12) General Principles & processes - Solutions

1. Magnetic , hematite , sideriteare such ores which can be separated by

magnetic separation method. In these ores, onecomponent is magnetic in nature.

2. This method removes the impurities of oxides of Fe, Si,Ti, etc., from the bauxite ore by various chemicalreactions stepwise either in the precipitated form or insoluble form.

3. Aluminum.4. Zinc blend (ZnS).5. Cr is present in ruby and Co in sapphire.6. Substances which ethane the non-wetteability of

mineral particles are known as collectors, e.g., pine oil.7. The process of heating of metal ore in the absence of air

is called calcinations. In calcinations ore is converted tooxide.

8. The process of heating of metal ore below its meltingpoint in the absence of air is called roasting. In roastingore is converted to oxide.

9. The process of extraction of a metal by heating themetal oxide with a suitable reducing agent is known aspyrometallurgy. It is used for the extraction of transitionmetals.

10. Froth stabilizer stabilizes the froth e.g. cresol, aniline.11. If either the ore or the gangue is capable of being

attracted by magnetic field then such separations arecarried out by magnetic separation.

12. Only sulphide ore particles are wetted by oil whilegangue particles are wetted by water. Therefore thefroth floatation method is selected for the concentrationof sulphide ores.

13. In case of low grade copper ores, hydrometallurgy isused for extraction purpose. Leaching is done by usingacid of bacteria. The solution containing is treatedwith scrap iron or .

14. Self (teacher)15. Self (teacher)16. The standard free energy of formation of (pyrites)

is greater than those of and . Hencecannot be reduced by carbon or hydrogen.

However the free energy of copper oxide is less thanthat of . That is the reason, extraction of copper iseasier from its oxide through reduction.

17. CO is a better reducing agent at 673 k. if we observe theEllingham diagram the curve involving

vs T intersect eachother at 983 K. is more as compared ot

.It means that at 983 K or above coke (C) is a betterreducing agent whereas below this temperature, thereverse is true (i.e. CO is better). Thus carbon monoxide(CO) is a better reducing agent at 673 K.

18. Role of cryolite (Na3AlF6) and/ or fluorspar (CaF2)alumina because(i) It lowers the melting point of the mixture.(ii) It also makes it a better conductor of electricity.

19. At low temperatures becomes negative for thereaction with C as compared to CO. hence C is a betterreducing agent for ZnO.

20. Iron obtained from last furnace contains 4% C is pig ironwhereas iron with 3% C is cast iron.

21. At temperature below 1700 K, magnesium will reducealumina.

22. Copper matte contains and little of FeO.

23. Self (teacher)24. Self (teacher)25. Self (teacher)26. Chromatography means colour writing. This is a

technique which is used to separate the colouredcomponents only form the mixture.

27. Antimony, selenium, tellurium silver gold and platinumare present in anode mud.

28. Zone refining is the method of purification for producingsemiconductor of other metals of very high purity.

29. is heated with impure Zr to form volatile compoundwhich on further heating decomposes to give purezirconium.

30. Aluminum foil is used as wrapper.31. Stationary phase (alumina).

Mobile phase solution of the substance to be purified.

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32. In general an adsorbent is used as a stationary phaseduring the process of chromatography. It should havethe following characteristics.(i) It should have high but selective adsorption capacity.(ii) The stationary phase should not react with themobile phase or with the components of the mixture tobe separated.(iii) It should be easily available.

33. Impure titanium is heated with to form volatilecomplex which on heating as highertemperature decomposes to give pure titanium

34. Self (teacher)35. Self (teacher)36. Self (teacher)37. Self (teacher)38. Self (teacher)39. Self (teacher)40. Self (teacher)41. Self (teacher)42. Self (teacher)43. The less reactive metals are likely to exist in native state.

These metals lie below hydrogen in the activity series.Some examples are gold, silver, platinum, copper, etc.

44. The reaction between sulphur and these metalsbecomes feasible at high temperature inside the earth.Hence, these metals exist as sulphides in addition to thefree or native state.

45. Self (teacher)46. Self (teacher)47. Self (teacher)48. Self (teacher)49. Self (teacher)50. Self (teacher)51. Self (teacher)52. Self (teacher)53. Self (teacher)

54. Difference between roasting and calcinationsRoasting CalcinationsOre is heated inthe excess of air.

Ore is heated in theabsence or limited supplyair.

This is used forsulphide ores.

