst used is Iron filling (Fe2O3) and Molybdenum (Mo) as a promoter. A catalyst enhances the rate of reaction. It does not have the value of equilibrium constant (kc).
Q. 2. How iron is extracted from Hematite ore is the blast furnace gives the reaction involved at different stages of blast furnace and also discusses the metallurgy involved ?
Ans⇒ Hematite ore (Fe2O3) is ore of Iron which have mainly Silica (SiO2) as main impurity. Hematite ore (Fe2O3) is put in a Blast furnace along with coke (c) and limestone.
There are different steps or stages for metallurgy first ore is calcined, roasted and then reduced or smelled to get pure iron from blast furnace (It is a double cup and cone arrangement).
Calcination : The ore is calcined first to remove earthly impurity and change ferrous oxide into ferric oxide.
Roasting : The ore is roasted to remove volatile impurity, to make the ore porous and roasted ore is easily reduced.
4 FeO +O2 (g)2Fe2O3
S+O2 (g) SO2 (g)
4AS+3O2 (g) 2 As2O3 (g)
The following reaction takes place at a blast furnace iron ore is put along with coke, limestone. In Blast furnaces the reaction takes place at the lower zone (zone of fusion), middle zone and upper zone.
The reaction taking place at blast furnace is
C(s)+O2 CO2 (g)
This is the reaction taking place at lower zone due to burning of coal with air and produces CO2 (g) . The temperature at this zone is 217
The reaction taking place at middle zone is
CaCO3 (s)CaO (s)+CO2 (g)
Limestone
CaO+SiO2 CaSiO3 (s)
Flux + Gangue Slag Calcium Silicate
The coke chemically combines with CO2 (g) to give Carbon monoxide and this reaction is an endothermic reaction which lowers the temperature of the blast furnace at this zone.
C(s)+CO2 (g) 2CO (g)
The reaction at upper zone in the temperature range 800-1100 k .
3 Fe2O3 (s)+CO (g)2Fe3O4 +CO2 (g)
Fe3O4 +CO (g)3FeO +CO2 (g)
FeO (s)+CO (g)Fe (s) +CO2 (g)
Carbon Monoxide produced in the middle zone acts as a reducing agent and reduced hematite in iron.
In the blast furnace the molten slag of Casio, (Calcium silicate) floats on Iron through different openings.
So the Iron obtained from blast furnaces contains about 4 % Carbon is known as pig iron and cast into a variety of shapes.
Q. 3. Explain : (i) Zone refining, (ii) Column Chromatography.
Ans⇒ (i) Zone refining : The method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. A circular mobile heater is fixed at one end of a rod of impure metal. A molten zone moves along with the hater which is moved forward. As the heater moves forward, the pure metal crystallizes out of the metal and the impurities pass on into the adjacent molten zone. The process is repeated several times and the heater is moved in the same direction. At one end impurities get concentrated. This end is cut off. In this way, the impurities are swept from one end of the bar to the other. By repeating the process ultra pure metal of silicon, germanium etc are obtained.
(ii) Column Chromatography : In column chromatography an adsorbent (e.g. Al2O3) is packed in a glass column. The mixture to be separated or purified, taken as a suitable solvent, is applied on the top of the column. The components of the mixture get adsorbed on the column. They are then eluted out with the suitable eluent (Solvent). The weakly adsorbed component is eluted first followed by the more strongly adsorbed and so on. The method is especially suitable for such elements which are available only in minute quantities and the impurities are not very much different in their chemical behavior from the element to be purified.
Q. 4. Outline the principles of refining of metals by the following methods :
(i) Electrolytic refining (ii) Vapour phase refining
Ans⇒ (i) Electrolytic refining : The impure metal is made of the anode and a pure strip of the same metal the cathode in a suitable electrolyte both.
Anode : M Mn+ + ne-
Cathode : Mn+ +ne- M
The net result is the transfer of pure metal from the anode to the cathode. The applied voltage is such that more electropositive metals (impurity) remain as ions in the bath whereas the less electropositive metals (impurities) remain unionized and fall done as anode mud.
(ii) Vapour phase refining : (a) Mond process : Nickel when heated in a stream of carbon monoxide forms volatile nickel carbonyl, Ni (CO4) . The carbonyl vapor when subjected to still higher temperature undergoes thermal decomposition giving pure metal.
