JEE Chemistry General Principles and Processes Of Isolation Of Elements MCQs Set 09

Answer: (d) Al

Explanation:
1. The thermite mixture is Al powder with a metal oxide, such as Fe₂O₃.
2. Aluminium acts as the reducing agent.

Teacher's Note:
1. The Fe₂O₃ + Al reaction is used to weld railway tracks.
2. Heat released is enough to melt the iron formed.

Answer: (d) All of these

Explanation:
1. Liquation separates a low melting metal from high melting impurities on a sloping hearth.
2. It is used for Sn, Pb and Bi.

Teacher's Note:
1. The pure metal melts and flows down; impurities remain behind.
2. Zinc and mercury are refined by distillation.

Answer: (a) Cu and Pb

Explanation:
1. Self-reduction needs a sulphide that gives an oxide which reacts with the remaining sulphide.
2. Cu₂S and PbS do this. Zn, Al and Fe do not.
3. Hg is self-reduced but Zn is not, so the pair Zn and Hg is wrong.

Teacher's Note:
1. Cu, Pb and Hg are the classic auto-reduction metals.
2. Zinc oxide needs carbon, so pairs containing Zn are wrong.

Answer: (b) Calcination

Explanation:
1. Heating a hydrated or hydroxide ore, usually in absence of air, to drive off water or CO₂ is calcination.
2. Al₂O₃·2H₂O → Al₂O₃ + 2H₂O.

Teacher's Note:
1. Calcination: water or CO₂ is removed.
2. Roasting: sulphur is removed as SO₂.

Answer: (a) Smelting

Explanation:
1. Smelting is reduction of the ore with carbon at high temperature, when the metal is produced in the molten state.
2. Roasting and calcination only change the ore chemically; froth flotation is a concentration step.

Teacher's Note:
1. Smelting = reduction in a furnace to a molten metal.
2. Flux is added to form slag.

Answer: (c) In the anodic mud

Explanation:
1. Ag and Au are less electropositive than Cu, so they are not oxidised at the anode.
2. They fall to the bottom as anode mud.

Teacher's Note:
1. Noble metals settle as anode mud.
2. More reactive impurities such as Zn and Fe dissolve into the electrolyte.

Answer: (d) Cupellation

Explanation:
1. Impure silver containing lead is heated in a bone-ash cupel in a current of air.
2. Lead is oxidised to PbO, which is absorbed by the cupel, leaving silver.

Teacher's Note:
1. Cupellation works when the impurity oxidises more easily than the metal.
2. Silver does not form a stable oxide at that temperature.

Answer: (b) Al

Explanation:
1. Oxides of Pb, Sn and Zn are reduced by carbon.
2. Al₂O₃ is far too stable and is reduced only by electrolysis.

Teacher's Note:
1. Carbon reduction works for metals less reactive than Zn or comparable to it.
2. Aluminium is obtained electrolytically.

Answer: (b) Magnetic separation

Explanation:
1. The ore and the gangue differ in magnetic character.
2. Chromite is separated from its non-magnetic gangue by magnetic separation.

Teacher's Note:
1. Magnetic separation is used for iron-containing ores like chromite.
2. Leaching and flotation are not used for oxides like this.

Answer: (c) Fe₂O₃ + CO → 2Fe + 3CO₂

Explanation:
1. Option 1 is roasting.
2. Option 2 is calcination.
3. Option 4 is the aluminothermic process.
4. Option 3 is reduction of an oxide with carbon (monoxide) in a furnace, which is smelting.

Teacher's Note:
1. Smelting uses carbon or CO to reduce an oxide.
2. Thermite is a different class of reduction.

Answer: (c) b, d

Explanation:
1. (a) is correct and (c) is correct.
2. The cyanide (MacArthur-Forrest) process is for Ag and Au, not Fe, so (b) is incorrect.
3. Van Arkel refining is for Ti and Zr, not Au, so (d) is incorrect.

Teacher's Note:
1. Mond gives Ni, Van Arkel gives Ti and Zr.
2. Cyanide is for noble metals.

Answer: (b) FeSiO₃

Explanation:
1. Roasting converts part of FeS to FeO.
2. FeO + SiO₂ → FeSiO₃, the slag removed during smelting.

Teacher's Note:
1. Iron goes out as a silicate slag.
2. Silica is the flux for the iron impurity.

Answer: (a) A molten mixture of fluorides of aluminium, sodium and barium

Explanation:
1. Hoope's process refines aluminium with a molten fluoride electrolyte (Na₃AlF₆, AlF₃ and BaF₂).
2. Aqueous solutions cannot be used because hydrogen would be released instead of aluminium.

Teacher's Note:
1. Aluminium cannot be deposited from water.
2. The middle layer of the Hoope cell is the molten fluoride electrolyte.

Answer: (c) Smelting : an oxidation process

Explanation:
1. Poling refines copper and cupellation refines silver, and roasting is oxidation of the ore.
2. Smelting is reduction of the oxide to the metal, so calling it an oxidation process is wrong.

