JEE Chemistry General Principles And Processes Of Isolation Of Elements MCQs Set 18

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Multiple Choice Questions

Question 1: Magnesium is obtained by electrolysing fused \( \ce{MgCl2} \) (with NaCl and \( \ce{CaCl2} \)) using
(a) an iron container as cathode and a graphite rod as anode
(b) a nickel cathode and a graphite anode
(c) an iron container as anode and a nickel cathode
(d) a nickel container as cathode and an iron anode
Show Answer & Explanation

Answer: (a) an iron container as cathode and a graphite rod as anode

Explanation:
1. Mg is liberated at the iron cathode (the cell wall), chlorine at the graphite anode.

Teacher's Note:
1. The graphite anode resists chlorine.

Question 2: The process of removing lighter gangue by washing in a current of water is called
(a) liquation
(b) leaching
(c) cupellation
(d) levigation
Show Answer & Explanation

Answer: (d) levigation

Explanation:
1. Levigation (gravity separation) washes away the lighter earthy matter and leaves the denser ore.

Teacher's Note:
1. It is used for oxide ores such as haematite and cassiterite.

Question 3: Which of the following is NOT an ore?
(a) Cerussite
(b) Salt cake
(c) Calamine
(d) Malachite
Show Answer & Explanation

Answer: (b) Salt cake

Explanation:
1. Salt cake is sodium sulphate, which is not used for extracting a metal economically.

Teacher's Note:
1. Malachite is a copper ore, calamine a zinc ore and cerussite a lead ore.

Question 4: Heating a mixture of \( \ce{Cu2O} \) and \( \ce{Cu2S} \) gives
(a) \( \ce{Cu2SO3} \)
(b) \( \ce{Cu + SO2} \)
(c) \( \ce{CuO + CuS} \)
(d) \( \ce{Cu + SO3} \)
Show Answer & Explanation

Answer: (b) \( \ce{Cu + SO2} \)

Explanation:
1. Self-reduction: \( \ce{2Cu2O + Cu2S -> 6Cu + SO2} \).

Teacher's Note:
1. The oxide acts as oxidant and the sulphide as reductant.

Question 5: The poling process of refining copper
(a) uses green poles of wood
(b) reduces SnO2 to Sn
(c) oxidises iron impurities and removes them as scum
(d) all of the above
Show Answer & Explanation

Answer: (a) uses green poles of wood

Explanation:
1. Molten impure copper containing cuprous oxide is stirred with green wood poles; the hydrocarbon gases formed reduce the oxide to copper.

Teacher's Note:
1. Poling is used for metals such as Cu and Sn that contain their own oxide as impurity.

Question 6: Sapphire is a mineral of
(a) Al
(b) Mg
(c) Zn
(d) Cu
Show Answer & Explanation

Answer: (a) Al

Explanation:
1. Sapphire and ruby are corundum, \( \ce{Al2O3} \), coloured by trace impurities.

Teacher's Note:
1. Corundum is a gem form of alumina.

Question 7: The set of metals mostly found as sulphide ores is
(a) Zn, Cu, Pb
(b) Fe, Al, Ti
(c) Zn, Cu, Mg
(d) Cu, Ag, Au
Show Answer & Explanation

Answer: (a) Zn, Cu, Pb

Explanation:
1. Zinc blende, copper pyrites and galena are sulphides. Mg occurs as chloride or carbonate, Al and Ti as oxides, and Ag and Au are partly native.

Teacher's Note:
1. Sulphide ores are concentrated by froth flotation.

Question 8: Which ore is concentrated by magnetic separation?
(a) Calamine
(b) Haematite
(c) Bauxite
(d) Chalcopyrite
Show Answer & Explanation

Answer: (b) Haematite

Explanation:
1. Magnetic separation uses the magnetic nature of the ore or its gangue. Iron oxides such as haematite and chromite respond to a magnet.

Teacher's Note:
1. It is also used for tin stone containing magnetic wolframite as impurity.

Question 9: Which mineral is named incorrectly?
(a) Feldspar : \( \ce{Be3Al2Si6O18} \)
(b) Corundum : \( \ce{Al2O3} \)
(c) Bauxite : \( \ce{Al2O3.2H2O} \)
(d) Cryolite : \( \ce{3NaF.AlF3} \)
Show Answer & Explanation

Answer: (a) Feldspar : \( \ce{Be3Al2Si6O18} \)

Explanation:
1. The formula \( \ce{Be3Al2Si6O18} \) is that of beryl. Feldspar is \( \ce{KAlSi3O8} \).