Ni + 4CO 
(b) Van Arkel Process : Zirconium (or titanium) is heated in iodine vapour at about 870 K to form volatile ZnI4. The latter when heated over a tungsten filament at 2975 K decomposes to give pur zirconium.
Zr + ZI2 
(Impure) I2 (Vaspir) (vapor)
. Pure
Q. 5. Name three ores which are concentrated by the froth-floatation process. What is meant by depresent ?
Ans⇒ The three ores which are concentrated by froth floatation process are : (i) Copper pyrites (CuFeS2). (ii) Zinc Sulphide (ZnS). (iii) Silver glance (Ag2S).
Depressants are used to prevent certain types of particles from forming the froth with bubbles.
Example : NaCN is used as a depressant in the separation of ZnS and PbS ore. NaCN forms a layer of Zinc complex Na2Zn (CN)4 on the surface of ZnS and thereby prevents it from formation of froth.
Q. 6. Write the conditions to maximize the yield of H2SO4 by contract process.
Ans⇒ Conditions are : (i) High concentration of reactants.
(ii) Low temperature: But an optimum temperature of 623-723 K must be maintained.
(iii) High pressure : Normally a pressure of about two atmospheres is maintained.
(iv) Presence of catalyst : In order to accelerate the reaction, the presence of catalyst is quite helpful. V2O5 is used as a catalyst.
(v) Purity of gasses : Gasses must be completely free from dust and poisonous gasses like arsenic oxide before they are passed through the catalyst.
Q. 7. Write down the reactions taking place in different zones in the blast furnace during the extraction of iron.
Ans⇒ Near the layers, coke and limestone are heated when C burns to give CO2.
C (s)+O2 (g) CO2 (g) ; H=-393.3 kj mol-1
Since the reaction is exothermic, lot of heat is produced and the temperature here is 1673 K.
As the gasses move up, they meet the descending charge. The coke present in the charge reduces CO2 to CO.
CO2+C 2CO ; H+=+163.2 KJ
Since the reaction is endothermic, therefore, the temperature falls gradually to about 1423 K.
Below 1123 K ,CO reduces the ores to FeO .
Fe2O3 +CO 
Reduction of FeO to Fe by CO occurs at about 1123 K .
FeO+CO 
In this region limestone also decomposes to form CaO and CO2.
CaCO3 (s) CaO (s)+CO2 (g)
CaO (s) + SiO2 CaSiO3
Silica (fusible slag)
Q.8. How is leaching carried out in the case of low-grade copper ores?
Ans⇒ When dealing with low-grade copper ores, bacteria or acids are used in the presence of air to leach the copper. In this method, copper enters the solution as Cu2+ ions.
Cu+2H+ +12O2 Cu2+ + 2H2O
The solution so obtained is then treated with H2 or scrap iron to get copper metal.
Cu+2H+2H2+ + Cu
Q.9. Giving examples, differentiate between ‘roasting’ and ‘calcination’.
Ans⇒Calcination is the conversion of carbonate and hydroxide ores to oxides by heating them at a temperature below their melting points and in the absence or in a very limited supply of air. For example, carbonates of Ca , Mg and Zn are turned into their respective oxides through this process.
ZnCO3 ZnO+CO2
Fe2O3.3H2OFe2O3+3H2O [Heat]
Roasting is the conversion of sulfide ores into their metallic oxides by heating at a temperature below their melting points in the excess presence of air.
For e.g., sulfide ores of Pb and Zn are turned into their respective oxides through this process.
2ZnS +3O2 2ZnO +2SO2
2PbS +3O2 2PbO +2SO2
Q.10. How can you separate alumina from silica in a bauxite ore associated with silica? Give equations, if any.
Ans⇒ The separation of alumina from silica in bauxite ore associated with silica is as follows :
Firstly, a concentrated NaOH solution is used to digest the powdered ore at 473-523 K and at a pressure of 35-36 bar . This causes alumina (Al2O3) and silica (SiO2) to leach as sodium aluminate and sodium silicate, respectively, abandoning the impurities behind.
