JEE Chemistry General Principles and Processes of Isolation of Elements MCQs Set 19

Chemistry Objective Questions and Answers: General Principles and Processes Of Isolation Of Elements

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Multiple Choice Questions

Question 1: In froth flotation, pine oil is added as a:
(a) collector
(b) frother
(c) depressant
(d) activator
Show Answer & Explanation

Answer: (b) frother

Explanation:
1. Pine oil and eucalyptus oil produce a stable froth that carries the ore particles to the surface, so they are frothers.
2. Collectors are xanthates, depressants include NaCN and an activator is CuSO4.
3. Answer: (b)

Teacher's Note:
1. Frother: stable froth. Collector: makes ore water repellent.
2. Pine oil, cresols and eucalyptus oil are frothers.

Question 2: In the separation of ZnS from PbS by froth flotation, the depressant used is:
(a) CuSO4
(b) NaCN
(c) pine oil
(d) sodium ethyl xanthate
Show Answer & Explanation

Answer: (b) NaCN

Explanation:
1. NaCN forms a soluble complex with ZnS surface and prevents it from floating, so PbS alone is carried in the froth.
2. CuSO4 is an activator and xanthate a collector.
3. Answer: (b)

Teacher's Note:
1. Depressant stops one sulphide from floating.
2. NaCN depresses ZnS.

Question 3: Calcination of zinc carbonate gives:
(a) ZnO and CO2
(b) ZnO and CO
(c) Zn and CO2
(d) ZnS and CO2
Show Answer & Explanation

Answer: (a) ZnO and CO2

Explanation:
1. Calcination is heating in limited or no air; carbonates decompose to oxides.
2. ZnCO3 → ZnO + CO2.
3. Answer: (a)

Teacher's Note:
1. Calcination removes water or CO2.
2. No major oxidation occurs in calcination.

Question 4: The molten mass called matte in the extraction of copper from copper pyrites consists mainly of:
(a) Cu2O and FeO
(b) Cu2S and a little FeS
(c) Cu and FeSiO3
(d) CuO and SiO2
Show Answer & Explanation

Answer: (b) Cu2S and a little FeS

Explanation:
1. During smelting, FeO combines with silica to give FeSiO3 slag.
2. The molten mass left, called matte, consists mainly of Cu2S with a little FeS.
3. Answer: (b)

Teacher's Note:
1. Matte is the sulphide mixture.
2. Slag floats over matte.

Question 5: In the extraction of copper, the iron impurity is removed as slag by the reaction:
(a) FeO + SiO2 → FeSiO3
(b) FeS + SiO2 → FeSiO3 + S
(c) Fe + SiO2 → FeSiO3
(d) Fe2O3 + SiO2 → FeSiO3
Show Answer & Explanation

Answer: (a) FeO + SiO2 → FeSiO3

Explanation:
1. Iron has more affinity for oxygen than copper, so it forms FeO.
2. Acidic flux SiO2 combines with basic FeO to give fusible FeSiO3 slag.
3. Answer: (a)

Teacher's Note:
1. Flux + gangue gives slag.
2. Silica is the flux in copper smelting.

Question 6: The reaction that occurs in the Bessemer converter in the final stage of copper extraction and is called self reduction is:
(a) Cu2S + 2Cu2O → 6Cu + SO2
(b) Cu2O + C → 2Cu + CO
(c) CuO + CO → Cu + CO2
(d) CuSO4 + Zn → Cu + ZnSO4
Show Answer & Explanation

Answer: (a) Cu2S + 2Cu2O → 6Cu + SO2

Explanation:
1. Part of Cu2S is oxidised to Cu2O; this oxide then reacts with the remaining sulphide: 2Cu2O + Cu2S → 6Cu + SO2.
2. No external reducing agent is needed, so this is self reduction, and the product is blister copper.
3. Answer: (a)

Teacher's Note:
1. Self reduction: oxide reacts with sulphide of the same metal.
2. Used for Cu, Pb and Hg.

Question 7: The principal ore of iron used in the blast furnace is:
(a) magnetite
(b) haematite
(c) siderite
(d) iron pyrites
Show Answer & Explanation

Answer: (b) haematite

Explanation:
1. Haematite, Fe2O3, is the main ore of iron for the blast furnace; magnetite Fe3O4 is also used but less commonly, and siderite is FeCO3.
2. Answer: (b)

Teacher's Note:
1. Ores: haematite Fe2O3, magnetite Fe3O4, siderite FeCO3, pyrites FeS2.
2. Haematite is the chief ore.

Question 8: In the zone of reduction of the blast furnace, the main reducing agent is:
(a) carbon
(b) carbon monoxide
(c) hydrogen
(d) silicon
Show Answer & Explanation

Answer: (b) carbon monoxide

Explanation:
1. Hot coke burns to CO2, which reacts with more carbon to give CO.
2. CO rising through the charge reduces iron oxides: Fe2O3 + 3CO → 2Fe + 3CO2.
3. Answer: (b)

Teacher's Note:
1. The reduction occurs in stages: Fe2O3, Fe3O4, FeO, Fe.
2. The zone temperature is about 675–975 K.

