JEE Chemistry General Principles And Processes Of Isolation Of Elements MCQs Set 15

Answer: (c) cathode: pure copper; anode: impure copper

Explanation:
1. In electrolytic refining the impure metal is made the anode and a strip of the pure metal is the cathode.
2. Cu\(^{2+}\) from the electrolyte deposits as pure copper on the cathode while the impure anode dissolves; the more electropositive zinc stays in solution.

Teacher's Note:
1. In electrolytic refining the anode is always the impure metal.

Answer: (a) 1 and 4 only

Explanation:
1. Cast iron is made by remelting pig iron with scrap iron and coke in a hot-blast cupola furnace (1 true).
2. Silver is extracted as the anionic complex \( \ce{[Ag(CN)2]^-} \) (2 false).
3. Nickel is refined by the Mond process, not zone refining (3 false).
4. Zr and Ti are refined by the van Arkel method (4 true).

Teacher's Note:
1. Learn the refining method for each metal: Mond (Ni), van Arkel (Ti, Zr), zone refining (Si, Ge, Ga).

Answer: (b) 1 and 3

Explanation:
1. Collectors such as pine oil and xanthates are adsorbed on the mineral particles and make them non-wettable by water (1 true, 2 false).
2. A depressant such as NaCN selectively prevents one sulphide from forming froth, so two sulphide ores can be separated (3 true).
3. Froth stabilisers such as cresols and aniline only stabilise the froth; they do not change the wettability of gangue (4 false).

Teacher's Note:
1. Collectors make the ore water-repelling, depressants keep one ore out of the froth.

Answer: (b) 2 and 3

Explanation:
1. ZnS and PbS are separated by selective flotation.
2. A depressant such as NaCN forms a complex with ZnS and prevents it from forming froth, and the oil to water ratio is adjusted so only PbS floats.

Teacher's Note:
1. NaCN is the classic depressant for separating ZnS from PbS.

Answer: (c) 3 and 4

Explanation:
1. Bauxite (\( \ce{Al2O3.xH2O} \)) is usually contaminated with iron(III) oxide and silica, and sometimes titania.
2. CuO and ZnO are not normal impurities of bauxite.

Teacher's Note:
1. The Bayer process removes Fe\(_2\)O\(_3\) and SiO\(_2\) from bauxite.

Answer: (b) 1 and 3

Explanation:
1. Calcination is heating in the absence or limited supply of air to remove volatile matter such as CO\(_2\) and water.
2. Reactions 1 and 3 do this.
3. Reactions 2 and 4 need oxygen and are roasting.

Teacher's Note:
1. Calcination is for carbonates and hydrated oxides; roasting is for sulphides.

Answer: (a) 1 and 2

Explanation:
1. Haematite (\( \ce{Fe2O3} \)) is already an oxide and calamine (\( \ce{ZnCO3} \)) gives ZnO on calcination, and both oxides are reduced by carbon.
2. Iron pyrites and sphalerite are sulphides, which are roasted rather than calcined.

Teacher's Note:
1. Carbonates are calcined to oxides, sulphides are roasted to oxides, and the oxides are then reduced with carbon.

Answer: (c) 1 and 4

Explanation:
1. In the blast furnace the main reduction is by carbon monoxide (reaction 1).
2. The limestone-derived CaO combines with silica impurity to form calcium silicate slag (reaction 4).
3. Direct reduction by carbon (3) is minor and FeO is not removed as silicate in the main reactions.

Teacher's Note:
1. Remember the roles: CO reduces the ore, CaO removes silica as slag.

Answer: (a) 1 and 2

Explanation:
1. In the Mond process the metal (Ni) forms volatile \( \ce{Ni(CO)4} \) with CO, which decomposes on heating to pure Ni.
2. In the van Arkel process the metal (Ti, Zr) forms a volatile iodide which decomposes on a hot filament.
3. Liquation and distillation do not involve a volatile compound that is decomposed.

Teacher's Note:
1. Mond and van Arkel are vapour phase refining methods.

Answer: (b) 2 and 3

Explanation:
1. The overall reaction \( \ce{2NaCl + 2H2O -> 2NaOH + Cl2 + H2} \) is non-spontaneous, so it needs electrical energy.
2. Hence \( \Delta G^\ominus \) is positive and \( E^\ominus \) is negative (about -2.2 V), which is 2 and 3.

Teacher's Note:
1. \( \Delta G^\ominus = -nFE^\ominus \): a positive \( \Delta G^\ominus \) goes with a negative \( E^\ominus \).

Answer: (d) Van Arkel

Explanation:
1. A metal is converted to a volatile iodide at a lower temperature and the iodide is decomposed on a heated filament at a higher temperature to give pure metal.
2. This is the van Arkel method, used for Ti and Zr.

Teacher's Note:
1. Van Arkel refining removes oxygen and nitrogen impurities from Ti and Zr.

