CBSE Class 12 Chemistry General principles and processes of isolation of elements Questions Answers

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GENERAL PRINCIPLES AND PROCESSES OF ISOLATION OF ELEMENTS

CHAPTER 6: GENERAL PRINCIPLES AND PROCESSES OF ISOLATION OF ELEMENTS

One-mark questions

Differentiate between a mineral and an ore.

Answer: The naturally occurring chemical substances present in the earth’s crust which can be obtained by mining are called minerals, while minerals from which metals can be extracted economically are called ores.

Why is it that only sulphide ores are concentrated by the froth flotation process?

Answer: Sulphide ore particles are preferentially wetted by oil, while gangue particles are wetted by water.

Name one acidic flux and one basic flux.

Answer: Silica (acidic flux) and lime (basic flux).

Name the chief ore of silver.

Answer: Argentite (silver glance), Ag₂S.

Name a reagent used during leaching of bauxite ore.

Answer: NaOH (sodium hydroxide).

Why is silica added to the sulphide ore of copper in the reverberatory furnace?

Answer: To remove the iron impurity as slag (iron silicate).

What is the role of flux in metallurgical processes?

Answer: Flux is used for making the molten mass more conducting. It also combines with the gangue (infusible impurities) to form an easily fusible slag, which is easily removed.

What is the thermodynamic relation between Gibbs free energy and the emf of a cell?

Answer: ΔG⁰ = −nFE⁰

What is the relation between Gibbs free energy and the equilibrium constant?

Answer: ΔG⁰ = −RT ln K

Give the expression for the Gibbs–Helmholtz equation.

Answer: ΔG = ΔH − TΔS

Name one chemical which can be used to concentrate galena selectively by the froth flotation process.

Answer: Sodium cyanide (NaCN), used as a depressant: it prevents ZnS from forming the froth, so galena (PbS) is separated.

What type of ores is roasted?

Answer: Sulphide ores.

Out of C and CO, which is a better reducing agent for ZnO?

Answer: C. The free energy of formation of CO from C becomes lower than that of ZnO above 1120 K, and that of CO₂ from C becomes lower above 1323 K. The free energy of formation of CO₂ from CO is always higher than that of ZnO. Therefore C can reduce ZnO to Zn better than CO.

On what chemical principle is chromatographic separation based?

Answer: Adsorption.

What are the products obtained during the electrolysis of brine solution? Also write the name of this process.

Answer: Chlorine, hydrogen and sodium hydroxide. The process is popularly known as the chlor-alkali process.

What is roasting?

Answer: The preliminary treatment of the concentrated ore in which the ore is heated in excess of air below its melting point.

What is calcination?

Answer: The process of heating the concentrated ore in the absence (or limited supply) of air below its melting point.

What is smelting?

Answer: Reduction of the metal oxide into metal in the presence of carbon or carbon monoxide.

What is blister copper? (copper matte)

Answer: Blister copper is the crude copper (about 98% pure) obtained after the extraction; it has a blistered appearance due to the evolution of SO₂. (In the source this is also called copper matte; strictly, copper matte is the mixture of Cu₂S and FeS from which blister copper is made.)

What is meant by the beneficiation process?

Answer: The process of removal of unwanted earthy and siliceous impurities from the ore is called the beneficiation process.

Name the chief ore of iron and of aluminium.

Answer: Haematite (Fe₂O₃) and bauxite (Al₂O₃.2H₂O).

At what temperature is CO a better reducing agent than carbon?

Answer: 673 K and above temperature.

What is meant by gangue?

Answer: The unwanted impurities associated with the ores.

Name the impurities associated with bauxite.

Answer: SiO₂, TiO₂ and Fe₂O₃.

What is the basis of zone refining?

Answer: Impurities are more soluble in the melt than in the solid metal.

What is the basis of vapour phase refining?

Answer: The metal should form a volatile compound which can be easily evaporated and easily decomposed to give the pure metal.

Name the refining method used for obtaining a very high degree of purity of a metal.

Answer: Electrolytic refining.

How is copper extracted from low-grade copper ore?

Answer: By hydrometallurgy: the ore is leached and the copper ions are displaced by a more reactive metal (or reduced by hydrogen).

Cu²⁺ + H₂ → 2H⁺ + Cu

What is the role of collectors in the froth flotation process?

