QA GEN-PRIN

8/13/2019 QA GEN-PRIN

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CLASS - XII CHEMISTRY(General principles and processes of isolation of elements)

Topic :- Occurrence of metals, concentration methods

1.Define the term - metallurgy?The scientific and technological process used for isolation of the metalfrom its ores is known as metallurgy.

2.What is gangue?The earthly or undesired materials which contaminate ore are calledgangue.

3.Give some important ores of Aluminum, Iron, Copper and Zinc?Metal Ore

Aluminum Bauxite

Iron Hematite, Magnetite

Copper Copper pyrite, Copper glance

Zinc Zinc blende, Calamine

4.Name the most abundant and second most abundant metal inearths crust.

The most abundant metal is Aluminum & second most abundant metalis iron.

5.What is the basis of hydraulic washing? For which type of ores is itused?

Hydraulic washing is based on the differences in the gravities of the oreand the gangue particles. It is used when ore is heavy and impurities arelighter.

6.When is magnetic separation used?If either of the ore or gangue is capable of being attracted by a magneticfield e.g. in case of iron ores, magnetic separation is used for its

concentration.7.Which of the ores mentioned in Table 6.1 can be concentrated by

magnetic separation method?

Iron ores such as Hematite, Magnetite concentrated by magneticseparation method


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8.Explain froth floatation method.Froth floatation is used for removing gangue from sulphide ores. Here a

suspension of the powdered ore is mixed with water. To it collectors and

froth stabilizer are added. A rotating paddle agitates the mixture and

draws air in it. As a result froth is formed. The mineral particles become

wet by froth (oil) and gangue particles by water. The froth is skimmed off

and dried for the recovery of ore.

9.What is the role of a depressant in the floatation process? Give anexample?

Depressant adjusts the proportion of oil and water so as to separate two

sulphide ores. e.g. in case of an ore containing ZnS and PbS, NaCN is used

an depressant which selectively prevents ZnS from coming to the froth.

10. What is used as collectors and froth stabilizers in froth floatationprocess? What is their role in the process?

Pine oil, fatty acids or xanthate can be used as collectors to enhance the

non- wettability of mineral particles whereas cresols or aniline can be used

as froth stabilizer to stabilize the froth in froth floatation process.

11. What is leaching? Explain with an example.(explain purification ofbauxite)

Leaching is used if the ore is soluble is suitable solvent.

e.g. Aluminum is leached from bauxite by using a conc. solution of NaOH at

473-523 K and 35-36 bar pressure. Al2O3 is leached out an sodium

aluminates leaving behind impuritie iron oxide.

Al2O3(S) + 2NaOH(aq)+3H2O 4 [2Na[Al(OH)4]

The aluminate solution is centralised by passing CO2 gas & hydrated Al2O3is precipitated The sodium silicate remains in the solution and hydrated

alumina is filtered, dried and heated to give back pure Al2O3:which is

filtered, dried and heated to give pure Al2O3.


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12.What is the significance of leaching in the extraction of aluminium?Leaching is used to remove impurities such as SiO2,iron oxides and

titanium oxide (TiO2) by dissolving bauxite ore with. a concentrated

solution of NaOH at 473 523 K and 35 36 bar pressure.thus the

significance of leaching in the extraction of aluminium is to prepare pure

aluminium from bauxite ore.

13.How can you separate alumina from silica in bauxiteore associated with silica? Give equations, if any.

To separate alumina from silica in bauxite ore associated withsilica, first the powdered ore is digested with a concentrated

NaOH solution at 473 523 K and 35 36 bar pressure. Thisresults in the leaching out of alumina (Al2O3) as sodiumaluminate and silica (SiO2) as sodium silicate leaving theimpurities behind.

Then, CO2 gas is passed through the resulting solution toneutralize the aluminate in the solution, which results in theprecipitation of hydrated alumina. To induce precipitation, thesolution is seeded with freshly prepared samples of hydratedalumina.

During this process, sodium silicate remains in the solution. Theobtained hydrated alumina is filtered, dried, and heated to get

back pure alumina.