This is used for carbonateores.

is producedalong with metaloxide.e.g.

55. This method is based upon the differences in themagnetic properties or ore components. If either the oreor gangue is magnetic in nature, then such separation iscarried out.

The powdered ore is placed on the conveyer let movingon two rollers on e of which is strong magnet. Theparticles which are magnetic in nature attracted by themagnets and fall closer to the roller while non magneticparticles fall away from the roller.This method is used to remove tungsten ore particlesfrom cassiterite (SnO2). It is also used to concentratemagnetite (Fe3O4), chromites (FeCr2O4) any pyrolusite(MnO2) from gangue.


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56. Difference between minerals and oresMinerals OresNaturally occurringcompounds of metalspresent in the earth’scrust.

Minerals why whichmetal can beextractedprofitability.

All minerals are not ores. All ores areessentially mineralstoo.

Bauxite (Al2O3.2H2O) andclay (Al2O3.2SiO2.2H2O).

Bauxite (Al2O3.2H2O)

57. This method is based upon the preferential wettingproperties with the frothing agent (collector) and water.In this process, powdered ore is mixed with watercontaining collectors and froth stabilisers.The minerals particle become wet with oil whiles thegangue particles by water. A stream of hot compressedair is passed which agitates the mixture and productsfroth, froth carries the mineral particles. The froth islight and is skimmed off. It is then dried for recovery ofthe ore particles. Water wets the gangue particles whichsink to bottom. This method is used for removinggangue from sulphide ores, e.g. copper pyrites, argentiteetc.

Sometimes it is possible to separate two sulphide oresby adjusting proportion of oil to water or by usingdepressant. For example in case of an ore containing Znsand pbS, the depressant use is NaCN. It selectivelyprevents ZnS from coming to the froth by forming alayer of zinc complex Na2[Zn(CN)4] on the surface of ZnSbut allows pbS to come with froth.58. Self (teacher)

59. Self (teacher)

60. value / Zn = -0.76 V is less than that ofit means that zinc is a stronger

reducing agent and can easily displace the ionspresent in the complex.

So zinc can be isolated by hydrometallurgy only whenstronger reducing agents than zinc are present. Butthese react with water to evolve hydrogen gas. Thusthese metals also do not solve the pupose and zinccannot be extracted by hydrometallurgy.

61. Pig iron is a from of iron obtained from blast furnace. Itcontains approximately 4% carbon and other impuritiesin small amounts. Cast iron on the other hand is formedby melting pig iron with coke and scraps of iron in veryhigh temperature range. The carbon content is 3% whichis lesser than that of pig iron.

62. Copper matte mainly contains and FeS. Forremoving the gangue, FeS, silica present in the lining ofthe Bessemer’s converter acts as a flux and forms slagon traction with FeO.

Therefore whole the iron present in matte is remove asslag.

63. for the conversion of Zn into ZnO is approx 600 KJand for the conversion of CO into CO2 is approx -500 kJi.e.

For the reaction

Positive value of shows that the reaction is notfeasible.That’s why CO cannot be used for the reduction of ZnOinto Zn.


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0E +2Zn.V34.0Cu/Cu2 +=+

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)ppt(Cu])CN(Zn[Zn])CN(Cu[ 22 +®+

SCu2

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64. The two thermo chemical equations may be written as

Subtracting the two equations we get

As comes out negative the reaction is feasible, i.e.Al can use for the reduction of

65. The statement is correct. We consider both reactions

For a particular reducing gent to work with a metallicoxide(i) The value of should be negative.(ii) Should be positive.(iii) A metal oxide place lower in the Ellingham diagramcannot be reduced by the metal involved in theformation of the oxide placed higher in the diagram.Hence both Al, Zn, can reduce FeO the Fe but Fe cannotreduce to Al and ZnO to Zn. Similarly C canreduce ZnO to Zn but CO cannot.The statement is definitely correct.

66. Down’s process is the name of the process which giveschlorine as the by product.Down’s process for the manufacture of Na metal whenmolten NaCl is subjected to electrolysis chlorine isobtained as by product at anode because in moltenstate only ions are present.

At cathodeAt anodeWhen aqueous solution of NaCl is electrolysed. H+ ionsare reduce in preference to at cathode and gasis released. remains in the solution and form NaOHwith ions. Chlorine gas is obtained at anode as aby product.

H+ (aq) + OH-(aq)At cathode

at anode

gas is obtained at cathode chlorine gas at anode (asby product) and NaOH is formed in the solution.