Answer: Collects the lighter sulphide ore in it: collectors (e.g. pine oil, fatty acids, xanthates) enhance the non-wetting of the ore particles by water, so they stick to the air bubbles and go into the froth.

Name two metals that occur in nature as oxides.

Answer: Iron and aluminium.

Out of C and CO, which is a better reducing agent for ZnO?

Answer: C (see the explanation given above).

What is the role of cryolite in the electrometallurgy of aluminium?

Answer: It lowers the melting point of alumina (and increases the conductivity of the melt).

Why is bauxite not heated to remove the water associated with it?

Answer: The water is chemically combined (Al₂O₃.2H₂O); the real impurities (SiO₂, Fe₂O₃, TiO₂) are not removed by heating but by leaching with NaOH, and strongly heated (anhydrous) alumina does not dissolve easily in NaOH.

Because aluminium has a low melting point and the water is chemically associated, so heating causes loss of metal.

Copper can be extracted by hydrometallurgy but not zinc. Why?

Answer: The E⁰ of Zn is lower than that of Cu, so Zn can displace Cu²⁺ from its solution. To displace Zn from Zn²⁺ a metal more reactive than Zn would be needed.

What is the role of CaF₂ in the electrometallurgy of aluminium?

Answer: It increases the conductivity of the electrolyte (and lowers the melting point).

Why is CaCO₃ added to the blast furnace during the reduction of haematite?

Answer: It decomposes to CaO, which acts as a flux and combines with silica to form slag.

Though thermodynamically feasible, the reaction 3TiO₂ + 4Al → 3Ti + 2Al₂O₃ (ΔG° negative) does not happen at room temperature. Explain why.

Answer: It needs activation energy, which is not available at room temperature.

What is the role of graphite rods in the electrometallurgy of aluminium?

Answer: They act as the anode.

Why is it not advisable to reduce MgO with carbon?

Answer: It requires a very high temperature, so it is not economical.

What is the percentage of carbon in cast iron and pig iron?

Answer: Cast iron: about 3%; pig iron: about 4%.

Two-mark questions

Write down the reactions taking place in the blast furnace related to the metallurgy of iron.

Answer:

3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂

Fe₃O₄ + 4CO → 3Fe + 4CO₂

Fe₂O₃ + CO → 2FeO + CO₂

Describe with chemical equations the extraction of silver from its ore.

Answer: Argentite ore is treated with a dilute solution of NaCN in the presence of oxygen to form a soluble complex.

2Ag₂S + 8CN⁻ + O₂ + 2H₂O → 4[Ag(CN)₂]⁻ + 2S + 4OH⁻

Zn acts as the reducing agent and displaces silver from the complex:

2[Ag(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2Ag

The crude silver obtained is refined by fusion with borax or by electrolysis.

Describe the role of (a) NaCN in the extraction of silver from a silver ore, (b) cryolite in the extraction of aluminium from pure alumina.

Answer: (a) NaCN dissolves the silver ore as a soluble complex:

2Ag₂S + 8NaCN + O₂ + 2H₂O → 4Na[Ag(CN)₂] + 2S + 4NaOH

or Ag₂S + 4NaCN → 2Na[Ag(CN)₂] + Na₂S

(b) Cryolite (i) lowers the melting point of the mixture and (ii) increases the electrical conductivity of the mixture.

Explain the role of carbon monoxide in the purification of nickel and of iodine in the purification of zirconium.

Answer: When nickel is heated with carbon monoxide it forms the volatile complex nickel tetracarbonyl, which on further heating at a higher temperature decomposes to give pure nickel.

Ni + 4CO → [Ni(CO)₄] (330–350 K)

[Ni(CO)₄] → Ni + 4CO (450–470 K)

Impure zirconium is heated with iodine to form volatile ZrI₄, which on further heating over a tungsten filament decomposes to give pure zirconium.

Zr + 2I₂ → ZrI₄ (870 K)

ZrI₄ → Zr + 2I₂ (2075 K)

(a) Name the method used for refining (i) nickel, (ii) zirconium. (b) The extraction of gold by leaching with NaCN involves both oxidation and reduction. Justify, giving equations.