Question 9: The carbon content of wrought iron is about:
(a) 2.5–5%
(b) 0.12–0.25%
(c) 0.25–2%
(d) 4–6%
Show Answer & Explanation

Answer: (b) 0.12–0.25%

Explanation:
1. Wrought iron is the purest commercial form of iron and has only 0.12–0.25% carbon.
2. Steel has 0.25–2% and pig/cast iron 2.5–5%.
3. Answer: (b)

Teacher's Note:
1. Order of carbon content: cast iron > steel > wrought iron.
2. Order of melting point is the reverse.

Question 10: Which of the following has the highest melting point?
(a) Cast iron
(b) Steel
(c) Wrought iron
(d) Pig iron
Show Answer & Explanation

Answer: (c) Wrought iron

Explanation:
1. Melting point rises as carbon content falls.
2. Wrought iron has the least carbon, so it has the highest melting point.
3. Answer: (c)

Teacher's Note:
1. Wrought iron > steel > cast iron.
2. Carbon lowers the melting point.

Question 11: Steel is made hard on the surface by heating it at about 1000 K in an atmosphere of ammonia. This process is called:
(a) nitriding
(b) case hardening
(c) annealing
(d) tempering
Show Answer & Explanation

Answer: (a) nitriding

Explanation:
1. Ammonia gives a hard iron nitride coating on the surface.
2. This surface treatment is called nitriding.
3. Answer: (a)

Teacher's Note:
1. Case hardening uses charcoal.
2. Nitriding uses NH3.

Question 12: In the cyanide process for extraction of silver, the metal is obtained from the soluble complex by treatment with:
(a) scrap zinc
(b) sodium
(c) carbon
(d) hydrogen
Show Answer & Explanation

Answer: (a) scrap zinc

Explanation:
1. Silver ore is leached with NaCN to form Na[Ag(CN)2].
2. Zinc then displaces silver: 2Na[Ag(CN)2] + Zn → Na2[Zn(CN)4] + 2Ag.
3. Answer: (a)

Teacher's Note:
1. Zn is more electropositive than Ag.
2. Gold is extracted in the same way.

Question 13: In the cyanide extraction of silver from argentite, air is blown through the solution to:
(a) oxidise Na2S formed and drive the reaction forward
(b) reduce Ag+
(c) remove CO2
(d) dissolve silver
Show Answer & Explanation

Answer: (a) oxidise Na2S formed and drive the reaction forward

Explanation:
1. Ag2S + 4NaCN ⇌ 2Na[Ag(CN)2] + Na2S is reversible.
2. Air oxidises Na2S to Na2S2O3 and Na2SO4, so the reaction goes to completion.
3. Answer: (a)

Teacher's Note:
1. Removal of a product shifts equilibrium forward.
2. Oxygen from air is the oxidant.

Question 14: Serpeck's process of purification of bauxite is used when the bauxite contains mainly the impurity of:
(a) Fe2O3
(b) SiO2
(c) TiO2
(d) CaO
Show Answer & Explanation

Answer: (b) SiO2

Explanation:
1. Serpeck's process is used for white bauxite containing silica: bauxite is heated with coke in nitrogen to give AlN, while silica is removed as volatile SiO.
2. AlN is hydrolysed to Al(OH)3 and NH3.
3. Answer: (b)

Teacher's Note:
1. Silica impurity: Serpeck's process.
2. Iron oxide impurity: Baeyer's or Hall's process.

Question 15: In the electrolytic extraction of aluminium, the graphite anode is replaced periodically because:
(a) it dissolves in the electrolyte
(b) oxygen liberated at the anode burns it to CO and CO2
(c) it is reduced
(d) it forms Al4C3
Show Answer & Explanation

Answer: (b) oxygen liberated at the anode burns it to CO and CO2

Explanation:
1. Oxygen liberated at the anode reacts with hot graphite to form CO and CO2.
2. So the anode is consumed gradually and must be replaced.
3. Answer: (b)

Teacher's Note:
1. Cathode is the carbon-lined tank.
2. Aluminium collects at the bottom.

Question 16: Hoop's process is used for the electrolytic refining of:
(a) aluminium
(b) copper
(c) zinc
(d) lead
Show Answer & Explanation

Answer: (a) aluminium

Explanation:
1. Hoop's process uses molten impure aluminium as the anode and molten pure aluminium as the cathode, with a molten cryolite-fluoride electrolyte in between.
2. Answer: (a)

Teacher's Note:
1. Three liquid layers of different densities.
2. Aluminium can be refined to 99.98%.

Question 17: Magnesium is manufactured by electrolysis of fused:
(a) MgCl2 with NaCl and NaF in an atmosphere of coal gas
(b) MgO in cryolite
(c) MgSO4 solution
(d) Mg(OH)2 in water
Show Answer & Explanation

Answer: (a) MgCl2 with NaCl and NaF in an atmosphere of coal gas

Explanation:
1. Fused MgCl2 (mixed with NaCl and NaF) is electrolysed at about 970 K in an iron pot, which acts as the cathode.
2. Coal gas prevents the attack of air on molten Mg.
3. Answer: (a)

Teacher's Note:
1. Aqueous electrolysis would not give Mg.
2. Chlorine is liberated at the graphite anode.

