JEE Chemistry General Principles and Processes Of Isolation Of Elements MCQs Set 12

Answer: (a) A - IV, B - II, C - I, D - III

Explanation:
1. A reverberatory furnace is used in smelting the sulphide ore of copper (IV).
2. Aluminium is extracted by electrolysis in the Hall-Heroult cell (II).
3. A blast furnace gives pig iron (I).
4. Zone refining is used for ultra-pure silicon (III).

Teacher's Note:
1. Match each furnace with its metal.
2. Zone refining is used for semiconductors such as Si and Ge.

Answer: (c) Cu

Explanation:
1. Copper is obtained from its sulphide ore, which is first roasted (oxidation) to give oxide and then reduced to the metal in the self-reduction step.
2. Al is obtained by electrolysis, Ag by cyanide leaching and Fe by direct reduction of the oxide.

Teacher's Note:
1. Sulphide ores are roasted to oxides before reduction.
2. Fe and Al ores are already oxides.

Answer: (c) CaO + SiO2 gives CaSiO3

Explanation:
1. In copper extraction the sulphide ore is roasted, and Cu2S and FeS are partly oxidised, so (a) and (b) occur.
2. Silica is added as flux to remove FeO as FeSiO3 slag, so (d) occurs.
3. Lime is not used as a flux in copper extraction because there is no acidic impurity to remove (it is used in the iron blast furnace).

Teacher's Note:
1. The flux for the basic impurity FeO is the acidic flux SiO2.
2. CaO is used where the impurity is acidic, such as SiO2 in iron ore.

Answer: (b) Statement I is false but Statement II is true

Explanation:
1. In froth floatation a rotating paddle agitates the mixture and draws air into it to form a froth, so Statement I is false.
2. Iron pyrites FeS2 give SO2 on roasting, which pollutes the air, so it is avoided for iron extraction and Statement II is true.

Teacher's Note:
1. Compressed air makes froth in which the sulphide ore particles float.
2. Iron is extracted from oxide ores, not pyrites.

Answer: (c) Sphalerite

Explanation:
1. Sphalerite is ZnS, a sulphide.
2. Calamine is ZnCO3, siderite is FeCO3 and malachite is CuCO3.Cu(OH)2, all carbonates.

Teacher's Note:
1. Sphalerite or zinc blende is ZnS.
2. Learn the names of ores with formulae.

Answer: (d) remove FeO as FeSiO3

Explanation:
1. The copper ore contains iron sulphide, which is partly oxidised to FeO.
2. Silica combines with FeO to give FeSiO3 slag, which floats above the matte.
3. So silica is added to remove FeO.

Teacher's Note:
1. SiO2 is the acidic flux for basic impurities.
2. The slag is lighter and is separated from the matte.

Answer: (a) Statement I is incorrect but Statement II is correct

Explanation:
1. In electrolytic refining the impure metal is the anode and the pure metal is the cathode, so Statement I is wrong.
2. Cryolite Na3AlF6 lowers the melting point of Al2O3 from about 2050 C to about 1000 C and increases conductivity, so Statement II is correct.

Teacher's Note:
1. Impure metal dissolves at the anode and pure metal deposits at the cathode.
2. Cryolite and fluorspar are added in the Hall-Heroult process.

Answer: (c) A and C only

Explanation:
1. Concentration (ore dressing) uses hydraulic washing, leaching, froth floatation and magnetic separation.
2. Liquation and electrolysis are methods of refining the metal, not of concentrating the ore.
3. So A and C are not involved.

Teacher's Note:
1. Concentration is for the ore and refining is for the metal.
2. Liquation is used for low melting metals such as Sn and Pb.

Answer: (b) Van Arkel method is used to purify tungsten

Explanation:
1. Zone refining is used for B, Ga, In, Si and Ge, so (a) is correct.
2. The van Arkel method is for very pure Ti and Zr (and Hf), not tungsten, so (b) is incorrect.
3. (c) and (d) are correct descriptions of cast iron and malleable (wrought) iron preparation.

Teacher's Note:
1. Van Arkel process uses the volatile iodide and decomposes it on a hot filament.
2. It is used for Ti, Zr, Hf and also for V, Th.

Answer: (d) A is correct but R is not correct

Explanation:
1. For 2C + O2 gives 2CO, the number of moles of gas increases, so entropy increases and \( \Delta G^\circ \) falls with temperature, giving a negative slope. A is correct.
2. CO does not decompose at high temperature, in fact CO becomes more stable. So R is incorrect.

Teacher's Note:
1. The slope of an Ellingham line equals minus the entropy change.
2. The C to CO line crosses most metal oxide lines at high temperature.

Answer: (a) 1

Explanation:
1. Copper matte contains Cu2S and FeS.
2. Of the compounds listed only Cu2S (B) is in the matte.
3. CuCO3 is an ore, Cu2O is formed in the converter and FeO goes into the slag.

Teacher's Note:
1. Matte is Cu2S with a little FeS.
2. Slag is FeSiO3.