Answer: (a) (i) Mond’s process; (ii) van Arkel method. (b) In the first reaction Au changes into Au⁺, i.e. oxidation takes place; in the second Au⁺ changes to Au, i.e. reduction takes place.

4Au + 8CN⁻ + O₂ + 2H₂O → 4[Au(CN)₂]⁻ + 4OH⁻

2[Au(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2Au

What criterion is followed for the selection of the stationary phase in chromatography?

Answer: The stationary phase is selected so that the impurities are more strongly adsorbed (or more soluble) in the stationary phase than the element to be purified. When the column is eluted, the impurities are retained by the stationary phase and the pure component is easily eluted.

Explain the electrolytic refining of copper with the thermodynamic principle involved in the process.

Answer: The impure metal is made the anode and a strip of pure copper the cathode, in an electrolytic bath containing a soluble salt of copper. On passing current, copper ions from the electrolyte are deposited at the cathode while an equivalent amount of copper dissolves from the anode into the solution. The thermodynamic principle is expressed by ΔG⁰ = −nFE⁰.

At cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)

At anode: Cu(s) → Cu²⁺(aq) + 2e⁻

What are the limitations of the Ellingham diagram?

Answer: (i) It only indicates whether a reaction is possible or not; it says nothing about the kinetics (rate) of the reduction. (ii) The interpretation of ΔG⁰ is based on K, so it is presumed that the reactants and products are in equilibrium, which is not always true.

What is the role of a depressant in the froth flotation process?

Answer: A depressant prevents certain types of particles from forming the froth with the air bubbles. For example, NaCN is used as a depressant to separate PbS from ZnS: NaCN forms a zinc complex Na₂[Zn(CN)₄] on the surface of ZnS, preventing it from forming the froth.

How are metals used as semiconductors refined? What is the principle of the method used?

Answer: They are refined by zone refining, which is based on the principle that impurities are more soluble in the melt than in the solid state of the metal.

Describe the method of refining of titanium.

Answer: In the van Arkel process, impure Ti is heated in a stream of iodine, forming the volatile compound TiI₄, which decomposes on a hot tungsten filament at a higher temperature to give pure Ti.

Ti + 2I₂ → TiI₄

TiI₄ → Ti + 2I₂

What is zone refining? Explain with an example.

Answer: Zone refining is a method of obtaining a metal in a very pure state. It is based on the principle that impurities are more soluble in the molten state of the metal than in the solid state. A rod of the impure metal is moved slowly over a circular heater; the portion being heated melts and forms the molten zone. As this portion of the rod moves out of the heater it solidifies, while the impurities pass into the molten zone. The process is repeated to obtain the ultrapure metal, and the end of the rod containing the impurities is cut off. It is used for semiconductors such as Ge, Si, Ga.

Write the principle of electro-refining.

Answer: The impure metal is made the anode and the pure metal the cathode. On passing electricity, the pure metal is deposited at the cathode, while the impurities dissolve in the solution or collect as anode mud. For example, electro-refining of copper:

At cathode: Cu²⁺ + 2e⁻ → Cu

At anode: Cu → Cu²⁺ + 2e⁻

Write the difference between calcination and roasting.

Answer: (1) Calcination is carried out in a limited supply or absence of air, whereas roasting is carried out in the presence of air. (2) In roasting, sulphide ore is converted to its oxide, while in calcination hydroxides, hydrates and carbonates are converted to oxides.

Describe the method of refining of zirconium.

Answer: The van Arkel process is used for obtaining ultrapure metal. The impure zirconium is converted into the volatile compound ZrI₄, which is then decomposed on an electrically heated tungsten filament to give pure zirconium.

Zr (impure) + 2I₂ → ZrI₄ (at 850 K)

ZrI₄ → Zr (pure) + 2I₂ (at 2075 K)

Out of C and CO, which is the better reducing agent for ZnO?

Answer: C. The free energy of formation of CO from C becomes lower than that of ZnO above 1120 K, and that of CO₂ from C becomes lower above 1323 K; however, the free energy of formation of CO₂ from CO is always higher than that of ZnO. Hence C is the better reducing agent for ZnO.

The value of Δ_fG⁰ for Cr₂O₃ is −540 kJ/mol and that of Al₂O₃ is −827 kJ/mol. Is the reduction of Cr₂O₃ with aluminium possible?