67. Graphite rods act as anode during electrolytic reductionof alumina. At anode gas is produced which reactwith carbon of anode (rods) to produce gas tothese graphite rods are consumed slowly and need to bereplaced from time to time.

At cathodeAt anodeOr


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;OCr32OCr

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68. The equation for the formation of two oxides are

If we observe the plots for the formation of the twooxides on the Ellingham diagram, we find the two curvesintersect each other at certain point. The correspondingvalue of becomes zero for the reduction of MgO byaluminium metal.

This means that the reduction of MgO by Al metalcannot occur below this temperature (1655 k) instead.Mg can reduce to all below 1655 K.Al can reduce MgO to Mg above 1655 k because

for is less as compared to that of MgO.

69. The reactants are solids so they cannot react in thepresent state. When the temperature is elevatedreactants melt and the reaction proceeds. Therefore ahigher temperature than room temperature is required.

70. Below 1693 k, the curve from the formation oflies above the curve for MgO so below

1693 K, mg can reduce to Si whereas above 1693 K,the curve for MgO lies above curve for

hence above 1693 K, Si can reduce MgO to Mg.

71. The chief ore of silver is argentite of silver glance .

Or

72. Zinc is extracted from zinc blende (ZnS). The ore isconcentrated by froth floatation process, then roastedto give oxide.

The zinc oxide is mixed with coke and clay to makebrikettes. These bickettes are heated at 1673 K to givezinc.

The metal is distilled off and collected by rapid chilling.

73. If the value of is positive for any reaction then tomake such reaction spontaneous, it is coupled withanother reaction of large negative value so that thesum of two becomes negative. This is known ascoupling of reaction.In the metallurgy, thermodynamically infeasible reactionis coupled with a reaction which has more negative

so that net becomes negative.74. The extraction of gold and silver involves leaching the

metal with which is also an oxidation reaction. Themetal is later recovered by displacement method using amore electro positive zinc metal.Extraction of gold

Or

In this reactions Zn acts as reducing agent.

75. Self (teacher)76. Self (teacher)77. Ni is purified by Mon’s process. Impure nickel is heated

in a steam of carbon monoxide forming a volatilecomplex nickeltetracarbonyl which is decomposed whenheated at 450 K to give nickel metal.


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S2)aq(OH4)aq(])CN(Ag[4)g(O 22 ++® --

)aq(])CN(Zn[)s(Ag2)s(Zn)aq(])CN(Ag[2 242

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+®+ 22 SO2ZnO2O3ZnS2

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GD

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+++ - )aq(OH2)aq(CN8)s(Au4 2

)aq(OH4)aq(])CN(Au[4)g(O 22-- +®

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¯+®+ Au2])CN(Zn[NaZn])CN(Au[Na2 422

Volatile PureimpureCO4Ni)CO(NiCO4Ni 4 +¾®¾¾®¾+

78. In this method all the oxygen and nitrogen present asimpurity are removed. The crude metals such as Zr andTi are heated in an evacuated vessel with the metaliodide being more covalent, volatilizes.

The zirconium iodide is decomposed on a tungstenfilament when heated at 1800 k electrically. The puremetal is thus deposited on the filament.

79. This method is used for low boiling metals which arevolatile in nature. Impure metal is heated in a retort andvapours are condensed. The non volatile impurities areleft behind e.g. Zn, Cd, Hg, etc.

80.Metals which are less reactive and voluble as silver, gold,platinum, etc. are found in anode mud the reason is thatbeing less reactive they do not lose electrons at anodeand collect under the anode as anode mud. Coppermetal anode less electrons in a usual manner to form

ions.At anode

Noble metals No reactions

81. This process is based upon the difference in fusibility ofmetal and impurities. The principal of refining byliquation is that the impurities whose melting points arehigher than the metal are left behind on melting theimpure metals. Hence pure metal separates out.The ingots of impure metal are heated (below itsmelting point) on the sloping hearth of a furnace. Themetal melt and flows down leaving behind theimpurities on hearth e.g. Sn, Pb, Hg, etc. in this way ametal is separated from higher melting impurities.

82. Self (teacher)83. Self (teacher)84. Self (teacher)85. Self (teacher)86. Self (teacher)87. Self (teacher)88. Self (teacher)89. Self (teacher)90. Self (teacher)91. Self (teacher)

92. Ores in which one of the components, either theimpurity or the ore, is magnetic can be concentrated bymagnetic separation. For example, haematite,magnetite, siderite and tin stone.

