CBSE Class 12 Chemistry General Principles and Process of Isolation of Elements Notes and Questions

Important Points :

• The chemical substances in the earth’s crust obtained by mining are called Minerals.
• Minerals, which act as source for metal, are called Ore. From ore metal can be obtained economically.
• The unwanted impurities present in ore are called Gangue.
• The entire process of extraction of metal from its ore is called Metallurgy.
• Removal of gangue from ore is called Concentration, Dressing or Benefaction of ore.
• Concentration by Hydraulic washing is based on the difference in gravities of ore and gangue particles.
• Concentration by Magnetic separation is based on differences in magnetic properties of ore components. If either of ore or gangue is capable of attracted by a magnet field, then such separation is carried out.
• Concentration by Froth Flotation Process is based on the facts that sulphide ore is wetted by oil & gangue particles are wetted by water.
• Concentration by Leaching is based on the facts that ore is soluble in some suitable reagent & gangue is insoluble in same reagent. e.g. Bauxite ore contains impurities of silica, iron oxide & \( \text{TiO}_2 \). The powdered ore is treated with \( \text{NaOH} \) which dissolve \( \text{Al} \) & impurities remains insoluble in it.
  \( \text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{Na}[\text{Al}(\text{OH})_4] \)
• Calcination involves heating of ore in absence of air below melting point of metal. In this process volatile impurities escapes leaving behind metal oxide.
  \( \text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O} \rightarrow \text{Fe}_2\text{O}_3 + x\text{H}_2\text{O} \br /> \( \text{ZnCO}_3 \rightarrow \text{ZnO} + \text{CO}_2 \)
  \( \text{CaCO}_3 \cdot \text{MgCO}_3 \rightarrow \text{CaO} + \text{MgO} + 2\text{CO}_2 \)
• Roasting involves heating of ore in presence of air below melting point of metal in reverberatory furnace. In this process volatile impurities escapes leaving behind metal oxide and metal sulphide converts to metal oxide.
  \( 2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2 \)
  \( 2\text{PbS} + 3\text{O}_2 \rightarrow 2\text{PbO} + 2\text{SO}_2 \)
  \( 2\text{Cu}_2\text{S} + 3\text{O}_2 \rightarrow 2\text{Cu}_2\text{O} + 2\text{SO}_2 \)
• Reduction of metal oxide involves heating of metal in presence of suitable reagent Coke or \( \text{CO} \).
• Reactions taking place at different zones of blast furnace in extraction of iron:-
  (i) Zone of reduction:- Temperature range \( 250^\circ\text{C}-700^\circ\text{C} \)
  \( 3\text{Fe}_2\text{O}_3 + \text{CO} \rightarrow 2\text{Fe}_3\text{O}_4 + \text{CO}_2 \)
  \( \text{Fe}_3\text{O}_4 + \text{CO} \rightarrow 3\text{FeO} + \text{CO}_2 \)
  \( \text{FeO} + \text{CO} \rightarrow \text{Fe} + \text{CO}_2 \)
  (ii) Zone of slag formation:- Temperature range \( 800^\circ\text{C}-1000^\circ\text{C} \)
  \( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)
  \( \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \),   \( \text{P}_4\text{O}_{10} + 10\text{C} \rightarrow 4\text{P} + 10\text{CO} \)
  \( \text{SiO}_2 + 2\text{C} \rightarrow \text{Si} + 2\text{CO} \),   \( \text{MnO}_2 + 2\text{C} \rightarrow \text{Mn} + 2\text{CO} \)
  (iii) Zone of fusion:- Temperature range \( 1150^\circ\text{C}-1350^\circ\text{C} \)
  \( \text{CO}_2 + \text{C} \rightarrow 2\text{CO} \)
  (iv) Zone of fusion:- Temperature range \( 1450^\circ\text{C}-1950^\circ\text{C} \)
  \( \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \)