Answer: Yes. The desired reaction is 2Al + Cr₂O₃ → Al₂O₃ + 2Cr, obtained by adding the following two reactions:

2Al + 3/2 O₂ → Al₂O₃ ΔfG° = −827 kJ/mol

Cr₂O₃ → 2Cr + 3/2 O₂ ΔG° = +540 kJ/mol

Therefore ΔG⁰ for the desired reaction is −827 + 540 = −287 kJ; it is negative, so the reduction is possible.

Why is copper matte put in a silica-lined converter?

Answer: Copper matte consists of Cu₂S and FeS. When a blast of air is passed through the molten matte in the silica-lined converter, FeS is oxidised to FeO, which combines with silica to form slag; Cu₂S is partly oxidised to Cu₂O, which reacts with the remaining Cu₂S to give copper.

2FeS + 3O₂ → 2FeO + 2SO₂

FeO + SiO₂ → FeSiO₃ (slag)

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂

2Cu₂O + Cu₂S → 6Cu + SO₂

What is meant by the term chromatography?

Answer: “Chroma” means colour and “graphy” means writing, because the method was first used for the separation of coloured substances. It is based on the selective distribution of the various constituents of a mixture between two phases, a stationary phase and a moving phase. The stationary phase can be a solid or a liquid on a solid support.

Why is the reduction of a metal oxide easier if the metal formed is in the liquid state at the temperature of reduction?

Answer: The entropy of a substance is higher in the liquid state than in the solid state. In the reduction of a metal oxide, the entropy change is more positive if the metal formed is a liquid, so ΔG⁰ becomes more negative and the reduction occurs easily.

Three-mark questions

Describe how the following changes are brought about: (i) pig iron into steel; (ii) zinc oxide into metallic zinc; (iii) impure titanium into pure titanium.

Answer: (i) Pig iron is converted into steel by heating in a converter. A blast of oxygen diluted with carbon dioxide is blown through the converter. Oxygen reacts with the impurities and raises the temperature to 2173 K. Carbon is oxidised to CO, which burns off at the mouth of the converter. Oxides of silicon and manganese form slag. When the flame stops, the slag is tapped out and other metals such as Mn, Cr, Ni, W may be added at the end. (ii) Zinc oxide is reduced using coke as the reducing agent. For heating, the oxide is made into briquettes with coke and clay. The metal is distilled off and collected by rapid chilling.

ZnO + C → Zn + CO

(iii) Impure titanium is heated with iodine to form volatile TiI₄, which decomposes on a hot tungsten filament at a higher temperature to give pure titanium.

Ti + 2I₂ → TiI₄

TiI₄ → Ti + 2I₂

Describe the role of (a) NaCN in the extraction of gold from gold ore, (b) SiO₂ in the extraction of copper from copper matte, (c) iodine in the refining of zirconium.

Answer: (a) NaCN dissolves gold as a soluble cyanide complex, from which it is displaced by zinc:

4Au + 8CN⁻ + O₂ + 2H₂O → 4[Au(CN)₂]⁻ + 4OH⁻

(b) The role of SiO₂ is to convert FeO into slag: 2FeS + 3O2 → 2FeO + 2SO2 and FeO + SiO2 → FeSiO3. (c) Impure zirconium is heated with iodine to form volatile ZrI₄, which on further heating over a tungsten filament decomposes to give pure zirconium: Zr + 2I2 → ZrI4 (870 K) → Zr + 2I2 (2075 K).

Describe how the following changes are brought about: (i) pig iron into steel; (ii) bauxite into pure alumina; (iii) impure copper into pure copper.

Answer: (i) As above: pig iron is heated in a converter with a blast of oxygen diluted with carbon dioxide; the impurities are oxidised (carbon to CO, silicon and manganese into slag), and Mn, Cr, Ni, W may be added at the end. (ii) Finely powdered bauxite is digested with an aqueous solution of sodium hydroxide. Al₂O₃ is leached out as sodium aluminate, leaving the impurities behind.

Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]

The aluminate is neutralised by passing CO₂ and hydrated Al₂O₃ is precipitated; the solution is seeded with freshly prepared hydrated Al₂O₃, which induces the precipitation. The sodium bicarbonate remains in solution; the hydrated alumina is filtered, dried and heated to get pure Al₂O₃.