Question 18: Metals of very high purity such as germanium, silicon and gallium are obtained by:
(a) liquation
(b) zone refining
(c) cupellation
(d) poling
Show Answer & Explanation

Answer: (b) zone refining

Explanation:
1. Zone refining depends on the impurities being more soluble in the molten metal than in the solid.
2. A moving heater carries impurities to one end of the rod.
3. Answer: (b)

Teacher's Note:
1. Semiconductor-grade silicon uses zone refining.
2. The impure end is cut off.

Question 19: Titanium and zirconium are purified by:
(a) Van Arkel method
(b) Mond process
(c) Parke's process
(d) electrolytic refining in aqueous solution
Show Answer & Explanation

Answer: (a) Van Arkel method

Explanation:
1. In the Van Arkel method the impure metal is heated with iodine at about 523 K to form volatile TiI4, which on a hot tungsten filament at about 1700 K decomposes to pure titanium.
2. Answer: (a)

Teacher's Note:
1. Vapour-phase refining.
2. Used for small quantities of very pure metal.

Question 20: Nickel is purified by Mond's process, which involves the formation of:
(a) Ni(CO)4
(b) NiO
(c) NiS
(d) Ni(NH3)6Cl2
Show Answer & Explanation

Answer: (a) Ni(CO)4

Explanation:
1. Impure nickel is heated with CO at about 330–350 K to form volatile Ni(CO)4.
2. The carbonyl decomposes at about 450–470 K to give pure nickel and CO.
3. Answer: (a)

Teacher's Note:
1. Mond's process: vapour phase refining.
2. CO is recycled.

Question 21: In the electrolytic refining of copper, the anode mud contains:
(a) Zn and Fe
(b) Ag, Au and Pt
(c) Cu only
(d) CuSO4
Show Answer & Explanation

Answer: (b) Ag, Au and Pt

Explanation:
1. Less electropositive impurities such as Ag, Au and Pt do not oxidise at the anode and fall as anode mud.
2. More electropositive impurities such as Zn and Fe go into solution.
3. Answer: (b)

Teacher's Note:
1. Anode: impure copper. Cathode: pure copper.
2. Electrolyte: acidified CuSO4.

Question 22: According to the Ellingham diagram, a metal can reduce the oxide of another metal if:
(a) its line lies below the line of the other metal oxide
(b) its line lies above the line of the other metal oxide
(c) the two lines are parallel
(d) its oxide is more stable than CO2
Show Answer & Explanation

Answer: (a) its line lies below the line of the other metal oxide

Explanation:
1. The metal whose oxide has the more negative ΔG° of formation (line lower down) can reduce oxides lying above it.
2. The overall ΔG° of the redox reaction is then negative.
3. Answer: (a)

Teacher's Note:
1. Lower line means more stable oxide.
2. Aluminium reduces Cr2O3 by this rule.

Question 23: Which of the following ores is correctly matched with its formula?
(a) Malachite – CuCO3·Cu(OH)2
(b) Zinc blende – ZnCO3
(c) Magnetite – Fe2O3
(d) Cryolite – Na3AlF6·CaF2
Show Answer & Explanation

Answer: (a) Malachite – CuCO3·Cu(OH)2

Explanation:
1. Malachite is the basic copper carbonate CuCO3·Cu(OH)2.
2. Zinc blende is ZnS, magnetite is Fe3O4 and cryolite is Na3AlF6.
3. Answer: (a)

Teacher's Note:
1. Calamine is ZnCO3.
2. Haematite is Fe2O3.

Question 24: Bauxite is an ore of aluminium with the formula:
(a) Al2O3·2H2O
(b) Na3AlF6
(c) Al2O3
(d) KAl(SO4)2·12H2O
Show Answer & Explanation

Answer: (a) Al2O3·2H2O

Explanation:
1. Bauxite is hydrated alumina, Al2O3·2H2O.
2. Cryolite is Na3AlF6 and corundum is anhydrous Al2O3.
3. Answer: (a)

Teacher's Note:
1. Bauxite is the main source of aluminium.
2. It is purified by Baeyer's or Hall's process.

Question 25: Liquation is used for refining metals which:
(a) have low melting points and are associated with high melting impurities
(b) have high boiling points
(c) are volatile
(d) are present with oxidisable impurities
Show Answer & Explanation

Answer: (a) have low melting points and are associated with high melting impurities

Explanation:
1. The impure metal is heated on a sloping hearth; the low melting pure metal flows down, leaving the infusible impurities behind.
2. It is used for Sn, Pb, Bi and Sb.
3. Answer: (a)

Teacher's Note:
1. Different melting points allow separation.
2. Distillation is used for volatile metals like Zn and Hg.

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