JEE Chemistry General Principles and Processes Of Isolation Of Elements MCQs Set 10

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

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Multiple Choice Questions

Question 1: Sulphide ores of metals are usually concentrated by froth flotation. Which one of the following sulphide ores is an exception and is concentrated by chemical leaching?
(a) Sphalerite
(b) Argentite
(c) Galena
(d) Copper pyrites
Show Answer & Explanation

Answer: (b) Argentite

Explanation:
1. Sphalerite (ZnS), galena (PbS) and copper pyrites (CuFeS2) are concentrated by froth flotation.
2. Argentite (Ag2S) is leached with NaCN solution (cyanide process).

Teacher's Note:
1. Silver and gold ores are leached with cyanide.
2. Zinc blende is ZnS = sphalerite.

Question 2: Which of the following elements is present as an impurity to the maximum extent in pig iron?
(a) Manganese
(b) Carbon
(c) Silicon
(d) Phosphorus
Show Answer & Explanation

Answer: (b) Carbon

Explanation:
1. Pig iron contains about 2.5 to 5 percent carbon.
2. Mn, Si and P are present only in small amounts, so carbon is the major impurity.

Teacher's Note:
1. Pig iron has the highest carbon content among iron types.
2. Wrought iron has the least.

Question 3: The following reactions take place in the blast furnace in the preparation of impure iron. Identify the reaction pertaining to the formation of the slag.
(a) 2C(s) + O2(g) → 2CO(g)
(b) Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)
(c) CaCO3(s) → CaO(s) + CO2(g)
(d) CaO(s) + SiO2(s) → CaSiO3(s)
Show Answer & Explanation

Answer: (d) CaO(s) + SiO2(s) → CaSiO3(s)

Explanation:
1. Slag forms when the flux CaO combines with the silica gangue.
2. CaO + SiO2 → CaSiO3 is that reaction.

Teacher's Note:
1. Option 3 produces the flux but is not itself slag formation.
2. Slag is calcium silicate.

Question 4: In the extraction of copper from its sulphide ore, the metal is finally obtained by the reduction of cuprous oxide with
(a) Iron(II) sulphide (FeS)
(b) Carbon monoxide (CO)
(c) Copper(I) sulphide (Cu2S)
(d) Sulphur dioxide (SO2)
Show Answer & Explanation

Answer: (c) Copper(I) sulphide (Cu2S)

Explanation:
1. In the Bessemer converter Cu2O reacts with Cu2S.
2. 2Cu2O + Cu2S → 6Cu + SO2.

Teacher's Note:
1. This is self reduction.
2. FeS is removed earlier as slag.

Question 5: Identify the alloy containing a non-metal as a constituent.
(a) Bell metal
(b) Bronze
(c) Invar
(d) Steel
Show Answer & Explanation

Answer: (d) Steel

Explanation:
1. Bell metal is Cu + Sn, bronze is Cu + Sn and invar is Fe + Ni.
2. Steel contains carbon, which is a non-metal.

Teacher's Note:
1. Steel is an alloy of iron and carbon.
2. Brass is Cu + Zn.

Question 6: Which of the following impurity components of iron ore is mainly separated as slag during the extraction of Fe in the blast furnace?
(a) SiO2
(b) Fe3O4
(c) CO
(d) C
Show Answer & Explanation

Answer: (a) SiO2

Explanation:
1. The main gangue in iron ore is silica.
2. It combines with CaO from the limestone flux to form CaSiO3 slag.

Teacher's Note:
1. Slag = calcium silicate.
2. Fe3O4 is an intermediate that is reduced, not slagged.