Teacher's Note:
1. Beryl is the ore of beryllium.

Question 10: In the Van Arkel method, if iodine is introduced at 1800 K over impure zirconium, the product is
(a) iodide of the metal
(b) pure metal
(c) impurities react with iodine
(d) none of these
Show Answer & Explanation

Answer: (b) pure metal

Explanation:
1. At 1800 K the iodide is unstable on the hot filament and decomposes to pure zirconium; the iodide forms at a lower temperature.

Teacher's Note:
1. The temperature decides whether the iodide forms (low) or decomposes (high).

Question 11: Which statement is incorrect?
(a) Argentite and cuprite are oxides
(b) Malachite and azurite are ores of copper
(c) Calamine and siderite are carbonates
(d) Zinc blende and iron pyrites are sulphides
Show Answer & Explanation

Answer: (a) Argentite and cuprite are oxides

Explanation:
1. Argentite is silver sulphide, \( \ce{Ag2S} \); only cuprite is an oxide.

Teacher's Note:
1. Remember argentite as 'silver glance'.

Question 12: Calamine is an ore of
(a) Zn
(b) Ca
(c) Pb
(d) Mg
Show Answer & Explanation

Answer: (a) Zn

Explanation:
1. Calamine is zinc carbonate, \( \ce{ZnCO3} \).

Teacher's Note:
1. Zinc ores to remember: zinc blende (ZnS), calamine (ZnCO3) and zincite (ZnO).

Question 13: The free energy change (kJ per mol) for preparing Mg from \( \ce{MgCl2} \) by the Dow process using a voltage of 2.02 V, divided by 10, is
(a) 78
(b) 19.5
(c) 39
(d) 3.9
Show Answer & Explanation

Answer: (c) 39

Explanation:
1. n = 2: \( \Delta G=nFE=2\times96500\times2.02=389.9 \) kJ per mole.

Teacher's Note:
1. Divided by 10 = 39.

Question 14: Match the metals with their ores: tin, zinc, iron, lead with (p) calamine, (q) cassiterite, (r) cerussite, (s) siderite. The correct sequence is
(a) tin-p, zinc-q, iron-r, lead-s
(b) tin-q, zinc-p, iron-s, lead-r
(c) tin-s, zinc-r, iron-q, lead-p
(d) tin-q, zinc-p, iron-r, lead-s
Show Answer & Explanation

Answer: (b) tin-q, zinc-p, iron-s, lead-r

Explanation:
1. Tin: cassiterite; zinc: calamine; iron: siderite (\( \ce{FeCO3} \)); lead: cerussite (\( \ce{PbCO3} \)).

Teacher's Note:
1. Learn ores in groups: carbonates (calamine, siderite, cerussite, malachite), oxides (haematite, cassiterite, bauxite) and sulphides (galena, blende, pyrites).

Question 15: During electro-refining of copper, metals settling as anode mud are
(a) Sn and Ag
(b) Fe and Ni
(c) Ag and Au
(d) Pb and Zn
Show Answer & Explanation

Answer: (c) Ag and Au

Explanation:
1. Ag, Au and Pt are less electropositive than copper and are not oxidised at the anode.

Teacher's Note:
1. Active metals like Zn and Fe go into solution.

Question 16: Gravity separation is based on
(a) preferential washing of ores and gangue
(b) difference in densities of ore particles and impurities
(c) difference in chemical properties
(d) none of these
Show Answer & Explanation

Answer: (b) difference in densities of ore particles and impurities

Explanation:
1. Heavier ore particles settle first and the lighter gangue is washed away.

Teacher's Note:
1. Gravity separation is a physical method.

Question 17: NaCN is added as a depressant in froth flotation of a mixture of ZnS and PbS because
(a) PbS forms a soluble complex while ZnS forms froth
(b) NaCN is never added in froth flotation
(c) Pb(CN)2 is precipitated and ZnS is unaffected
(d) ZnS forms the soluble complex \( \ce{Na2[Zn(CN)4]} \) while PbS forms froth
Show Answer & Explanation

Answer: (d) ZnS forms the soluble complex \( \ce{Na2[Zn(CN)4]} \) while PbS forms froth

Explanation:
1. Cyanide forms a soluble tetracyanozincate with ZnS, which prevents it from floating. PbS does not react and floats in the froth.

Teacher's Note:
1. A depressant selectively stops one sulphide from entering the froth, which allows separation of two sulphide ores.