Al2O3(s)+2NaOH (aq)+3H2O (f)2Na[Al(OH)4] (aq)
Alumina sodium aluminate
SiO2 +2NaOH (aq)Na2SiO(aq)+H2O (I)
Silica Sodium silicate
Now, to neutralize the aluminate present in the solution, carbon dioxide gas is passed through the solution we obtained. This causes the sodium meta-aluminate to precipitate as hydrated alumina.
2Na [Al(OH)4](aq)+CO2 Al2O3.xH2O(s)+2NaHCO3 (aq)
hydrated alumina
Sodium silicate cannot be precipitated, so it is filtered off. The hydrated alumina is dried, heated and filtered to obtain pure alumina.
Al2O3.xH2O (s)Al2O3 (s)+ xH2O (g) [1470 K]
Q.11. Write Chemical Reactions Taking Place in the Extraction of Zinc from Zinc Blende.
Ans⇒ The following processes are involved in the extraction of zinc from zinc blende:
(i) Concentration: Zinc blende ore is crushed and the concentration done by froth- floatation process.
(ii) Roasting: The concentrated ore is then roasted in presence of excess air at about 1200 K As a result zinc oxide is formed.
2ZnS + 3O2
Zinc blende Zinc oxide
(iii) Reduction: Zinc oxide obtained above is mixed with powdered coke and heated to 1673 K in a fire clay retort where it is reduced to zinc metal.
ZnO+ C 
At 1673 K , zinc metal being volatile (boiling point 1180 K), distills over and is condensed.
(iv) Electrolytic Refining: Impure zinc is made the anode while pure zinc strip is made the cathode. ZnSO4 Solution acidified with dil. H2SO4 is the electrolyte used. On passing electric current, pure zinc gets deposited on the cathode.
Q.12. The Choice of a Reducing Agent in a Particular Case Depends on Thermodynamic Factors. How Far Do You Agree With This Statement? Support Your Opinion With Two Examples.
Ans⇒ We can study the choice of a reducing agent in a particular case using the Ellingham diagram. It is evident from the diagram that metals for which the standard free energy of formation of their oxides is more negative can reduce those metal oxides for which the standard free energy of formation of their respective oxides is less negative it means that any metal will reduce the oxides of other metals which lie above it in the Ellingham diagram. This is because the standard free energy change (r G0) of the combined redox reaction will be negative by an amount equal to the difference in f G0 of the two metal oxides. Thus both Aluminum and Zinc can reduce FeO to Iron but Fe cannot reduce Al2O3 to Aluminum and ZnO to Zinc.
Note: Only that reagent will be preferred as a reducing agent which will lead to decrease in free energy value G0 at a certain specific temperature.
Q.13. Write chemical reactions taking place in the extraction of aluminum from bauxite ore.
Ans⇒ Chemical reaction taking place in extraction of aluminum from bauxite ore: Aluminum is mainly isolated from bauxite ore (impurities are SiO2 , FeO , TiO2) which involves two steps
Step I- Concentration of ore (Leaching of alumina) Concentration of bauxite ore is carried out by leaching (Baeyer's process). In this process, pure alumina (Al2O3) is obtained. The principal ore of aluminum, i.e. bauxite, usually contains SiO2 , iron oxides and titanium oxide (TiO2) as impurities. Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at 473-523 K and 35-36 bar pressure. Al2O3 is leached out as sodium aluminate (and SiO2 as sodium silicate) leaving the impurities behind.
Al2O3 (s)+2NaOH (aq)+3H2O 2Na[Al(OH)4](aq)
The aluminate in solution is neutralized by passing CO2 gas and hydrated Al2O3 is precipitated. Here, the solution is seeded with freshly prepared samples of hydrated Al2O3 which induces precipitation.
2 Na[Al(OH)4](aq)+CO2 (g)Al2O2.xH2O(s)+2NaHCO3(aq)
The sodium silicate (impurities of silica) remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al2O3 .
Al2O3.xH2O (s) 
Step II - Electrolytic reduction (Hall's process) It is also known as the Hall-Hernult process. In this process, purified alumina is mixed with Na3AlF6 or CaF2 (cryolite or fluorspar), which lowers the melting point of alumina and brings conductivity. In the electrolytic cell, steel cathode and series of graphite anode are used. The overall reaction is
2Al2O3 +3C4Al+3CO2
The electrolytic reactions are as follows
Al2O32Al3++3O2-
At cathode
Al3++ 3e-Al(l)
At anode
C(s)+O2- (melt) CO (g)+2e-
C(s)+2O2- (melt) CO2 (g)+4e-
Q.14. Describe how the following changes can be brought about?