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14.Which reagent is used for leaching of Gold or silver? Write theequations involved.

A dil. Solution of NaCN or KCN in the presence of air is used as reagentfor leaching of silver and gold. from which the metal is obtained later byreplacement:The equations are-

Topic:- Extraction of crude metal from concentrated ore,

1. Sulphide and carbonate ores are converted to oxidebefore reduction. Why?

Since the reduction of oxide ores involves a decrease in Gibbs freeenergy making G value more negative, it is easier to reduce oxidestherefore suphide and carbonate ores are converted to oxides beforereduction.

2.What is calcinations and roasting? Give one example

3.What is slag? Give an example.Slag is the substance obtained after flux reacts with impurity.Flux + Impurity -----Slag.For example silica is added as flux to remove iron oxide during extractionof copper as ferrous silicate


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Thermodynamic principles of metallurgy

1.What is the condition for a reduction reaction to occur in terms of freeenergy change? How can it be achieved?

When the value of G is negative , the reduction reaction is said to bespontaneous

1) It S is positive , on increasing the temperature, the value of T S

would increase & G will become negative.

2) For a process which is otherwise having G positive & is non-

spontaneous, , it can be coupled with a reaction having highly negative

G value so that the overall G is negative and the process can take

place.2.What are Ellingham diagrams?

Ellingham diagrams are graphical representation of

variation of G vs T for the formation of oxides of

elements i.e., for the reaction

3.State one limitation of Ellingham diagrams.Ellingham diagrams only tell us about the feasibility of a

reaction. They do not tell anything about the reaction

kinetics.

4.Suggest a condition under which magnesium couldreduce alumina.


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5.Although thermodynamically feasible, in practice,magnesium metal is not used for the reduction ofalumina in the metallurgy of aluminium. Why ?

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

7.The reaction,

Cr2 O3+ 2 Al Al2O3+ 2 Cr (G0=421 kJ)is thermodynamically feasible as is apparent fromthe Gibbs energy value.Why does it not take place atroom temperature?It does not take place room temperature as G0 ispositive


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8.Is it true that under certain conditions, Mg can reduceSiO2 and Si can reduce MgO? What are thoseconditions?

9.Out of C and CO, which is a better reducing agent at673 K?

At 673 K, the value of G(CO,CO2)is less than that of G (C,CO)Therefore, CO can be reduced more easily to CO2than C to CO. Hence,CO is a better reducing agent than C at 673 K.

10.Why is zinc not extracted from zinc oxide throughreduction using CO?

The standard Gibbs free energy of formation of ZnO from Zn is lower thanthat of CO2from CO. Therefore, CO cannot reduce ZnO to Zn. Hence, Zn isnot extracted from ZnO through reduction using CO

11.Out of C and CO, which is a better reducing agent forZnO ?

Reduction of ZnO to Zn is usually carried out at 1673 K. From the abovefigure, it can be observed that above 1073 K, the Gibbs free energy offormation of CO from C and above 1273 K, the Gibbs free energy offormation of CO2from C is lesser than the Gibbs free energy of formationof ZnO. Therefore, C can easily reduce ZnO to Zn.

On the other hand, the Gibbs free energy of formation of CO2from CO isalways higher than the Gibbs free energy of formation of ZnO. Therefore,

CO cannot reduce ZnO. Hence, C is a better reducing agent than CO forreducing ZnO.

12. The value of for formation of Cr2O3 is 540 kJmol1 andthat of Al2 O3 is 827 kJmol

1. Is the reduction of Cr2O3possible with Al?


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The value of for the formation of Cr2O3from Cr (540 kJmol1) is

higher than that of Al2O3 from Al (827 kJmol1). Therefore, Al can

reduce Cr2O3to Cr. Hence, the reduction of Cr2O3with Al is possible.

Alternatively,

Subtracting equation (ii) from (i), we have

As for the reduction reaction of Cr2O3by Al is negative, this reaction ispossible

Topic:- Extraction of copper

1. Explain the extraction of copper?The sulphide ores of copper are roasted to give oxides:

The oxide can then be easily reduced to metallic copperusing coke.