93. Self (teacher)94. Self (teacher)95. Self (teacher)96. Self (teacher)97. Self (teacher)98. Even those reactions, which are thermodynamically

feasible, require certain amount of activation energy tocross over the energy barrier between reactants andproducts. For the given reaction this energy is notavailable at room temperature and hence the reactiondoes not take place at room temperature.

99. Self (teacher)100. Self (teacher)101. Self (teacher)102. Self (teacher)103. Self (teacher)104. Coke or CO is use as reducing agent. When ore is

heated with C or CO, it reduces ore into metal.For example,Reduction of PbO and Fe2O3

105. Cinnabar is roasted in excess of air mercury isobtained.

106. This method is used for highly electropositive metalssometimes a small amount of flux is added whichdecreases melting point and increases conductivity, e.g.Na, K, Mg, etc.In this method fused metal ore is electrolyzed and puremetal is deposited at cathode.

107. In froth floatation process, depressant prevent theformation of froth. It is used to separate two sulphideores by preventing the formation of froth of onesulphide ore and allowing prevents ZnS from coming inforth but allows PbS to come with the froth.


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.2I

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24 I2ZrZrI +¾®¾

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232 CO3Fe2CO3OFe +®+

+®+ 22 SO2HgO2O3HgS2+®+ 2SOHg3HgSHgO2

+® 2OHg2HgO2

cathode anodeClNa2NaCl2 2+®

108. (i) The green colour substance is the compoundof copper. Its chemical formula is

(ii) Awareness and scientific knowledge.109. Self (teacher)110. Self (teacher)111. Reaction at the lower temperature range

(5000-8000 K)

Reaction at the higher temperature range (9000-15000K)

112. The following reactions take place during extractionof zinc from blende.RoastingReductionBoth this reactions are taking place in very hightemperature conditions.

113. Alumina is separated from silica in a bauxite orethrough Baeyer’s process.Baeyer’s process bauxite ore is concentrated by themethod of leaching of chemical separeatioon.

114. (i) The graphical representation of Gibbs energy vsabsolute temperature for a process is known asEllingham diagram.(ii) (a) Care and concern(b) Scientific knowledge(c) Responsibility

115. (i) Pig iron contains 4% C and small amount of S P Siand Mn when it is heated strongly in Bessemerconverter in the presence of oxygen C gets oxidized to

, P to , S to which are removed as gasesSi is converted into and Mn into MnO both reactwith each other and form MnSiO3 slag which is removednow wrought iron is prepared. Then add 0.5% of carbonand heat to get steel.(ii) Zinc oxide is obtained from zinc blende, and then zincis extracted from zinc oxide. The whole process is asfollows.The zinc blende (ZnS) ore is concentrated by frothfloatation process, then roasted to give oxide.

The zinc oxide is mixed with coke and clay to makebikcettes. These brikettes are heated at 1673 K to givezinc. The temperature in this case is higher than that incase of copper.

The metal is distilled off and collected by rapid chilling.(iii) This method is used for highly electropositive metals,sometimes a small amount of flux is added whichdecreases melting point and increases conductivity e.g.Na, K, Mg, etc.In this method fused metal ore is electrolysed and puremetal is deposited at cathode.

116. (i) Role of NaCN in the extraction of gold is to do theleaching of gold ore in the presence of air from whichthe gold is obtained later by displacement method.

(ii) is added to copper matte to convert the leftout FeS, FeO into slag.(iii) Role of cryolite ( ) and or fluorspar ( )alumina because(a) It lowers to melting point of mixture.(b) It also makes it a better conductor of electricity.


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.)OH(Cu.CuCO 23

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243 CO4Fe3CO4OFe +®+

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22 SO2ZnO2O3ZnS2 +®+COZn)coke(CZnO +®+

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117. During leaching of Au it is first oxidized to Au+ by O2

which then combines with CN- ions to form the solublecomplex.

By displacement method, using more electropositivezinc metal this gold is extracted, where Zn acts as sreducing agent, it reduces Au+ to Au while it itself getsoxides to zn2+ which then combine with CN- ions formingsoluble complex, sodium tetracyanozincate (II)

Therefore, extraction of Au by leaching with NaCNinvolves both oxidation as well as reduction.