2Na[Al(OH)₄] + 2CO₂ → Al₂O₃.xH₂O + 2NaHCO₃ (balanced for x = 3)

Al₂O₃.xH₂O → Al₂O₃ + xH₂O

(iii) The impure copper is made the anode and a strip of pure copper the cathode in an electrolytic bath containing a soluble salt of copper (electrolytic refining). Copper ions are deposited at the cathode while an equivalent amount of copper dissolves from the anode.

At cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)

At anode: Cu(s) → Cu²⁺(aq) + 2e⁻

Describe the principle behind each of the following processes: (i) vapour phase refining of a metal; (ii) electrolytic refining of a metal; (iii) recovery of silver after the silver ore was leached with NaCN.

Answer: (i) The metal is converted into its volatile compound and collected; it is then decomposed to give the pure metal. (ii) The impure metal is made the anode and a strip of the pure metal the cathode in a bath containing a soluble salt of the same metal; on passing current the metal ions from the electrolyte are deposited at the cathode while an equivalent amount of metal dissolves from the anode. (iii) During leaching Ag is oxidised to Ag⁺, which combines with CN⁻ to form a soluble complex. Silver is then recovered by displacement using a more electropositive metal: Zn acts as the reducing agent.

2[Ag(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2Ag

Write the reactions involved in the following processes: (i) leaching of bauxite ore to prepare pure alumina; (ii) refining of zirconium by the van Arkel method; (iii) recovery of gold after the gold ore has been leached with NaCN.

Answer:

(i) Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]

2Na[Al(OH)₄] + 2CO₂ → Al₂O₃.xH₂O + 2NaHCO₃ (balanced for x = 3)

Al₂O₃.xH₂O → Al₂O₃ + xH₂O

(ii) Zr + 2I₂ → ZrI₄ (870 K)

ZrI₄ → Zr + 2I₂ (2075 K)

(iii) 4Au + 8CN⁻ + O₂ + 2H₂O → 4[Au(CN)₂]⁻ + 4OH⁻

2[Au(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2Au

Write the reactions involved in the following processes: (i) Mond’s process; (ii) MacArthur–Forrest cyanide process; (iii) Hall–Héroult process.

Answer:

(i) Ni + 4CO → [Ni(CO)₄] (330–350 K)

[Ni(CO)₄] → Ni + 4CO (450–470 K)

(ii) 4M + 8CN⁻ + O₂ + 2H₂O → 4[M(CN)₂]⁻ + 4OH⁻

2[M(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2M (M = Ag or Au)

(iii) Al₂O₃ → 2Al³⁺ + 3O²⁻

Cathode: Al³⁺ + 3e⁻ → Al

Anode: C + O²⁻ → CO + 2e⁻

C + 2O²⁻ → CO₂ + 4e⁻

Overall: 2Al₂O₃ + 3C → 4Al + 3CO₂

Account for the following facts: (a) Reduction of a metal oxide is easier if the metal formed is in the liquid state at the temperature of reduction. (b) The reduction of Cr₂O₃ with aluminium is thermodynamically feasible, yet it does not occur at room temperature. (c) Pine oil is used in the froth flotation method.

Answer: (a) In the liquid state the entropy is higher than in the solid form, which makes ΔG more negative. (b) By increasing the temperature the fraction of activated molecules increases, which helps them cross the activation-energy barrier. (c) Pine oil enhances the non-wetting property of the ore particles and acts as a collector.

State briefly the principles which serve as the basis for the following operations in metallurgy: (a) froth flotation process; (b) zone refining; (c) refining by liquation.

Answer: (a) Sulphide ore particles are preferentially wetted by pine oil, whereas the gangue particles are wetted by water. (b) The impurities are more soluble in the melt than in the solid state of the metal. (c) The metal has a lower melting point than the impurities; on melting the impure metal, the pure metal flows away and the impurities with higher melting points are left behind.

Explain the basic principles of the following metallurgical operations: (a) zone refining; (b) vapour phase refining; (c) electrolytic refining.

Answer: (a) The impurities are more soluble in the melt than in the solid state of the metal. (b) The metal is converted into a volatile compound and collected; it is then decomposed to give the pure metal. (c) The impure metal is the anode and the pure metal the cathode in an electrolytic bath containing a soluble salt of the metal; metal ions deposit on the cathode while an equivalent amount of metal dissolves from the anode.

