JEE Chemistry General Principles And Processes Of Isolation Of Elements MCQs Set 07

Answer: (d) Azurite — CaCO₃·CuCO₃

Explanation:
1. Pyrolusite is MnO₂, Mohr's salt is FeSO₄(NH₄)₂SO₄·6H₂O and cinnabar is HgS
2. Azurite is 2CuCO₃·Cu(OH)₂ (basic copper carbonate), not CaCO₃·CuCO₃. Answer: (d)

Teacher's Note:
1. Azurite and malachite are the two ores of copper that are basic carbonates.
2. Check each name against its formula.

Answer: (c) Van Arkel process

Explanation:
1. The impure metal forms a volatile iodide, which is decomposed on a hot tungsten filament to give the pure metal
2. This is the Van Arkel method, used for Zr and Ti. Answer: (c)

Teacher's Note:
1. Van Arkel uses iodine; Mond's process uses carbon monoxide for nickel.

Answer: (a) roasting as well as auto-reduction

Explanation:
1. Heating Cu₂S in air is roasting, giving Cu₂O
2. Cu₂O then reacts with the remaining Cu₂S without any external reducing agent, giving Cu: this is auto-reduction (self-reduction). Answer: (a)

Teacher's Note:
1. Roasting is heating in excess air; auto-reduction needs no added reductant.

Answer: (d) Chromatographic method

Explanation:
1. Leaching, hydraulic washing and froth floatation are used to concentrate ores
2. Chromatography is a method of purification (refining) of metals. Answer: (d)

Teacher's Note:
1. Concentration separates the ore from gangue; refining purifies the crude metal.

Answer: (d) NaCN in the presence of O₂

Explanation:
1. \( \mathrm{Ag_2S + 4NaCN \to 2Na[Ag(CN)_2] + Na_2S} \)
2. Oxygen oxidises Na₂S to sulphate so that the reaction goes to completion
3. Silver is then precipitated by zinc. Answer: (d)

Teacher's Note:
1. The cyanide process dissolves silver as a soluble complex.
2. Oxygen removes sulphide and drives the reaction forward.

Answer: (b) \( \mathrm{MgCO_3 \to MgO + CO_2} \)

Explanation:
1. Calcination is strong heating of an ore in a limited supply or absence of air, so that it decomposes
2. Decomposition of the carbonate MgCO₃ into oxide and CO₂ is calcination. Answer: (b)

Teacher's Note:
1. Carbonates and hydrated oxides are calcined.
2. Sulphide ores are roasted.

Answer: (d) Malachite

Explanation:
1. Carnallite is KCl·MgCl₂·6H₂O, dolomite is MgCO₃·CaCO₃ and magnesite is MgCO₃
2. Malachite is CuCO₃·Cu(OH)₂ and contains no magnesium. Answer: (d)

Teacher's Note:
1. Malachite is a copper ore.

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

Explanation:
1. Frothers are pine oil and cresols: a-(iv)
2. KCN is a depressant (it selectively prevents ZnS from floating): b-(i)
3. Potassium ethyl xanthate is a collector: c-(ii)
4. CaCO₃ is a basic flux that removes acidic impurities such as silica: d-(iii). Answer: (b)

Teacher's Note:
1. Collectors help the ore particles stick to froth, frothers stabilise the froth and depressants stop one ore from floating.

Answer: (d) All are correct

Explanation:
1. A is \( \mathrm{Na[Ag(CN)_2]} \) (soluble)
2. \( \mathrm{2Na[Ag(CN)_2] + Zn \to Na_2[Zn(CN)_4] + 2Ag} \), so B is \( \mathrm{Na_2[Zn(CN)_4]} \) with coordination number 4
3. Silver is oxidised and then reduced by zinc, so the process is redox. All statements are correct. Answer: (d)

Teacher's Note:
1. Cyanide leaching of silver involves oxidation by oxygen and reduction by zinc.

Answer: (c) \( \mathrm{C(s) + \tfrac12O_2(g) \to CO(g)} \)

Explanation:
1. \( \Delta G^\circ = \Delta H^\circ - T\Delta S^\circ \), so the slope is \( -\Delta S^\circ \)
2. The slope is negative when \( \Delta S^\circ > 0 \), that is when the number of moles of gas increases
3. In \( \mathrm{C + \frac12O_2 \to CO} \), 0.5 mol of gas gives 1 mol, so entropy rises. Answer: (c)

Teacher's Note:
1. Reactions that increase the amount of gas have positive entropy change.
2. In the Ellingham diagram, the C to CO line slopes downward.

Answer: (d) All of these

Explanation:
1. A reverberatory furnace is used for calcination and roasting of ores
2. (a) is calcination, and (b) and (c) are roasting reactions. All of them occur in it. Answer: (d)

Teacher's Note:
1. The furnace heats the charge by reflected flame and does not mix it with fuel.

Answer: (a) Mg is a good reducing agent below 1673 K

Explanation:
1. A metal can reduce the oxide of another metal if its own oxide line is lower on the diagram
2. Below 1673 K the MgO line is lower, so Mg can reduce Al₂O₃
3. Above 1673 K the Al₂O₃ line is lower, so Al is the better reducing agent there
4. An oxide (Al₂O₃) does not act as a reducing agent. Answer: (a)

Teacher's Note:
1. The lower the line, the more stable the oxide.
2. Reduction is feasible when the combined ΔG is negative.

Answer: (c) Cu₂S + FeS

Explanation:
1. Copper matte is the molten mixture of Cu₂S and FeS obtained from the reverberatory furnace. Answer: (c)

Teacher's Note:
1. Matte is converted to blister copper in the Bessemer converter.

Answer: (c) Wrought iron

Explanation:
1. Wrought iron is the purest form of commercial iron, with about 99.5% iron
2. Cast iron and pig iron contain 2 to 4.5% carbon, and steel contains 0.1 to 1.5% carbon. Answer: (c)

Teacher's Note:
1. Lower carbon content means a higher iron percentage.

Answer: (b) a only

Explanation:
1. Nickel is purified by the Mond process, not by the Van Arkel method, so (a) is incorrect
2. Low-boiling metals Zn, Cd and Hg are distilled, semiconductors Si, Ge and Ga are zone refined and lanthanoids are separated by ion-exchange chromatography, so (b), (c) and (d) are correct. Answer: (b)

Teacher's Note:
1. Van Arkel is used for Zr and Ti.
2. Mond process uses CO to form volatile Ni(CO)4.