118. (i) Zinc oxide is mixed with coke and clay to makebrickettes. These brickettes are heated at 1673 K to givezinc.(ii) Zinc is the extraction of silver: silver is present in theore is leached with dilute solution of NaCN in thepresence of air or oxygen to form a soluble complex.

Silver is then recovered from the complex bydisplacement method using complex using a moreelectropositive zinc metal.

The extraction of gold and silver involves leaching themetal with which is also an oxidation reaction. Themetal is later recovered by displacement method using amore electro positive zinc metal.Extraction of gold

Or

In this reactions Zn acts as reducing agent.The chief ore of silver is argentite of silver glance .

Or

(iii) Silica in the extraction of copper: During smeltingand bessemeriation the impurity ferrous sulphide isoxidizes to ferrous oxide which is then reacted with silica(flux) to form slag ferrous silicate.

119. Self (teacher)120. Self (teacher)


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-- +®+

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)slag(silicateFerrousFluxFeSiOSiOFeO 32 ®+

121. (i) Zone refining: This method is base on theprinciple that the impurities are more soluble in the meltthan in the solid state of the metal.A movable circular heater is fixed at one end of the rodof impure metal. The molten zones moves along with theheater which is moved forward as the heater movesforward, the pure metal crystallizes while the impuritiespass on the molten zone and finally fall down. Thisprocess is repeated several times. It gives metals of highpurity e.g. Si, Ge, Ga, B and In,etc.

(ii) Electrolytic refining: In this method the impure metalis made anode and a strip of pure form of same metal ismade cathode. Aqueous solution of salt of same metal istaken as electrolyte. On passing electric current, metalsions from the electrolyte are deposited at the cathode inthere form of pure metal while and equivalent amountof metal dissolves from the anode and goes into theelectrolyte solution as metal ions i.e. pure metal isdissolved from anode and deposited at cathode throughelectrolyte in equivalent amount.Soluble impurities pas into the solution and insolubleimpurities collect below that anode as anode mud. Themore basic metal remains in the solution while the lessbasic metals go to the anode mud.The reactions are

Anode

CathodeElectrolyte refining of copper: here anodes are of impurecopper and pure copper strips are taken as cathode. Theelectrolyte is acidified solution of copper sulphate. Dueto electrolysis pure copper form anode is transferred tocathode.

Anode

CathodeImpurities from the blister copper deposit as anode mudwhich contains impurities like antimony, selenium,tellurium, silver, gold, and platinum, Zn may also berefined this way.(iii) Vapour phase refining: In this method the metal isconverted into its volatile compound and collected

elsewhere then it is decomposed to give pure metal.The two necessary conditions are follows(i) The metal should from a volatile compound with anavailable reagent.(ii) The volatile compound should be easilydecomposable for the easy recovery of metal.

122. (i) Zone refining: This method is base on theprinciple that the impurities are more soluble in themelt than in the solid state of the metal.A movable circular heater is fixed at one end of the rodof impure metal. The molten zones moves along withthe heater which is moved forward as the heater movesforward, the pure metal crystallizes while the impuritiespass on the molten zone and finally fall down. Thisprocess is repeated several times. It gives metals of highpurity e.g. Si, Ge, Ga, B and In,etc.

(ii) Column chromatography: In this method a glasscolumn is filled with a solid adsorbent like . Asolution of components is allowed to pass through thecolumn different components are adsorbed at differentlevels of the column. The adsorbed components areremoved (eluted) by using suitable solvent (eluant).It is very useful for purification of elements which areavailable in minure quantities and the impurities are notvery different in chemical properties from the elementsto be purified.


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32OAl

123. Electrolytic refining: In this method the impuremetal is made anode and a strip of pure form of samemetal is made cathode. Aqueous solution of salt of samemetal is taken as electrolyte. On passing electric current,metals ions from the electrolyte are deposited at thecathode in there form of pure metal while andequivalent amount of metal dissolves from the anodeand goes into the electrolyte solution as metal ions i.e.pure metal is dissolved from anode and deposited atcathode through electrolyte in equivalent amount.Soluble impurities pas into the solution and insolubleimpurities collect below that anode as anode mud. Themore basic metal remains in the solution while the lessbasic metals go to the anode mud.The reactions areAnodeCathodeElectrolyte refining of copper: here anodes are ofimpure copper and pure copper strips are taken ascathode. The electrolyte is acidified solution of coppersulphate. Due to electrolysis pure copper form anode istransferred to cathode.AnodeCathodeImpurities from the blister copper deposit as anode mudwhich contains impurities like antimony, selenium,tellurium, silver, gold, and platinum, Zn may also berefined this way.