Complete the following reactions: (i) Al₂O₃ + NaOH + H₂O; (ii) Au + CN⁻ + O₂ + H₂O; (iii) [Ni(CO)₄] (at 450–470 K).

Answer:

(i) Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]

(ii) 4Au + 8CN⁻ + O₂ + 2H₂O → 4[Au(CN)₂]⁻ + 4OH⁻

(iii) [Ni(CO)₄] → Ni + 4CO

Explain the following: (a) Zinc but not copper is used for the recovery of Ag from the complex [Ag(CN)₂]⁻. (b) Partial roasting of the sulphide ore is done in the metallurgy of copper. (c) Extraction of Cu from pyrites is more difficult than that from its oxide ore through reduction.

Answer: (a) Zn is a more powerful reducing agent than copper (and is also cheaper). (b) Partial roasting of the sulphide ore forms some oxide; this oxide then reacts with the remaining sulphide ore to give copper, i.e. self-reduction occurs. (c) Carbon is a good reducing agent for oxides, but it is a poor reducing agent for sulphides, because the reduction of a metal sulphide does not have a large negative ΔG⁰ value.

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂

2Cu₂O + Cu₂S → 6Cu + SO₂

Explain the role of each of the following in the extraction of metals from their ores: (a) CO in the extraction of nickel; (b) Zn in the extraction of Ag; (c) silica in the extraction of copper.

Answer: (a) CO forms the volatile compound Ni(CO)₄ with nickel, which decomposes on heating to give pure nickel. (b) Zn displaces silver from the leached cyanide complex (it acts as the reducing agent). (c) Silica acts as a flux and reacts with FeO to form slag (FeSiO₃).

Describe the principle behind each of the following processes: (a) vapour phase refining of a metal; (b) electrolytic refining; (c) froth flotation process.

Answer: (a) The metal should form a volatile compound, and that compound should be easily decomposable to give the pure metal. (b) The impure metal is the anode and the pure metal the cathode in an electrolytic solution of its salt; the less reactive impurities fall off as anode mud and the pure metal is deposited at the cathode. (c) Sulphide ore particles are wetted by oil and carried up in the froth, while the gangue particles are wetted by water and settle down.

Describe the principles of extraction of zinc from zinc blende.

Answer: The chief ore of zinc is zinc blende, ZnS. (1) Enrichment of the ore by froth flotation. (2) Roasting to convert the sulphide into the oxide. (3) Reduction of ZnO with carbon (coke). (4) Refining by distillation.

2ZnS + 3O₂ → 2ZnO + 2SO₂

ZnO + C → Zn + CO

Predict the modes of occurrence of the following three types of metals: (a) highly reactive metals; (b) moderately reactive metals; (c) noble metals.

Answer: (a) Highly reactive metals occur as oxides and halides. (b) Moderately reactive metals occur as sulphides, oxides and carbonates. (c) Noble metals occur in the free (native) state.

What happens when (a) cinnabar is roasted; (b) silver sulphide is shaken with a dilute solution of NaCN; (c) HgO is heated?

Answer:

(a) 2HgS + 3O₂ → 2HgO + 2SO₂

(b) Ag₂S + 4NaCN → 2Na[Ag(CN)₂] + Na₂S

(c) 2HgO → 2Hg + O₂

What is the Ellingham diagram? What is its use?

Answer: The plots of ΔG⁰ of formation of oxides of elements against temperature are called Ellingham diagrams. They are useful in deciding the suitable reducing agent in metallurgical processes.

What is the role of a depressant in the froth flotation process?

Answer: Depressants are used to separate two sulphide ores. For example, PbS and ZnS are separated using NaCN as a depressant, which selectively prevents ZnS from coming into the froth.

Define the following terms: (a) roasting; (b) calcination; (c) smelting.

Answer: (a) Roasting: heating of sulphide ores in the presence of air to obtain the oxide. (b) Calcination: heating of metal carbonates, hydroxides or hydrates to obtain oxides in the absence (or limited supply) of air. (c) Smelting: reduction of a metal oxide to the metal using carbon as the reducing agent.

How are metals used as semiconductors refined? What is the principle of the method used?

Answer: By zone refining. The impurities are more soluble in the melt than in the solid metal.