Impure copper into pure copper.
Ans⇒ Impure copper into pure copper: Copper is refined using an electrolytic method. Anodes are of impure copper and pure copper strips are taken as cathode. Acidified aqueous solution of copper sulfate acts as an electrolyte. The net result of electrolysis is the transfer of copper in pure form from the anode to cathode.
At anode Cu Cu2++2e
At cathode Cu2++2 e Cu
Impurities deposit as anode mud below anode which contains Sb , Se , Te , Ag , Au , Pt etc.
Q.15. Describe how the following changes are brought about?
(i) Pig iron into steel.
(ii) Zinc oxide into metallic zinc.
Ans⇒ Pig iron contains 4% C and small amounts of S , P , Si and Mn. When it is heated strongly in a bessemer converter in the presence of oxygen, C gets oxidized to CO2 , P to P2O5S to SO, which are removed as gasses. Si is converted into SiO2 and Mn into MnO. Both react with each other and form MnSiO2(slag) which is removed. This results in the formation of wrought iron. Then, wrought iron is heated by adding 0.5 % of carbon to get steel. \
(ii) Reduction of zinc oxide is done using coke. The temperature in this case is higher than that in the case of copper. For the purpose of heating, the oxide is made into briquettes with coke and clay.
ZnO+C 
The metal is distilled off and collected by rapid chilling.
Q.16. Write the chemical reactions involved in the process of extraction of gold. Explain the role of dilute NaCN and Zn in this process.
Ans⇒ In extraction of gold, NaCN is used for leaching of metal. The chemical reactions involved in the process are given below
4Au (s)+8 CN- (aq)+2 H2O (aq)+O2 (g4[Au(CN)2]- (aq)+4OH- (aq)
2[Au(CN)2]- (aq)+Zn (s) 2Au (s)+[Zn(CN)4]2- (aq)
or
4Au +8NaCN+O2+2H2O4Na[Au(CN)2] + 4 NaOH
2Na[Au(CN)2]+Zn Na2[Zn(CN)4]+2Au
In this process, Zn acts as a reducing agent.
Q.17. (i) Why does copper obtained in the extraction from copper pyrites have a blistered appearance?
(ii) What is the role of depressants in the Froth floatation process?
Ans⇒ (i) Copper obtained in the extraction from copper pyrites have a blistered appearance due to evolution of SO2. Thus, it is also called blistered copper.
1. 2Cu2S+3O2 - 2Cu2O+2SO2
2. 2Cu2O+Cu2S6Cu+SO2
(ii) The substances which selectively prevent certain types of particles from forming the froth with the bubbles are called depressants. e.g. in case of an ore containing ZnS and PbS , NaCN is used as a depressant.
Q.18. How is chemical reduction different from electrolytic reduction? Name a metal of each which is obtained by
(i) electrolytic reduction.
(ii) chemical reduction.
Ans⇒
In chemical reduction, reduction is done by heating with a reducing agent while in electrolytic reduction, reduction is done by electrolysis. Chemical reduction is the process involving electron gain. Usually, reduction of the metal oxide involves heating with a reducing agent. The reducing agent combines with oxygen of the metal oxide.
M2O y+yC xM+yCO
Some metal oxides get reduced easily while others are very difficult to be reduced.
In electrolytic reduction, metal in solution or montent state is reduced by electrolysis. In simple electrolysis, Mn+ ions are discharged at the negative electrode (cathode) and deposited there.
Sodium is extracted from fused NaCl by electrolytic reduction.
Iron is obtained by chemical reduction.
Q.19. Describe the principle controlling each of the following processes.
(i) Vapour phase refining of titanium metal.
(ii) Froth floatation method of concentration of a sulfide ore.
Ans⇒
(i) van-Arkel method is used for refining titanium. Crude titanium is heated in an evacuated vessel with iodine, titanium iodide is formed which is more covalent, volatilises.