Question 7: Match the items of Column I with those of Column II.
(a) Cyanide process (b) Froth flotation process (c) Electrolytic reduction (d) Zone refining
(i) Ultrapure Ge (ii) Dressing of ZnS (iii) Extraction of Al (iv) Extraction of Au (v) Purification of Ni
(a) (a)-(iv), (b)-(ii), (c)-(iii), (d)-(i)
(b) (a)-(ii), (b)-(iii), (c)-(i), (d)-(v)
(c) (a)-(i), (b)-(ii), (c)-(iii), (d)-(iv)
(d) (a)-(iii), (b)-(iv), (c)-(v), (d)-(i)
Show Answer & Explanation

Answer: (a) (a)-(iv), (b)-(ii), (c)-(iii), (d)-(i)

Explanation:
1. The cyanide process extracts Au (iv).
2. Froth flotation is used for dressing sulphide ores such as ZnS (ii).
3. Electrolytic reduction extracts Al (iii).
4. Zone refining gives ultrapure Ge (i).

Teacher's Note:
1. Ni is purified by Mond's process.
2. Zone refining suits semiconductors.

Question 8: Extraction of gold and silver involves leaching with CN- ion. Silver is later recovered by
(a) Distillation
(b) Zone refining
(c) Displacement with Zn
(d) Liquation
Show Answer & Explanation

Answer: (c) Displacement with Zn

Explanation:
1. Silver is present as [Ag(CN)2]-.
2. Zinc dust displaces it: 2[Ag(CN)2]- + Zn → [Zn(CN)4]2- + 2Ag.

Teacher's Note:
1. Zn is more electropositive than Ag.
2. The same step is used for gold.

Question 9: In the fusion of chromite ore with sodium carbonate in excess of air, which of the following is not formed?
(a) Fe2O3
(b) Na2CrO4
(c) CO2
(d) Na2Cr2O7
Show Answer & Explanation

Answer: (d) Na2Cr2O7

Explanation:
1. 4FeCr2O4 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2.
2. The sodium chromate is formed, but sodium dichromate forms only on subsequent acidification.

Teacher's Note:
1. Fusion with alkali gives chromate; acidification gives dichromate.
2. This is the first step of dichromate manufacture.

Question 10: Considering the Ellingham diagram, which of the following metals can be used to reduce alumina?
(a) Fe
(b) Zn
(c) Mg
(d) Cu
Show Answer & Explanation

Answer: (c) Mg

Explanation:
1. A metal can reduce an oxide that lies above it on the Ellingham diagram.
2. Only Mg (at high temperature, above about 1623 K where the lines cross) lies below Al2O3. Fe, Zn and Cu form less stable oxides.

Teacher's Note:
1. Reductant line must be below the oxide line.
2. The Mg and Al lines cross at high temperature.

Question 11: For the smelting of copper, the correct statement is
(a) Cu is reduced
(b) FeS is converted into FeO
(c) Fe is reduced
(d) Cu2S is converted into Cu2O
Show Answer & Explanation

Answer: (b) FeS is converted into FeO

Explanation:
1. In the blast furnace smelting step, Cu2O reacts with FeS: Cu2O + FeS → Cu2S + FeO.
2. FeO then forms slag with silica.
3. So the correct statement is that FeS is converted into FeO.

Teacher's Note:
1. Smelting removes iron as slag and gives matte.
2. Reduction of copper occurs later in the converter.

Question 12: The method of zone refining of metals is based on the principle of
(a) Greater mobility of the pure metal than that of the impurity
(b) Higher melting point of the impurity than that of the pure metal
(c) Greater noble character of the solid metal than that of the impurity
(d) Greater solubility of the impurity in the molten state than in the solid
Show Answer & Explanation

Answer: (d) Greater solubility of the impurity in the molten state than in the solid

Explanation:
1. A molten zone is moved along a rod.
2. Impurities are more soluble in the melt than in the solid, so they travel with the molten zone to the end of the rod.

Teacher's Note:
1. Used for Ge, Si, Ga and B.
2. The impure end is cut off.

