JEE Chemistry General Principles and Processes Of Isolation Of Elements MCQs Set 05

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

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Multiple Choice Questions

Question 1: In the extraction of copper from its sulphide ore, the metal is finally obtained by the reduction of cuprous oxide with
(a) copper(I) sulphide (Cu2S)
(b) sulphur dioxide
(c) iron(II) sulphide
(d) carbon monoxide
Show Answer & Explanation

Answer: (a) copper(I) sulphide (Cu2S)

Explanation:
1. In the Bessemer converter, 2Cu2O + Cu2S → 6Cu + SO2.
2. This is self-reduction, so Cu2S is the reductant.

Teacher's Note:
1. The product is blister copper (about 98% Cu).

Question 2: Heating a mixture of Cu2O and Cu2S gives
(a) Cu + SO2
(b) Cu + SO3
(c) CuO + CuS
(d) Cu2O + S
Show Answer & Explanation

Answer: (a) Cu + SO2

Explanation:
1. 2Cu2O + Cu2S → 6Cu + SO2.
2. Sulphide ion is oxidised to SO2 while Cu+ is reduced to Cu.

Teacher's Note:
1. Self-reduction needs no external reductant.

Question 3: The slag obtained during extraction of copper from copper pyrites is mainly
(a) CaSiO3
(b) FeSiO3
(c) CuSiO3
(d) SiO2
Show Answer & Explanation

Answer: (b) FeSiO3

Explanation:
1. Iron is oxidised to FeO in roasting and combines with the added silica: FeO + SiO2 → FeSiO3.

Teacher's Note:
1. Silica is an acidic flux for the basic FeO.

Question 4: What is the composition of matte in copper extraction?
(a) Cu2O + FeS
(b) Cu2O + Cu2S
(c) FeS + SiO2
(d) Cu2S + FeS
Show Answer & Explanation

Answer: (d) Cu2S + FeS

Explanation:
1. Matte, the product of smelting the roasted ore with silica, is a molten mixture of Cu2S and FeS.

Teacher's Note:
1. Matte is then treated in the Bessemer converter.

Question 5: Blister copper contains about
(a) 60% Cu
(b) 90% Cu
(c) 98% Cu
(d) 100% Cu
Show Answer & Explanation

Answer: (c) 98% Cu

Explanation:
1. It is called blister copper because SO2 escapes during solidification, making a blistered surface; purity about 98%.

Teacher's Note:
1. It is refined by electrolysis or poling.

Question 6: Which of the following reactions does not occur in the Bessemer converter?
(a) FeS + Cu2O → Cu2S + FeO
(b) 2Cu2S + 3O2 → 2Cu2O + 2SO2
(c) 2CuFeS2 + O2 → Cu2S + 2FeS + SO2
(d) FeO + SiO2 → FeSiO3
Show Answer & Explanation

Answer: (c) 2CuFeS2 + O2 → Cu2S + 2FeS + SO2

Explanation:
1. Partial roasting of the concentrated ore (c) is done in a reverberatory furnace before the converter stage.
2. The other three occur in the converter.

Teacher's Note:
1. The converter handles matte, not raw ore.

Question 7: Silica is added in the extraction of copper to remove
(a) cuprous sulphide
(b) ferrous oxide
(c) cupric oxide
(d) cuprous oxide
Show Answer & Explanation

Answer: (b) ferrous oxide

Explanation:
1. FeO + SiO2 → FeSiO3 (slag).

Teacher's Note:
1. Limestone, not silica, is used as a flux in iron extraction.

Question 8: Which species is desirable in the extraction of copper but not in the extraction of iron?
(a) CaSiO3
(b) FeSiO3
(c) SiO2
(d) Coke
Show Answer & Explanation

Answer: (c) SiO2

Explanation:
1. Copper extraction adds SiO2 to slag away FeO.
2. Iron extraction already contains silica as gangue and needs CaO to remove it.

Teacher's Note:
1. Flux is chosen to match the impurity: acidic impurities need basic flux and basic ones need acidic flux.

Question 9: Which metal is obtained directly by roasting its sulphide ore?
(a) Cu
(b) Pb
(c) Hg
(d) Zn
Show Answer & Explanation

Answer: (c) Hg

Explanation:
1. HgS + O2 → Hg + SO2: HgO formed is unstable and decomposes at roasting temperature.

Teacher's Note:
1. Hg is the only common metal that comes from roasting alone.

Question 10: After partial roasting, the sulphide of lead is reduced by
(a) carbon
(b) electrolysis
(c) self-reduction
(d) cyanide process
Show Answer & Explanation

Answer: (c) self-reduction

Explanation:
1. 2PbS + 3O2 → 2PbO + 2SO2 and then PbS + 2PbO → 3Pb + SO2.

Teacher's Note:
1. Lead is also obtained by carbon reduction of PbO.

Question 11: Which of the following sulphides is not commercially extracted by self-reduction?
(a) Ag2S
(b) PbS
(c) CuFeS2
(d) HgS
Show Answer & Explanation

Answer: (a) Ag2S

Explanation:
1. Pb, Cu and Hg are obtained by self-reduction.
2. Ag2S is leached with NaCN and displaced by zinc.

Teacher's Note:
1. Silver is a noble metal that is extracted by cyanide leaching.

Question 12: Pb and Sn are extracted from their chief ores by
(a) carbon reduction and self-reduction respectively
(b) self-reduction and carbon reduction respectively
(c) electrolysis and self-reduction
(d) self-reduction and electrolysis
Show Answer & Explanation

Answer: (b) self-reduction and carbon reduction respectively

Explanation:
1. Lead from galena: partial roasting then self-reduction.
2. Tin from cassiterite SnO2: reduced with carbon.

Teacher's Note:
1. Remember Pb = self, Sn = carbon.