Answer: (c) Galena

Explanation:
1. Froth floatation is used for sulphide ores.
2. Galena (PbS) is a sulphide, whereas magnetite, cassiterite (SnO\(_2\)) and malachite (a carbonate) are not.

Teacher's Note:
1. Sulphide ores are wetted by oil and float in the froth.

Answer: (d) alumina mixed with molten cryolite

Explanation:
1. Pure alumina has a very high melting point and is a poor conductor.
2. So it is dissolved in molten cryolite, which lowers the melting point and increases the conductivity, and the mixture is electrolysed.

Teacher's Note:
1. Cryolite is a solvent and a conductor, not the source of aluminium.

Answer: (b) silver

Explanation:
1. Silver (and gold) ores are leached with dilute NaCN solution in the presence of air to form the soluble complex \( \ce{Na[Ag(CN)2]} \).
2. Zinc dust then displaces silver from the complex.

Teacher's Note:
1. MacArthur-Forest cyanide process is used for Ag and Au.

Answer: (b) copper(I) sulphide

Explanation:
1. In the Bessemer converter, part of Cu\(_2\)S is oxidised to Cu\(_2\)O.
2. The self-reduction reaction \( \ce{2Cu2O + Cu2S -> 6Cu + SO2} \) then gives the metal.

Teacher's Note:
1. Copper extraction is a self-reduction process.

Answer: (d) Magnetite

Explanation:
1. Magnetite is \( \ce{Fe3O4} \).
2. Malachite is a copper ore, cassiterite is tin oxide and pyrolusite is manganese dioxide.

Teacher's Note:
1. Common iron ores are haematite, magnetite, siderite and iron pyrites.

Answer: (d) Argentite

Explanation:
1. Galena, copper pyrites and zinc blende (sphalerite) are concentrated by froth floatation.
2. Argentite (Ag\(_2\)S) is concentrated by leaching with dilute sodium cyanide solution.

Teacher's Note:
1. Silver and gold ores are leached, not floated.

Answer: (d) Carbon and hydrogen are suitable reducing agents for metal sulphides

Explanation:
1. Because the \( \Delta G_f^\circ \) of most sulphides is greater than that of CS\(_2\) and H\(_2\)S, carbon and hydrogen cannot reduce metal sulphides.
2. Oxidation of the sulphide to the oxide is thermodynamically favourable, so roasting is done first.
3. Statement (d) is therefore not true.

Teacher's Note:
1. Sulphides are converted to oxides because C and H\(_2\) cannot reduce sulphides directly.

Answer: (b) 2

Explanation:
1. Chalcocite (\( \ce{Cu2S} \)) and iron pyrites (\( \ce{FeS2} \)) are sulphides.
2. Azurite (\( \ce{Cu3(CO3)2(OH)2} \)) is a basic carbonate and limonite (\( \ce{Fe2O3.3H2O} \)) is a hydrated oxide.

Teacher's Note:
1. Learn the formula of each common ore.

Answer: (b) 3

Explanation:
1. The van Arkel method is used for Ti, Zr and B (to obtain very pure metal/metalloid).
2. Pb and Hg are not purified this way, so three of the five.

Teacher's Note:
1. Van Arkel is for metals that form volatile iodides.

Answer: (c) 3

Explanation:
1. Calcination removes CO\(_2\) and water by heating in the absence of air.
2. Reactions (i), (ii) and (iii) are of this type.
3. Reaction (iv) uses oxygen and is roasting.

Teacher's Note:
1. Carbonates, hydroxides and hydrated oxides are calcined.

Answer: (c) 4

Explanation:
1. A basic flux is a basic oxide (or a compound giving one on heating) that removes acidic impurities such as silica as slag.
2. MgO, CaO, FeO and \( \ce{CaCO3.MgCO3} \) (which gives CaO and MgO) are basic fluxes.
3. SiO\(_2\) and B\(_2\)O\(_3\) are acidic.

Teacher's Note:
1. A basic flux is used against acidic gangue like SiO\(_2\).

Answer: (b) 3

Explanation:
1. Cu, Fe and Sn are obtained by smelting (reduction at high temperature).
2. Au is obtained by hydrometallurgy, and K and Na by electrolysis.

Teacher's Note:
1. Pyrometallurgy uses heat and a reducing agent.

Answer: (b) 2

Explanation:
1. In the Goldschmidt process aluminium reduces oxides of metals that are hard to reduce with carbon, such as Cr\(_2\)O\(_3\) and Mn\(_3\)O\(_4\).
2. Na and Al are obtained by electrolysis and Pb and Sn by carbon reduction.

Teacher's Note:
1. Aluminothermy is for Cr, Mn and for welding with iron oxide.

Answer: (c) 4

Explanation:
1. Slag formation is the combination of a flux with the gangue (acidic with basic).
2. Reactions (i), (ii), (iii) and (v) form slags.
3. Reaction (iv) is the aluminothermite reduction of an oxide.

Teacher's Note:
1. Slag = flux + gangue.