Question 13: Which of the following sulphides, when heated strongly in air, gives the corresponding metal?
(a) ZnS
(b) CuS
(c) FeS
(d) HgS
Show Answer & Explanation

Answer: (d) HgS

Explanation:
1. ZnS gives ZnO and FeS gives iron oxide on roasting.
2. Roasting HgS gives HgO + SO2, and HgO decomposes at about 673 K to Hg.

Teacher's Note:
1. Mercury is obtained by simple roasting of cinnabar.
2. Its oxide is thermally unstable.

Question 14: Stainless steel does not rust because
(a) Chromium and nickel combine with iron
(b) Chromium forms an oxide layer and protects iron from rusting
(c) Nickel present in it does not rust
(d) Iron forms a hard chemical compound with the chromium present in it
Show Answer & Explanation

Answer: (b) Chromium forms an oxide layer and protects iron from rusting

Explanation:
1. Stainless steel contains 12 to 14 percent Cr and 2 to 4 percent Ni.
2. Chromium forms a thin, tough, adherent layer of oxide on the surface that protects the iron beneath.

Teacher's Note:
1. The oxide layer is self-healing.
2. Steel is an alloy of iron and carbon.

Question 15: Purification of Ge, which is a semiconductor, is done by
(a) Cyanide process
(b) Van Arkel process
(c) Alumino thermite process
(d) Zone refining
Show Answer & Explanation

Answer: (d) Zone refining

Explanation:
1. Ultrapure semiconductors such as Ge, Si and Ga are obtained by zone refining.

Teacher's Note:
1. Zone refining gives the extremely high purity needed for semiconductors.
2. Van Arkel is for Ti and Zr.

Question 16: The metal that cannot be obtained by electrolysis of an aqueous solution of its salts is
(a) Cu
(b) Cr
(c) Ag
(d) Ca
Show Answer & Explanation

Answer: (d) Ca

Explanation:
1. Cu, Cr and Ag lie below hydrogen-releasing potentials in water and are deposited at the cathode.
2. Calcium is very electropositive, so water is reduced instead and hydrogen is evolved.

Teacher's Note:
1. Active metals (Na, K, Ca, Mg, Al) need fused salt electrolysis.
2. Chromium is plated from aqueous chromic acid baths.

Question 17: In which of the following extractions is no reducing agent required?
(a) Iron from haematite
(b) Aluminium from cinnabar
(c) Magnesium from carnallite
(d) Zinc from zinc blende
Show Answer & Explanation

Answer: (c) Magnesium from carnallite

Explanation:
1. Fe and Zn oxides are reduced by carbon.
2. Cinnabar is a mercury ore, not an aluminium ore.
3. Magnesium is obtained by electrolysis of fused carnallite-derived MgCl2, where the electric current does the reduction and no chemical reducing agent is needed.

Teacher's Note:
1. Electrolytic reduction uses electrons as the reductant.
2. Cinnabar HgS is a mercury ore.

Question 18: Of the following reduction processes, the correct process(es) is/are
(a) B2O3 + Al →(heat) B
(b) Cr2O3 + 2Al →(heat) Cr
(c) TiCl4 + Mg →(heat) Ti
(d) All of the above
Show Answer & Explanation

Answer: (d) All of the above

Explanation:
1. Aluminothermic reduction gives Cr from Cr2O3 (balanced: Cr2O3 + 2Al → 2Cr + Al2O3) and boron from B2O3 (B2O3 + 2Al → 2B + Al2O3).
2. The Kroll process gives Ti: TiCl4 + 2Mg → Ti + 2MgCl2.
3. All three are valid.

Teacher's Note:
1. Each of the three is a real industrial reduction.
2. The Kroll process uses Mg to reduce TiCl4.

Question 19: Which of the following is/are correctly matched?
(a) Copper : Bessemer converter
(b) Iron : blast furnace
(c) Chromium : aluminothermic process
(d) All of the above
Show Answer & Explanation

Answer: (d) All of the above

Explanation:
1. Blister copper is made in the Bessemer converter, iron in the blast furnace and chromium by reduction of Cr2O3 with aluminium.
2. All three pairs are correct.