Five-mark questions

Describe the principle behind each of the following processes: (i) vapour phase refining; (ii) electrolytic refining of a metal; (iii) recovery of silver after the silver ore was leached with NaCN; (iv) preparation of cast iron from pig iron; (v) preparation of pure alumina from bauxite.

Answer: (i) The metal is converted into a volatile compound and collected; it is then decomposed to give the pure metal, e.g. Mond’s process: nickel heated with carbon monoxide forms volatile nickel tetracarbonyl, which on further heating at a higher temperature decomposes to give pure nickel. (ii) The impure metal is the anode and a strip of pure metal the cathode in an electrolytic bath of a soluble salt of the same metal; metal ions are deposited at the cathode while an equivalent amount of metal dissolves from the anode. (iii) During leaching Ag is oxidised to Ag⁺, which combines with CN⁻ to form a soluble complex; silver is recovered by displacement with a more electropositive metal, Zn. (iv) Pig iron is melted with scrap iron and coke using a hot air blast; the impurities carbon, sulphur and phosphorus are removed as CO₂, SO₂ and P₂O₅, and the carbon content is reduced to about 3%. (v) Bauxite dissolves in concentrated NaOH solution whereas the impurities do not.

Ni + 4CO → [Ni(CO)₄] (330–350 K)

[Ni(CO)₄] → Ni + 4CO (450–470 K)

2[Ag(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2Ag

Explain the role of each of the following in the extraction of metals from their ores: (i) CO in the extraction of nickel; (ii) zinc in the extraction of silver; (iii) silica in the extraction of copper; (iv) iodine in the extraction of titanium; (v) cryolite in the extraction of aluminium.

Answer: (i) CO converts nickel into the volatile Ni(CO)₄, which is decomposed to give pure nickel. (ii) Zinc acts as the reducing agent and displaces silver from the complex. (iii) SiO₂ converts FeO present in the matte into slag. (iv) Iodine forms volatile TiI₄ with impure titanium, which decomposes on a tungsten filament to give pure titanium. (v) Cryolite lowers the melting point of the mixture and increases its electrical conductivity.

Ni + 4CO → [Ni(CO)₄] → Ni + 4CO

2[Ag(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2Ag

2FeS + 3O₂ → 2FeO + 2SO₂

FeO + SiO₂ → FeSiO₃

Ti + 2I₂ → TiI₄ → Ti + 2I₂

Explain the following: (a) Generally sulphide ores are converted into oxides before reduction. (b) Carbon and hydrogen are not used as reducing agents at high temperature. (c) Silica is added to the sulphide ore of copper in the reverberatory furnace. (d) NaCN acts as a depressant in preventing ZnS from forming the froth. (e) Role of cryolite in the metallurgy of aluminium.

Answer: (a) Sulphide ores are not reduced easily, whereas oxide ores are easily reduced. (b) At high temperature carbon and hydrogen react with the metals to form carbides and hydrides respectively. (c) The role of SiO₂ is to convert FeS/FeO present in the matte into slag (2FeS + 3O2 → 2FeO + 2SO2; FeO + SiO2 → FeSiO3). (d) In froth flotation a depressant prevents certain particles from forming the froth with the air bubbles; NaCN forms a zinc complex Na₂[Zn(CN)₄] on the surface of ZnS, preventing it from forming the froth, so PbS can be separated from ZnS. (e) Cryolite lowers the melting point of the mixture and increases its electrical conductivity.

(a) Describe the principle of the froth flotation process. What is the role of the depressant? Give an example. (b) Define leaching. How is this process used in the benefaction of silver and gold ores?

Answer: (a) Sulphide ore particles are preferentially wetted by pine oil, whereas the gangue particles are wetted by water. The depressant prevents certain types of particles from forming the froth with the air bubbles; e.g. NaCN is used as a depressant to separate PbS from ZnS, since it forms a zinc complex Na₂[Zn(CN)₄] on the surface of ZnS. (b) Leaching consists of treating the powdered ore with a suitable reagent which selectively dissolves the ore but not the impurities. For silver and gold, the powdered ore is treated with sodium cyanide to obtain a soluble complex, which is then treated with zinc metal, which displaces the less active metal from the complex:

4M + 8CN⁻ + O₂ + 2H₂O → 4[M(CN)₂]⁻ + 4OH⁻

2[M(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2M (M = Ag or Au)

Write the chemical reactions which take place in the following operations: (a) electrolytic reduction of alumina; (b) Mond’s process; (c) van Arkel method; (d) MacArthur–Forrest cyanide process; (e) electrolysis of brine.