Question 13: The alumino-thermite (Goldschmidt) process is used for the extraction of
(a) Al
(b) Cr
(c) Cu
(d) Na
Show Answer & Explanation

Answer: (b) Cr

Explanation:
1. Cr2O3 + 2Al → Al2O3 + 2Cr.
2. Aluminium acts as the reducing agent for oxides that cannot be reduced by carbon.

Teacher's Note:
1. Cr and Mn are extracted this way.

Question 14: In the alumino-thermite process, aluminium acts as
(a) an oxidising agent
(b) a flux
(c) a reducing agent
(d) an ignition agent
Show Answer & Explanation

Answer: (c) a reducing agent

Explanation:
1. Al is oxidised to Al2O3 and reduces the metal oxide.

Teacher's Note:
1. The strongly exothermic reaction is ignited with magnesium ribbon.

Question 15: Aluminium is extracted in large quantities by
(a) heating cryolite in a limited supply of air
(b) reducing aluminium oxide with coke
(c) reducing aluminium oxide with sodium
(d) electrolysing aluminium oxide dissolved in fused cryolite
Show Answer & Explanation

Answer: (d) electrolysing aluminium oxide dissolved in fused cryolite

Explanation:
1. Al2O3 is electrolysed in molten Na3AlF6 with CaF2 (Hall-Heroult process).

Teacher's Note:
1. Carbon reduction of Al2O3 is not feasible below very high temperature.

Question 16: The electrolyte used in the commercial extraction of aluminium is
(a) Al(OH)3 in NaOH solution
(b) molten mica and CaF2
(c) a molten mixture of Al2O3, Na3AlF6 and CaF2
(d) molten potash alum
Show Answer & Explanation

Answer: (c) a molten mixture of Al2O3, Na3AlF6 and CaF2

Explanation:
1. Alumina is dissolved in cryolite with fluorspar, lowering the melting point to about 1230 K and improving conductivity.

Teacher's Note:
1. Pure alumina melts at about 2323 K.

Question 17: During the extraction of aluminium, the carbon anodes are replaced periodically because
(a) they form Al4C3
(b) they melt and mix with the electrolyte
(c) oxygen liberated at the anode reacts with the carbon to form CO and CO2
(d) carbon anodes reduce Al2O3
Show Answer & Explanation

Answer: (c) oxygen liberated at the anode reacts with the carbon to form CO and CO2

Explanation:
1. O2− is discharged at the anode as oxygen, which burns the graphite away as CO and CO2.

Teacher's Note:
1. Cathode: Al3+ + 3e− → Al.

Question 18: Which statement about the Hall-Heroult process is incorrect?
(a) Cryolite lowers the working temperature
(b) Al is obtained at the cathode
(c) Anode rods are consumed by oxidation
(d) Pure aluminium floats above the molten electrolyte
Show Answer & Explanation

Answer: (d) Pure aluminium floats above the molten electrolyte

Explanation:
1. Molten aluminium is denser than the electrolyte, so it collects at the bottom of the cell and is tapped from there.

Teacher's Note:
1. The cell lining acts as the cathode.

Question 19: Bauxite is leached with
(a) N2 and coke
(b) NaCN
(c) NaOH
(d) Na2CO3
Show Answer & Explanation

Answer: (c) NaOH

Explanation:
1. Al2O3·2H2O + 2NaOH → 2NaAlO2 + 3H2O at 500 K and 35 bar.

Teacher's Note:
1. Fe2O3 and TiO2 do not dissolve.

Question 20: Bauxite containing Al2O3, SiO2, TiO2 and Fe2O3 is treated with concentrated NaOH at 500 K and 35 bar. The species present in the filtrate are
(a) NaAlO2 only
(b) Na2Ti(OH)6 only
(c) both NaAlO2 and Na2SiO3
(d) Na2SiO3 only
Show Answer & Explanation

Answer: (c) both NaAlO2 and Na2SiO3

Explanation:
1. Amphoteric Al2O3 and acidic SiO2 dissolve in NaOH.
2. Fe2O3 and TiO2 remain as residue.

Teacher's Note:
1. Aluminate is later precipitated as Al(OH)3 by CO2.

Question 21: Magnesium is commercially extracted by
(a) self-reduction
(b) Dow's sea water process
(c) thermite reduction
(d) leaching followed by metal displacement
Show Answer & Explanation

Answer: (b) Dow's sea water process

Explanation:
1. In Dow's process Mg(OH)2 precipitated from sea water with lime is converted into MgCl2 and electrolysed in the fused state.

Teacher's Note:
1. Carnallite can also be electrolysed.

Question 22: Fused carnallite on electrolysis gives
(a) Ca and Cl2
(b) Na and CO2
(c) Al and Cl2
(d) Mg and Cl2
Show Answer & Explanation

Answer: (d) Mg and Cl2

Explanation:
1. Carnallite KCl·MgCl2 contains Mg2+, which is reduced at the cathode more readily than K+; Cl2 is evolved at the anode.

Teacher's Note:
1. NaCl and CaCl2 are added to lower the melting point.

Question 23: NaCl and CaCl2 are added to fused MgCl2 during the extraction of Mg because
(a) the melting point of the electrolyte decreases and its conductivity increases
(b) the melting point increases and conductivity decreases
(c) both melting point and conductivity decrease
(d) both melting point and conductivity increase
Show Answer & Explanation

Answer: (a) the melting point of the electrolyte decreases and its conductivity increases

Explanation:
1. The mixture is a eutectic-type melt with a lower melting point and more mobile ions.

Teacher's Note:
1. Fused MgCl2 alone is a poor conductor.

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Multiple Choice Questions (MCQs) for JEE Chemistry General Principles and Processes Of Isolation Of Elements

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