FLOW SHEET FOR EXTRACTION OF IRON

Iron ore (Magnetite \( \text{Fe}_3\text{O}_4 \))(Haematite \( \text{Fe}_2\text{O}_3 \))
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Concentration is done by Gravity separation followed by magnetic separation
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Calcination & Roasting i.e. Ore + Air + Heat → Moisture, \( \text{CO}_2 \), \( \text{SO}_2 \), \( \text{As}_2\text{O}_3 \) removed And FeO oxidized to \( \text{Fe}_2\text{O}_3 \)
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Smelting of charge i.e. mixture of ore, coke & \( \text{CaCO}_3 \) takes place in long BLAST FURNACE. Following reaction take place at different zones:- (refer to point 13)
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Pig iron is obtained, which is remelted and cooled then cast iron is obtained

• Pig Iron: It contains Fe 93-95%, Carbon 2.5-5%, and Impurities 3%.
• Cast Iron: It contains Fe 99.5-99.8%, Carbon 0.1-0.2% Impurities 0.3%.
• Spongy iron: Iron formed in the zone of reduction of blast furnace is called spongy iron. It contains impurities of C, Mn, Si, etc.

FLOW SHEET FOR EXTRACTION OF COPPER

Copper Pyrites \( \text{CuFeS}_2 \)
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Concentration is done by Froth floatation process
Powdered ore + water + pine oil + air → Sulphide ore in the froth
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Roasting in presence of air. following reactions take place:-
\( \text{S} + \text{O}_2 \rightarrow \text{SO}_2 \),   \( 4\text{As} + 3\text{O}_2 \rightarrow 2\text{As}_2\text{O}_3 \),   \( 2\text{CuFeS}_2 + \text{O}_2 \rightarrow \text{Cu}_2\text{S} + 2\text{FeS} + \text{SO}_2 \)
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Smelting in small blast furnace of a mixture of Roasted ore, coke, and silica.
\( 2\text{FeS} + 3\text{O}_2 \rightarrow 2\text{FeO} + 2\text{SO}_2 \),   \( \text{FeO} + \text{SiO}_2 \rightarrow \text{FeSiO}_3 \)(slag)
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A mixture of \( \text{Cu}_2\text{S} \), \( \text{FeS} \) & silica is obtained from blast furnace known as Copper matte
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Bessemerisation of copper matte is done in Bessemer converter in presence of air. Following reactions take place:-
\( 2\text{FeS} + 3\text{O}_2 \rightarrow 2\text{FeO} + 2\text{SO}_2 \),   \( \text{FeO} + \text{SiO}_2 \rightarrow \text{FeSiO}_3 \) (slag),
\( 2\text{Cu}_2\text{S} + 3\text{O}_2 \rightarrow 2\text{Cu}_2\text{O} + 2\text{SO}_2 \),   \( 2\text{Cu}_2\text{O} + 2\text{Cu}_2\text{S} \rightarrow 6\text{Cu} + \text{SO}_2 \)
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Melted copper is cooled, and then \( \text{SO}_2 \) is evolved. such copper is known as BLISTER COPPER (98% Cu + 2% impurities)

FLOW SHEET FOR EXTRACTION OF ALUMINIUM

Bauxite \( \text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O} \)
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Concentration of ore is done by leaching. Bauxite is treated with \( \text{NaOH} \). Following reaction takes place:-
\( \text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{Na}[\text{Al}(\text{OH})_4] \) and impurities of \( \text{Fe}_2\text{O}_3 \), \( \text{TiO}_2 \) & \( \text{SiO}_2 \) are removed.
↓
\( \text{Na}[\text{Al}(\text{OH})_4] \) then reacts with \( \text{CO}_2 \) then pure Alumina is obtained.
\( \text{Na}[\text{Al}(\text{OH})_4] + 2\text{CO}_2 \rightarrow \text{Al}_2\text{O}_3 \cdot x\text{H}_2\text{O} + 2\text{NaHCO}_3 \)
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Electrolytic reduction of pure alumina takes place in iron box (cathode) with cryolite (\( \text{Na}_3\text{AlF}_6 \)) & fluorspar \( \text{CaF}_2 \). Graphite rods act as anode. Following reactions take place:-
At cathode:- \( \text{Al}^{3+} + 3\text{e}^- \rightarrow \text{Al} \),   At Anode:- \( 2\text{O}^{2-} \rightarrow \text{O}_2 + 4\text{e}^- \)
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By this process 98.8% pure Aluminum is obtained.