Answer:

(a) Al₂O₃ → 2Al³⁺ + 3O²⁻

Cathode: Al³⁺ + 3e⁻ → Al

Anode: C + O²⁻ → CO + 2e⁻

C + 2O²⁻ → CO₂ + 4e⁻

Overall: 2Al₂O₃ + 3C → 4Al + 3CO₂

(b) Ni + 4CO → [Ni(CO)₄] (330–350 K)

[Ni(CO)₄] → Ni + 4CO (450–470 K)

(c) Ti + 2I₂ → TiI₄ → Ti + 2I₂

(d) 4M + 8CN⁻ + O₂ + 2H₂O → 4[M(CN)₂]⁻ + 4OH⁻

2[M(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2M (M = Ag or Au)

(e) 2NaCl + 2H₂O → 2NaOH + H₂ + Cl₂

Explain the method for obtaining pig iron from magnetite.

Answer: Extraction of iron takes place in the following steps:

1. Concentration of the ore: by gravity separation followed by magnetic separation.
2. Calcination: heating, in which the volatile matter escapes leaving the metal oxide, e.g. Fe₂O₃.xH₂O → Fe₂O₃ + xH₂O.
3. Roasting: heating the ore in the presence of air; moisture, CO₂, SO₂ and As₂O₃ are removed and FeO is oxidised to Fe₂O₃.
4. Smelting of the roasted ore: a mixture of ore, coke and CaCO₃ is smelted in a blast furnace. The following reactions take place in the different temperature zones.

Zone of reduction (250 °C–700 °C):

3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂

Fe₃O₄ + 4CO → 3Fe + 4CO₂

Fe₂O₃ + CO → 2FeO + CO₂

FeO + CO → Fe + CO₂

Zone of slag formation (800 °C–1000 °C):

CaCO₃ → CaO + CO₂

CaO + SiO₂ → CaSiO₃

P₄O₁₀ + 10C → 4P + 10CO

SiO₂ + 2C → Si + 2CO

MnO₂ + 2C → Mn + 2CO

Zone of fusion (heat absorption) (1150 °C–1350 °C):

CO₂ + C → 2CO

Zone of combustion (1450 °C–1950 °C):

C + O₂ → CO₂

Thus pig iron is obtained from the blast furnace.

Name the principal ore of aluminium and describe how Al is extracted from its ore.

Answer: The principal ore is bauxite, Al₂O₃.2H₂O. Step 1: bauxite is treated with NaOH; the impurities Fe₂O₃, TiO₂ and SiO₂ are removed, and the sodium aluminate formed reacts with CO₂ to give pure alumina. Step 2: electrolytic reduction of pure alumina (dissolved in molten cryolite, Na₃AlF₆, with fluorspar, CaF₂) takes place in an iron box (cathode), with graphite rods as the anode. By this process 98.8% pure aluminium is obtained.

Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]

2Na[Al(OH)₄] + 2CO₂ → Al₂O₃.xH₂O + 2NaHCO₃ (balanced for x = 3)

At cathode: Al³⁺ + 3e⁻ → Al

At anode: 2O²⁻ → O₂ + 4e⁻ (the oxygen reacts with the graphite anode giving CO and CO₂)

Given is the Ellingham diagram. With the help of this diagram answer the following questions. (a) Which of Al and Mg is the better reducing agent below and above the temperature of point A? (b) Which of C and CO can reduce MgO at 2000 K? (c) Suggest a condition under which magnesium could reduce alumina.

Answer:

1. Below 1673 K – Mg is the better reducing agent, and above 1673 K – Al is the better reducing agent.
2. C. Near 2000 K and above, the line for 2C + O2 → 2CO falls below the MgO line, so carbon can reduce MgO; CO cannot, because the line for 2CO + O2 → 2CO2 lies far above the MgO line.
3. Below the temperature of 1600 K.

14. What is the chemical principle on which chromatography separation based on?

Ans : Adsorption.