Teacher's Note:
1. Smelting is reduction.
2. Roasting is oxidation.

Answer: (d) The enthalpy of formation of Al₂O₃ is too high

Explanation:
1. The oxidation of Al to Al₂O₃ is far more exothermic than that of carbon to CO or CO₂.
2. At attainable temperatures the Al line stays below the carbon lines, so carbon cannot reduce Al₂O₃.

Teacher's Note:
1. Reduction needs the reductant's oxide line to lie below the metal's line.
2. Option 2 states the reverse of the truth.

Answer: (d) All of these

Explanation:
1. Ag, Au and Pt are all less reactive than copper.
2. They do not dissolve and settle as anode mud.

Teacher's Note:
1. Anode mud is a valuable by-product.
2. Also contains Se and Te.

Answer: (c) 2Fe + Al₂O₃ → 2Al + Fe₂O₃

Explanation:
1. In the thermite process aluminium reduces other metal oxides.
2. Reduction of Al₂O₃ by iron (option 3) is not possible, since Al₂O₃ is more stable than Fe₂O₃.

Teacher's Note:
1. Al is the reducing agent, never the product metal.
2. The reverse reaction is the thermite reaction.

Answer: (b) Not easily reduced by carbon

Explanation:
1. Cr₂O₃, Mn₃O₄ and similar oxides cannot be reduced by carbon without forming carbides or needing very high temperatures.
2. Aluminium, which has greater affinity for oxygen, is therefore used.

Teacher's Note:
1. Aluminium is the next choice when carbon fails.
2. Used for Cr, Mn and for welding iron.

Answer: (b) B

Explanation:
1. In A an external reducing agent (carbon) is used.
2. In B the PbO formed reacts with unchanged PbS to give Pb and SO₂ with no added reductant. This is self reduction.

Teacher's Note:
1. Self reduction means the ore reduces itself.
2. A needs coke, so it is smelting.

Answer: (b) a-3, b-4, c-1, d-2

Explanation:
1. Gold is leached with cyanide, which is hydrometallurgy: a-3.
2. Bauxite is purified by Bayer's process: b-4.
3. Lead sulphide is self reduced: c-1.
4. Tin is refined by liquation: d-2.

Teacher's Note:
1. Hydrometallurgy is for Au and Ag.
2. Liquation is a refining step for low melting Sn.

Answer: (c) CO in most parts of the furnace

Explanation:
1. CO, formed by C + CO₂ → 2CO, reduces Fe₂O₃ to Fe₃O₄, FeO and Fe through most of the furnace.
2. Direct reduction by carbon occurs only in the small hottest region.

Teacher's Note:
1. CO is the main reductant over most of the stack.
2. Statements 2 and 4 reverse or restrict the zones.

Answer: (b) Aluminium is extracted by Hall's process, which involves carbon reduction

Explanation:
1. Tin is smelted with carbon and silver is obtained by the cyanide process.
2. Aluminium is obtained by electrolytic reduction of alumina (Hall-Heroult), not by carbon reduction, so statement 2 is incorrect.

Teacher's Note:
1. Carbon only provides the anode in the aluminium cell.
2. Bessemerisation is for copper (and steel).

Answer: (b) Roasting followed by reduction with carbon

Explanation:
1. ZnS is roasted to ZnO: 2ZnS + 3O₂ → 2ZnO + 2SO₂.
2. ZnO is reduced with coke at about 1673 K: ZnO + C → Zn + CO.

Teacher's Note:
1. Zinc distils over and is condensed.
2. Self-reduction is not possible for zinc.

Answer: (b) 1-R, 2-S, 3-Q, 4-P

Explanation:
1. Zincite is ZnO, an oxide ore (R).
2. Malachite is a carbonate ore (S).
3. Horn silver AgCl is a halide ore (Q).
4. Iron pyrites FeS₂ is a sulphide ore (P).

Teacher's Note:
1. Classify ores by anion.
2. Memorise the formulae of ores along with names.

Answer: (d) Separate two sulphide ores

Explanation:
1. A depressant prevents one sulphide from forming froth.
2. For a mixture of PbS and ZnS, NaCN depresses ZnS so that PbS alone goes into the froth.

Teacher's Note:
1. Depressants allow selective flotation.
2. Collectors, not depressants, enhance water repellency.

Answer: (c) Ag₂S + 4NaCN → 2Na[Ag(CN)₂] + Na₂S

Explanation:
1. The cyanide process dissolves silver ore with NaCN solution as a soluble dicyanoargentate complex.
2. Option 3 is that reaction.
3. Options 1 and 2 are dissolution of silver halides in hypo and ammonia, which are not part of the cyanide process.

Teacher's Note:
1. Add Zn afterwards to precipitate silver.
2. Air oxidises the Na₂S formed so that the reaction goes to completion.