Teacher's Note:
1. Learn the equipment or process for each metal.
2. Chromium cannot be made by carbon reduction because of carbide formation.

Question 20:
Assertion (A): Ores are generally converted into oxides prior to reduction.
Reason (R): Metal oxides can be reduced easily.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Assertion is true: sulphides and carbonates are first roasted or calcined to oxides.
2. Reason is true: carbon, CO or Al can reduce oxides readily, while sulphides and carbonates are harder to reduce.
3. The reason explains the assertion.

Teacher's Note:
1. Reduction of sulphides by carbon is thermodynamically unfavourable.
2. Hence roast or calcine first.

Question 21:
Assertion (A): In the froth flotation process, sodium ethyl xanthate is used as a floating agent.
Reason (R): Sulphide ores are water soluble.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (c) A is true, but R is false.

Explanation:
1. Assertion is true: xanthate collectors make the sulphide particles water repellent.
2. Reason is false: sulphide ores are insoluble in water.

Teacher's Note:
1. Collectors adsorb on the ore surface.
2. Insolubility is what lets the ore reach the froth.

Question 22:
Assertion (A): CuFeS2 is concentrated by the froth flotation method.
Reason (R): CuFeS2 is the main ore of copper.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Assertion is true: CuFeS2 is a sulphide ore and sulphides are concentrated by froth flotation.
2. Reason is true: copper pyrites is the main ore of copper.
3. Being the main ore does not explain why flotation is used.

Teacher's Note:
1. Flotation is chosen because of the sulphide nature of the ore.
2. Importance of an ore is irrelevant to the method.

Question 23:
Assertion (A): Wolframite impurities are separated from cassiterite by electromagnetic separation.
Reason (R): Cassiterite is attracted by the magnet.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (c) A is true, but R is false.

Explanation:
1. Assertion is true: wolframite (FeWO4·MnWO4) is magnetic while SnO2 is not.
2. Reason is false: cassiterite is non-magnetic and it is the wolframite that is attracted.

Teacher's Note:
1. Magnetic separation of tin stone removes the magnetic impurity.
2. Check which component is the magnetic one.

Question 24:
Assertion (A): In the smelting of copper ore, coke is added to the blast furnace.
Reason (R): Coke reduces CuO into Cu.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (c) A is true, but R is false.

Explanation:
1. Assertion is true: coke is added.
2. Reason is false in this context: in copper smelting the coke serves mainly as a fuel, and copper is obtained later by self reduction in the Bessemer converter; the CuO is in effect converted to Cu2S by FeS.

Teacher's Note:
1. Cu2S is not reduced by carbon.
2. Blister copper arises from self-reduction.

Question 25:
Assertion (A): The reaction Cr2O3 + 2Al → Al2O3 + 2Cr does not start by itself at room temperature.
Reason (R): The ΔG value for this reaction is negative.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Assertion is true: the thermite mixture needs strong initial heating (an igniter) to start.
2. Reason is true: the reaction is thermodynamically favourable, with a large negative ΔG.
3. But a negative ΔG does not explain why the reaction fails to start at room temperature (that is a kinetic matter, activation energy).

Teacher's Note:
1. Feasibility (ΔG) and speed (kinetics) are separate ideas.
2. Thermite reactions need ignition.

Question 26:
Assertion (A): According to the Ellingham diagram, FeO can be reduced by carbon more easily than Cu2O.
Reason (R): The ΔG° of formation of FeO is −300 kJ whereas that of Cu2O is −450 kJ.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (d) A is false, but R is true.

Explanation:
1. The assertion is false: Cu2O lies higher on the diagram (less stable), so it is reduced more easily than FeO.
2. The reason is false: ΔG° of formation of Cu2O is less negative than that of FeO.

Teacher's Note:
1. Higher on the Ellingham diagram means easier reduction.
2. Copper oxides are reduced even by weak reductants.

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