124. (i) Graphite is a good conductor of electricity due tothe presence of free electrons within its layers whereasdiamond is a bad conductor of electricity due to theabsence of free electrons in its structure thereforediamond cannot be used as anode.(ii) When CO is passed over a heated mixture of Ni andCo at temperature 330-350 K, Ni forms volatile Ni(CO)4

but Co does not the volatile Ni(CO)4 is separated it isthen heated at 450-470 K to get pure Ni.

125. (i) Self (teacher)(ii) Zinc in the extraction of silver: silver is present in theore is leached with dilure solution of NaCN in thepresence of air or oxygen to from a soluble complex.

Silver is then recovered for the complex by displacementmethod using a more electropositive zinc metal

(iii) Silica in the extraction of copper: during smeltingAnd bessemerisation the impurity ferrous sulphide isoxidized to ferrous oxide which is then reached withsilica (flux) to form slag ferrous silicate.

126. (i) Zone refining(ii) Zone refining: This method is base on the principlethat the impurities are more soluble in the melt than inthe solid state of the metal.A movable circular heater is fixed at one end of the rodof impure metal. The molten zones moves along withthe heater which is moved forward as the heater movesforward, the pure metal crystallizes while the impuritiespass on the molten zone and finally fall down. Thisprocess is repeated several times. It gives metals of highpurity e.g. Si, Ge, Ga, B and In,etc.

(iii) Knowledge and scientific temperament.


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MneMn ¾®¾+ -+

-+ +¾®¾ e2CuCu 2

Cue2Cu 2 ¾®¾+ -+

nickelpurenyltetracarboNickelnickelimpureCO4Ni)CO(NiCO4Ni K470450

4K350300 +¾¾¾¾ ®¾¾¾¾¾ ®¾+ --

)(4])([)()(2)(8)(4 222 aqOHCNAggOlOHaqCNsAg --- +¾®¾+++


-- +¾®¾+

)Slag(silicateFerrousFluxFeSiOSiOFeO 32 ¾®¾+

127. (i) Chromatography(ii) Chromatography means colour writing. This is atechnique used to separate the coloured componentsonly form the mixture.This method is based on the principle of sele4ctiveadsorption of different components in a mixture on anadsorbent i.e. different components of a mixture aredifferently adsorbed on an adsorbent.There are different chromatographic techniques(1) Column chromatography(2) Gas c chromatography(3) Paper chromatographyPrincipal it involves a mobile phase and a stationaryphase. The sample extract is dissolved in mobile phasewhich may be a liquid of gas the stationary phase isimmobile and immiscible. The mobile phase andstationary phase have different solubility for thecomponent to be purified. The mobile phase is forced topass through the stationary phase. A component whichis soluble in stationary phase takes longer time to travelthrough it than a component which is not very soluble.In other words the mixture is put in a liquid or gaseousmedium which is moved through the adsorbent.Different components are absorbed at different level atthe column later the adsorbed components areremoved (eluted) by using suitable solvents (eluant).

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147.

148. Certain amount of activations energy is essentialeven for such reactions which are thermodynamicallyfeasible. Therefore for the reactions heating is required.

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Cr2OAlAl2OCr 3232 +®+

152.

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154. (i) Highly reactive metals occur in combined stategenerally as their chlorides, sulphates or carbonates.(ii) Moderately reactive metals generally occur as theiroxides.(iii) Noble metals exist in free state or native state.

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179. Depressants are the chemical which are used forpreventing certain type of particles from forming theforth. Zinc sulphide ore can be separated from leadsulphide ore by froth floatation process using sodiumcyanide (NaCN) as depressant. Sodium cyanide acts asdepressant for ZnS but does not prevent PbS from theformation of froth. Thus, the two ores can be separated.

180. Oxides can be more easily reduced to the respectivemetals using carbon or hydrogen as reducing agent. Onthe other hand sulphides cannot be reduced by carbonor hydrogen because DrG for such reactions are positiveand hence these reactions are not feasible.


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181.

182. Even thermodynamically feasible reactions requiresome activation energy to cross over the energy barrierbetween reactants and products. Therefore, the abovereaction does not take place at room temperature butoccurs on heating because on heating the activationenergy becomes available.

183.

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12) General Principles & processes - Solutions

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