Question 18: Which of the following is a self-reduction process?
(a) \( \ce{HgS + O2 -> HgO + SO2; HgO + HgS -> Hg + SO2} \)
(b) All of these
(c) \( \ce{PbS + O2 -> PbO + SO2; PbO + PbS -> Pb + SO2} \)
(d) \( \ce{Cu2S + O2 -> Cu2O + SO2; Cu2S + Cu2O -> Cu + SO2} \)
Show Answer & Explanation

Answer: (b) All of these

Explanation:
1. In each, the partly oxidised ore reacts with the unreacted sulphide to give the metal, with no external reducing agent.

Teacher's Note:
1. Cu, Pb and Hg are the metals commonly obtained by self-reduction.

Question 19: Silver is separated from lead (argentiferous lead) by
(a) Parke's process (addition of zinc)
(b) cupellation
(c) fractional crystallisation
(d) liquation
Show Answer & Explanation

Answer: (a) Parke's process (addition of zinc)

Explanation:
1. Molten zinc dissolves silver (and not lead); the zinc-silver alloy is then distilled to leave silver.

Teacher's Note:
1. Cupellation is used afterwards to remove the last traces of lead from silver.

Question 20: For the Hall-Heroult cell reaction \( \ce{3C + 2Al2O3 -> 4Al + 3CO2} \) (\( \Delta G_f(\ce{Al2O3})=-1520 \) and \( \Delta G_f(\ce{CO2})=-394 \) kJ/mol), the minimum voltage multiplied by 10 is
(a) 8
(b) 16
(c) 32
(d) 24
Show Answer & Explanation

Answer: (b) 16

Explanation:
1. Net \( \Delta G=3(-394)-2(-1520)=+1858 \) kJ.

Teacher's Note:
1. Electrons transferred: 12. \( E=1858000/(12\times96500)=1.60 \) V, and 1.60 x 10 = 16.

Question 21: Which metal cannot be displaced from aqueous solution by zinc?
(a) Al
(b) Hg
(c) Ag
(d) Cu
Show Answer & Explanation

Answer: (a) Al

Explanation:
1. Aluminium lies above zinc in the electrochemical series, so zinc cannot reduce its ions.

Teacher's Note:
1. Zn displaces metals lower in the series.

Question 22: Magnesium is extracted from carnallite by
(a) cyanide leaching followed by reduction
(b) electrolytic reduction
(c) carbon reduction
(d) self-reduction
Show Answer & Explanation

Answer: (b) electrolytic reduction

Explanation:
1. Magnesium is very electropositive and its oxide is stable, so electrolysis of the fused chloride is used.

Teacher's Note:
1. In the Dow process the electrolyte is molten \( \ce{MgCl2} \) with NaCl and \( \ce{CaCl2} \).

Question 23: Goldsmiths heat gold with borax in a flame through which air is blown because
(a) borax makes gold softer
(b) gold oxide is reduced by borax
(c) heat and air oxidise the impurities, and the oxides combine with molten borax to form a slag
(d) air increases the temperature at which gold dissolves in borax
Show Answer & Explanation

Answer: (c) heat and air oxidise the impurities, and the oxides combine with molten borax to form a slag

Explanation:
1. Impurities (base metals) are oxidised and fluxed away by borax as metal borate slag, leaving gold.

Teacher's Note:
1. Borax is a flux for basic oxides.

Question 24: Carbon cannot reduce \( \ce{Al2O3} \) because
(a) it is a non-metal
(b) the heat of formation of \( \ce{CO2} \) is more than that of \( \ce{Al2O3} \)
(c) pure carbon is not available
(d) the heat of formation of \( \ce{Al2O3} \) is too high
Show Answer & Explanation

Answer: (d) the heat of formation of \( \ce{Al2O3} \) is too high

Explanation:
1. Alumina is so stable that its free energy of formation is more negative than that of CO or \( \ce{CO2} \) at normal furnace temperatures.

Teacher's Note:
1. Hence electrolysis is needed.

Question 25: High purity germanium is obtained by a technique based on
(a) fractional distillation
(b) recrystallisation
(c) diffusion
(d) fractional crystallisation (zone refining)
Show Answer & Explanation

Answer: (d) fractional crystallisation (zone refining)

Explanation:
1. Zone refining relies on different solubility of impurities in the molten and solid phases, which is fractional crystallisation.

Teacher's Note:
1. It is used for semiconductors.

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