The impurities like iron oxide are removed as slag byreacting with SiO2, added as flux.

2. What is Blister copper?The solidified copper obtained after extraction hasblistered appearance due to evolution of SO2 is calledblister copper.


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3.Copper can be extracted by hydrometallurgy but notzinc. Explain.

The reduction potentials of zinc and iron are lower thanthat of copper. In hydrometallurgy, zinc and iron can beused to displace copper from their solution.

But to displace zinc, more reactive metals i.e., metalshaving lower reduction potentials than zinc such as Mg,Ca, K, etc. are required. But all these metals react withwater with the evolution of H2gas.

As a result, these metals cannot be used in hydrometallurgyto extract zinc.Hence, copper can be extracted by hydrometallurgy but notzinc

4.How is copper extracted from low grade ores?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 orH2

5.At a site, low grade copper ores are available and zinc

and iron scraps are also available. Which of the twoscraps would be more suitable for reducing the leachedcopper ore and why?Zinc being above iron in the electrochemical series (morereactive metal is zinc), the reduction will be faster in casezinc scraps are used. But zinc is costlier metal than iron sousing iron scraps will be advisable and advantageous.


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6.State the role of silica in the metallurgy of copper.

During the roasting of pyrite ore, a mixture of FeO andCu2O is obtained.

The role of silica in the metallurgy of copper is to removethe iron oxide obtained during the process of roasting asslag. If the sulphide ore of copper contains iron, then silica(SiO2) is added as flux before roasting. Then, FeO combineswith silica to form iron silicate, FeSiO3(slag).

7.Why copper matte is put in silica lined converter?

Copper mattecontains Cu2S and FeS. Copper matteis put ina silica-lined converter to remove the remaining FeO and FeSpresent in the matte as slag (FeSiO3). Also, some silica is

added to the silica-lined converter. Then, a hot air blast isblown. As a result, the remaining FeS and FeO are convertedto iron silicate (FeSiO3) and Cu2S is converted into metalliccopper.

8.Why is the extraction of copper from pyrites moredifficult than that from its oxide ore through reduction?


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The Gibbs free energy of formation (fG) of Cu2S is less than

that of and . Therefore, H2and C cannot reduce Cu2S toCu.

On the other hand, the Gibbs free energy of formation ofis greater than that of . Hence, C can reduce Cu2O to Cu.

Hence, the extraction of copper from its pyrite ore is difficultthan from its oxide ore through reduction

Topic:- Extraction of zinc

1. Write chemical reactions taking place in the extraction of zinc fromzinc blende.

The different steps involved in the extraction of zinc from zinc blende (ZnS) aregiven below:

(i) Concentration of ore

First, the gangue from zinc blende is removed by the froth floatation method.

(ii) Conversiontooxide (Roasting)

Sulphide ore is converted into oxide by the process of roasting. In this

process, ZnS is heated in a regular supply of air in a furnace at atemperature, which is below the melting point of Zn.

(iii)Extraction of zinc from zinc oxide (Reduction)

Zinc is extracted from zinc oxide by the process of reduction. The reductionof zinc oxide is carried out by mixing it with powdered coke and then,

heating it at 673 K.

(iv) Electrolytic Refining

Zinc can be refined by the process of electrolytic refining. In this process,impure zinc is made the anode while a pure copper strip is made thecathode. The electrolyte used is an acidified solution of zinc sulphate


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(ZnSO4). Electrolysis results in the transfer of zinc in pure from theanode to the cathode.

Topic:- Extraction of iron1.Write down the reactions taking place in different

zones in the blast furnace during the extraction ofiron.

During the extraction of iron, the reduction of iron oxides takesplace in the blast furnace. In this process, hot air is blown from the

bottom of the furnace and coke is burnt to raise the temperature upto 2200 K in the lower portion itself. The temperature is lower in theupper part. Thus, it is the lower part where the reduction of ironoxides (Fe2O3and Fe3O4) takes place.