Ti + 2I2 
(impure) (pure)
Titanium iodide is decomposed on a tungsten filament electrically heated to about 1800 K . Pure titanium is deposited on the filament.
(ii) The principle of froth flotation process is that sulfide ores are preferentially wetted by the pine oil, whereas the gangue particles are wetted by the water. Hence the method used for the concentration of sulfide ores is froth flotation.
Q.20. Which methods are usually employed for purifying the following metals?
(i) Nickel
(ii) Germanium
Mention the principle behind each one of them.
Ans⇒
(i) Purification of nickel:
The Mond process, sometimes known as the carbonyl process, is a technique created by Ludwig Mond in 1890, to extract and purify nickel.
This process involves the fact that carbon monoxide combines with nickel readily and reversibly to give nickel carbonyl.It is purified by Mond's process.
(ii) Purification of germanium: Zone-refining is used to obtain metal of very high purity. The principle on which it is based is that an impure molten metal on gradual cooling will deposit its crystals of pure metal, while the impurities will be left in the remaining part of molten metal. Germanium metal can be purified by this method.
Q.21. Describe the role of the following:
(i) Sil ,in the extraction of copper from copper matte.
(ii) NaCN in the froth floatation process.
Ans⇒
(i) Silica in the extraction of copper:
Silica (SiO2) is added in the reverberatory furnace during the extraction of Cu to remove impurities of iron oxide (FeO) present in the ore. Silica here acts as flux and reacts with iron oxide gangue to remove it as slag, iron silicate.
FeO+SiO2 FeSiO3
Iron oxide Silica Iron Silicate
(ii) NaCN acts as depressant, e.g. it prevents ZnS from coming in froth but allows PbS to come with the froth.
Q.22. Out of C and CO, which is a better reducing agent for ZnO?
Ans⇒ The two reduction reactions are :
ZnO (s)+C(s) Zn (s)+CO(g) …………(1)
ZnO (s)+CO(g) Zn (s)+CO2(g) ………………(2)
In the first case, there is an increase in the magnitude of S while in the second case, it almost remains the same. In other words G will have more negative value in the first case when C(s) is used as the reducing agent than in the second case when CO(g) acts as the reducing agent. Therefore, C(s) is a better reducing agent.
Q.23. What criterion is followed for the selection of the stationary phase in chromatography?
Ans⇒In chromatography, particularly in adsorption chromatography, the stationary phase is the adsorbent. It should fulfill certain criteria for better results.
(i) It should have high but selective adsorption power.
(ii) The particles should be spherical in shape and of uniform size.
(iii) The adsorbent should not react chemically with the solvents used for elution or with the components of the mixture under investigation.
(iv) The adsorbent should contain as small amount of the soluble components as possible.
(v) The adsorbent should be catalytically inactive and must have a neutral surface.
(vi) The adsorbent should be easily available.
(vii) The adsorbent should be perfectly white.
Q.24. Name the processes from which chlorine is obtained as a by-product What will happen if an aqueous solution of NaCl is subjected to electrolysis?
Ans⇒ Down process is used for the preparation of sodium metal, where chlorine is obtained as a by- product. This -process involves the electrolysis of a fused mixture of NaCl and CaCl2 at 873 K .Sodium is discharged at the cathode while Cl2 is obtained at the anode as a by-product.
NaCl (l) 
At cathode : Na+ + e- Na
At anode : Cl-12Cl2 + e-
If, an aqueous solution of NaCl is electrolysed, H2 is evolved at the cathode while Cl2 is obtained at the anode.
Q.25. Predict conditions under which Al might be expected to reduce MgO.
Ans⇒ The equations for the formation of the two oxides are
43Al (s)+O2 (s)23Al2O3 (s)
2 Mg (s)+O2 (s)2MgO (s)
If we look at the plots for the formation of the two oxides of the Ellingham diagram, we find that they intersect at certain points. The corresponding value of G becomes zero for the reduction of MgO by Al metal.
2MgO (s)+43Al (s)⇌2Mg (s)+23Al2O3 (s)
This means that the reduction of MgO by Al metal can occur below this temperature. Aluminum (Al) metal can reduce MgO to Mg above this temperature because G for Al2O3 is less as compared to that of MgO.
3MgO (s)+2Al (s) 