• Vapour phase refining is used for extraction of Nickel (MOND PROCESS) and Zirconium & Titanium (VAN ARKEL PROCESS).
• Zone refining is used for extraction of Si, Ge, Ga, etc.
• Chromatography method is based on selective distribution of various constituents of a mixture between two phases, a stationary phase and a moving phase. The stationary phase can be either solid or liquid on solid support.
• Column chromatography is based on adsorption phenomenon. This method is useful for those elements, which are available in small amounts and the impurities are not much different in chemical properties from the element to be purified.

VERY SHORT ANSWER TYPE QUESTIONS (1 Mark)

Question. What is slag?
Answer: It is easily fusible material, which is formed when gangue still present in roasted ore combines with the flux.
e.g. \( \text{CaO (flux)} + \text{SiO}_2\text{ (gangue)} \rightarrow \text{CaSiO}_3\text{ (slag)} \)

Question. Which is better reducing agent at 983K, carbon or CO?
Answer: CO, (above 983K CO being more stable & does not act as a good reducing agent but carbon does.)

Question. At which temperature carbon can be used as a reducing agent for Foe?
Answer: Above \( 1123\text{K} \), carbon can reduce \( \text{FeO} \) to \( \text{Fe} \).

Question. What is the role of graphite rods in electrometallurgy of aluminium?
Answer: Graphite rods act as anode, are attacked by oxygen to form \( \text{CO}_2 \) and so to be replace time to time.

Question. What is the role of cryolite in electrometallurgy of aluminium?
Answer: alumina cannot be fused easily because of high melting point. Dissolving of alumina in cryolite furnishes \( \text{Al}^{3+} \) ions, which can be electrolyzed easily.

Question. What are depressants?
Answer: It is possible to separate two sulphide ore by adjusting proportion of oil to water in froth flotation process by using a substance known as depressant.
e.g. \( \text{NaCN} \) is used to separate \( \text{ZnS} \) and \( \text{PbS} \).

Question. Copper can be extracted by hydrometallurgy but not Zn. Why?
Answer: The \( E^0 \) of \( \text{Zn} \) is lower than that of \( \text{Cu} \) thus \( \text{Zn} \) can displace \( \text{Cu}^{2+} \) ion from its solution. On other hand side to displace \( \text{Zn} \) from \( \text{Zn}^{2+} \) ion, we need a more reactive metal than it.

Question. Give name and formula of important ore of iron.
Answer: Haematite – \( \text{Fe}_2\text{O}_3 \), Magnetite – \( \text{Fe}_3\text{O}_4 \), Iron pyrites \( \text{FeS}_2 \).

Question. Give name and formula of important ore of Copper.
Answer: Copper pyrites \( \text{CuFeS}_2 \), Malachite \( \text{CuCO}_3 \cdot \text{Cu(OH)}_2 \), Cuprite \( \text{Cu}_2\text{O} \).

Question. Give name and formula of important ore of Zinc.
Answer: Zinc blende - \( \text{ZnS} \), Calamine - \( \text{ZnCO}_3 \), Zincite – \( \text{ZnO} \).

SHORT ANSWER TYPE QUESTIONS (2 Marks)

Question. Describe the method of refining of nickel.
Answer: In the Mond Process, \( \text{Ni} \) is heated in a stream of \( \text{CO} \) forming a volatile complex, which then decomposes at higher temperature to give \( \text{Ni} \).
At \( 330-350\text{K} \):   \( \text{Ni} + 4\text{CO} \rightarrow \text{Ni(CO)}_4 \)
At \( 450-470\text{K} \):   \( \text{Ni(CO)}_4 \rightarrow \text{Ni} + 4\text{CO} \)

Question. What is Zone Refining? Explain with example.
Answer: Zone refining is a method of obtaining a metal in very pure state. It is based on the principal that impurities are more soluble in molten state of metal than solidified state.
In this method, a rod of impure metal is moved slowly over circular heater. The portion of the metal being heated melts & forms the molten zone. As this portion of the rod moves out of heater, it solidified while the impurities pass into molten zone. The process is repeated to obtain ultrapure metal and end of rod containing impure metal cutoff.