The reactions taking place in the lower temperature range (500 800 K) in the blast furnace are:

The reactions taking place in the higher temperature range (900 1500 K) in the blast furnace are:

The silicate impurity of the ore is removed as slag by calcium oxide(CaO), which is formed by the decomposition of limestone (CaCO3).

2.What is pig iron?The iron which is obtained from blast furnace and containsabout 4% carbon and many other impurities in smalleramounts like S, P, Si, Mn etc, is called pig iron.


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3.What is cast iron?Iron obtained by melting pig iron with scrap iron and cokeusing hot air blast is cast iron.4.What is wrought iron?Wrought iron and malleable iron is the purest form of

commercial iron which is prepared from cast iron byoxidizing impurities in a reverberalory furnace lined withhematite.5.What is added as flux in extraction of iron?Limestone is used as flux in extraction of iron.

6.How is cast iron different from pig iron?

The iron obtained from blast furnaces is known as pig iron.It contains around 4% carbon and many impurities such asS, P, Si, Mn in smaller amounts.

Cast iron is obtained by melting pig iron and coke using ahot air blast. It contains a lower amount of carbon (3%) thanpig iron. Unlike pig iron, cast iron is extremely hard andbrittle

Topic:- Electrochemical Principles of Metallurgy

What is the basis of reduction of a molten metal salt?ExplainIn the reduction of molten metal salt, electrolysis is done which is based on

electrochemical principles following equation G nFE0

Here n is the number of electrons and E0 is the electrode potential of redox

couple. More reactive metals have large negative values of the electrodepotential and are difficult to reduce


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Electrolytic cell for the extraction of aluminium

Na3AlF6or CaF2is added to alumina to lower the melting point, and toincrease the conductivity of the electrolyte.

The overall reaction is

This electrolytic process is known as HallHeroult process. The oxygen liberated at the anode reacts with the carbon of the anode to

produce CO and CO2. The cell reactions are What is the role of cryolite in the metallurgy of aluminium?

Cryolite (Na3AlF6) has two roles in the metallurgy of aluminium:

1.To decrease the melting point of the mixture from 2323 K to 1140 K.

2.To increase the electrical conductivity of Al2O3.

What is the role of graphite rod in the electrometallurgy of aluminium?

In the electrometallurgy of aluminium, a fused mixture of purified alumina(Al2O3), cryolite (Na3AlF6) and fluorspar (CaF2) is electrolysed. In thiselectrolysis, graphite is used as the anode and graphite-lined iron is used asthe cathode. During the electrolysis, Al is liberated at the cathode, while CO

and CO2are liberated at the anode, according to the following equation.

If a metal is used instead of graphite as the anode, then O2will be liberated.This will not only oxidise the metal of the electrode, but also convert some of


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the Al liberated at the cathode back into Al2O3. Hence, graphite is used forpreventing the formation of O2 at the anode. Moreover, graphite is cheaperthan other metals.

At Cathode:At Anode:

Topic:- Oxidation Reduction; Refining.

1. Give an example of extraction based on oxidation reduction.An example based on extraction by oxidation is extraction of chlorine from brine.

2. How are gold and silver extracted? Gold and silver are extracted by leaching the metal with CN -. The metal is later recovered by

displacement method in which zinc acts as reducing agent.

3. Give two examples of metal refined bya) Distillation [3]

b) Liquation

c) Electrolytic refining

a) Distillation Zinc and Mercury

b) Liquation Tin and Antimony

c) Electrolytic refining

Copper and Zinc

4. Explain electrolytic refining of copper.In this method impure copper acts as anode and a strip of the same metal in the pure form is used

as cathode. The electrolyte is acidified solution of copper sulphate. The net result is the transfer of

copper in pure form from the anode to cathode.

Impurities from the blister copper like antimony, selenium, tellurium, silver, gold and platinum

deposit as anode mud.

5. Which method is used for refining of silicon or gallium?The refining of Silicon or gallium is done by Zone refining.

6. What is the principle behind zone refining?The principle of zone refining is that impurities are more soluble in the melt then in the solid state

of the metal.

7. Explain Zone refining:This method is based on the principle that impurities are more soluble in the molten state of metal (the melt)than in the solid state. In the process of zone refining, a circular mobile heater is fixed at one end of a rod of

impure metal. As the heater moves, the molten zone of the rod also moves with it. As a result, pure metacrystallizes out of the melt and the impurities pass onto the adjacent molten zone. This process is repeatedseveral times, which leads to the segregation of impurities at one end of the rod. Then, the end with theimpurities is cut off. Silicon, boron, gallium, indium etc. can be purified by this process.


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(ii) Column chromatography:

8.Give the requirements for vapour phase refining?The two requirements of vapour phase refining are :-1) The metal should from a volatile compound with an available

reagent,2) The volatile compound should be easily decomposable so thatrecovery is easy.

9.Write a short note on Monds process.Monds Process-In this process, nickel is heated in a stream of carbon monoxideto give a volatile complex, nickel tetra carbonyl.

The carbonyl is heated to higher temperature

10. Which method is used for refining of zirconium? Explain.Zirconium and Titanium are refined by van Ankle process, Herethe crude metal is heated in an evacuated vessel with iodine.

The metal iodide is decomposed on a tungsten filament,electrically heated to about 1800K and pure metal is depositedon the filament.

11. What is the principle behind chromatography? Name sometypes of chromatographic techniques.

The principle behind chromatography is that differentcomponents of a mixture are differently adsorbed on anadsorbent. Some of the chromatographic techniques are paperchromatography, column chromatography, gas chromatography etc.


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Refining: It is the process of converting an impure metal into pure metaldepending upon the nature of metal.

Distillation

Involves evaporation of pure metal Used for metals having low boiling points, such as Zn, Hg

Liquation

Involves flowing of low melting metal like tin on a sloping surface so thathigher melting impurities are left behind

Electrolytic Refining

Impure metal is refined using electricity. The impure metal is made the anode, and a strip of pure metal is made

the cathode. A solution of a soluble salt of the same metal is taken as the electrolyte. Impurities get collected below the anode and are known as anode mud. At anode: At cathode:

Anode mud of blister copper contains antimony, selenium, tellurium,silver, gold and platinum.

Zone Refining

Principle Impurities are more soluble in the molten state of metal (themelt) than in the solid state.

In this process, a circular heater is fixed at one end of a rod of impuremetal.


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As the heater moves, the molten zone of the rod also moves along with it

As a result, pure metal crystallises out of the melt, and the impuritiespass to the adjacent molten zone.

This process is repeated several times, which leads to the segregation ofimpurities at one end of the rod. Then, the end with the impurities is cutoff

.Vapour Phase Refining

In this process, the impure metal is converted into its volatilecompound, which is decomposed to obtain the pure metal.

To carry out this process o The metal should form a volatile compound with an available

reagento The volatile compound should be easily decomposable so that

the metal can be easily recovered

Nickel, zirconium and titanium are refined using this process

Mond process for refining nickelNickel is purified by Monds process. Nickel, when heated in stream of carbon monoxide formsvolatile Ni(CO)4 which on further subjecting to higher temperature decomposes to give puremetal.

van Arkel Method for refining zirconium or titaniumo All the oxygen and nitrogen present as impurity are removed.o Crude metal is heated with iodine in an evacuated vessel.


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o Metal iodide is decomposed on a tungsten filament, electricallyheated to about 1800 K to give the pure metal.

Chromatographic Methods

Principle Different components of a mixture are differentlyadsorbed on an adsorbent.

Some chromatographic techniques are o Column chromatographyo Paper chromatographyo Gas chromatography

There are two phases in chromatography: mobile phase and stationary

phase.

Column chromatography is useful for the purification of elementsavailable in minute quantities. It is also used for removing theimpurities that are not very different in chemical properties fromthe element to be purified.

Adsorbed components are removed (eluted) using suitable solvents(eluents).

Schematic diagrams of column chromatography in industrial andlaboratory methods are as follows:


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