Question. Write the principal of electro-refining.
Answer: In this method of purification impure metal is made Anode and pure metal is made the cathode. On passing electricity, pure metal is deposited at the cathode while the impurities dissolve in solution as anode mud. E.g. electro- refining of copper:-
At Cathode: - \( \text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu} \)
At Anode: - \( \text{Cu} \rightarrow \text{Cu}^{2+} + 2\text{e}^- \)

Question. Describe the method of refining of Zirconium and Titanium.
Answer: Van Arkel process is used for obtaining ultrapure metal. The impure metal is converted into volatile compound, which then decomposes electrically to get pure metal.
At \( 850\text{K} \): - \( \text{Zr (impure)} + 2\text{I}_2 \rightarrow \text{ZnI}_4 \)
At \( 2075\text{K} \): - \( \text{ZnI}_4 \rightarrow \text{Zr (pure)} + 2\text{I}_2 \)

Question. Out of C & CO, which is better reducing agent for ZnO?
Answer: Since free energy of formation of \( \text{CO} \) from \( \text{C} \) is lower at temperature above \( 1120\text{K} \) while that of \( \text{CO}_2 \) from carbon is lower above \( 1323\text{K} \) than free energy of formation of \( \text{ZnO} \). However, the free energy of formation of \( \text{CO}_2 \) from \( \text{CO} \) is always higher than that of \( \text{ZnO} \). Hence, \( \text{C} \) is better reducing agent of \( \text{ZnO} \).

Question. The value of \( \Delta_f G^\circ \) for \( \text{Cr}_2\text{O}_3 \) is \( -540\text{ kJ/mole} \) & that of \( \text{Al}_2\text{O}_3 \) is \( -827\text{ kJ/mole} \). Is the reduction of \( \text{Cr}_2\text{O}_3 \) possible with aluminium?
Answer: The desired conversion is
\( 4\text{Al} + 2\text{Cr}_2\text{O}_3 \rightarrow 2\text{Al}_2\text{O}_3 + 4\text{Cr} \)
It is obtained by addition of following two reactions:-
\( 4\text{Al} + 3\text{O}_2 \rightarrow 2\text{Al}_2\text{O}_3 \quad \Delta_f G^\circ = -827\text{ kJ/mole} \)
\( 2\text{Cr}_2\text{O}_3 \rightarrow 4\text{Cr} + 3\text{O}_2 \quad \Delta_f G^\circ = +540\text{ kJ/mole} \)
Therefore, \( \Delta G^\circ \) for desired reaction is \( -827 + 540 = -287\text{ kJ/mole} \), as a result reduction is possible.

Question. Why copper matte is put in silica lined converter?
Answer: Copper matte consists of \( \text{Cu}_2\text{S} \) and \( \text{FeS} \). When blast of air is passed through molten matte in silica-lined converter, \( \text{FeS} \) present in matte is oxidized to \( \text{FeO} \custom \), which combines with silica to form slag.
(i) \( 2\text{FeS} + 3\text{O}_2 \rightarrow 2\text{FeO} + 2\text{SO}_2 \),
(ii) \( \text{FeO} + \text{SiO}_2 \rightarrow \text{FeSiO}_3\text{ (slag)} \),
(iii) \( 2\text{Cu}_2\text{S} + 3\text{O}_2 \rightarrow 2\text{Cu}_2\text{O} + 2\text{SO}_2 \),
(iv) \( 2\text{Cu}_2\text{O} + 2\text{Cu}_2\text{S} \rightarrow 6\text{Cu} + \text{SO}_2 \)

Question. What is meant by term chromatography?
Answer: Chromato means Colour and graphy means writing because the method was first used for separation of coloured substance. It is based on selective distribution of various constituents of a mixture between two phases, a stationary phase and a moving phase. The stationary phase can be either solid or liquid on solid support.

Question. Why is reduction of metal oxide easier if metal formed is in liquid state at temperature of reduction?
Answer: The entropy of a substance is higher in liquid state than solid state. In the reduction of metal oxide, the entropy change will be positive if metal formed is in liquid state. Thus, the value of \( \Delta G^\circ \) becomes negative and reduction occurs easily.

SHORT ANSWER TYPE QUESTIONS (3 Marks)

Question. Explain the following:-
(i) Zinc but not copper is used for recovery of Ag from the complex \( [\text{Ag(CN)}_2]^- \).
(ii) Partial roasting of sulphide ore is done in the metallurgy of copper.
(iii) Extraction of Cu from pyrites is difficult than that from its oxide ore through reduction.
Answer:
(i) \( \text{Zn} \) is more powerful reducing agent in comparison to copper. \( \text{Zn} \) is also cheaper than \( \text{Cu} \).
(ii) Partial roasting of sulphide ore forms some oxide. This oxide then reacts with remaining sulphide ore to give copper i.e. self-reduction occurs.
\( 2\text{Cu}_2\text{S} + 3\text{O}_2 \rightarrow 2\text{Cu}_2\text{O} + 2\text{SO}_2 \),
\( 2\text{Cu}_2\text{O} + 2\text{Cu}_2\text{S} \rightarrow 6\text{Cu} + \text{SO}_2 \).
(iii) Though carbon is good reducing agent for oxide but it is poor reducing agent for sulphides. The reduction of metal sulphide does not have large negative value.

Question. Explain the method for obtaining pig iron from magnetite.
Answer: Extraction of iron from Magnetite takes place in following steps:-
(i) Concentration of ore: It is done by Gravity separation followed by magnetic separation process.
(ii) Calcination: It involve heating when the volatile matter escapes leaving behind metal oxide.
\( \text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O} \rightarrow \text{Fe}_2\text{O}_3 + x\text{H}_2\text{O} \).
(iii) Roasting: It involves heating of ore in presence of air, thus moisture, \( \text{CO}_2 \custom, \( \text{SO}_2 \), \( \text{As}_2\text{O}_3 \) removed and \( \text{FeO} \) oxidized to \( \text{Fe}_2\text{O}_3 \).
(iv) Smelting of roasted ore: A mixture of ore, coke & \( \text{CaCO}_3 \) is smelted in long BLAST FURNACE. Following reaction takes place at different temperature zones:-
• Zone of reduction: Temperature range \( 250^\circ\text{C}-700^\circ\text{C} \)
\( 3\text{Fe}_2\text{O}_3 + \text{CO} \rightarrow 2\text{Fe}_3\text{O}_4 + \text{CO}_2 \custom
\( \text{Fe}_3\text{O}_4 + \text{CO} \rightarrow 3\text{FeO} + \text{CO}_2 \custom
\( \text{FeO} + \text{CO} \rightarrow \text{Fe} + \text{CO}_2 \)
• Zone of slag formation: Temperature range \( 800^\circ\text{C}-1000^\circ\text{C} \custom
\( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \custom
\( \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \),   \( \text{P}_4\text{O}_{10} + 10\text{C} \rightarrow 4\text{P} + 10\text{CO} \)
\( \text{SiO}_2 + 2\text{C} \rightarrow \text{Si} + 2\text{CO} \),   \( \text{MnO}_2 + 2\text{C} \rightarrow \text{Mn} + 2\text{CO} \)
• Zone of fusion: Temperature range \( 1150^\circ\text{C}-1350^\circ\text{C} \)
\( \text{CO}_2 + \text{C} \rightarrow 2\text{CO} \)
• Zone of fusion: Temperature range \( 1450^\circ\text{C}-1950^\circ\text{C} \)
\( \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \)
Thus, Pig iron is obtained from Blast Furnace.

Question. Name the principal ore of aluminium and describe how Al is extracted from its ore.
Answer: Important ores – (i) Bauxite \( \text{Al}_2\text{O}_3 \cdot x\text{H}_2\text{O} \), (ii) Corundum \( \text{Al}_2\text{O}_3 \). Bauxite is commercially important ore of Al.
Extraction from Bauxite ore involves the following two stages:-
(i) Purification of bauxite to get pure alumina (\( \text{Al}_2\text{O}_3 \))
(ii) Electrolysis of pure alumina in molten cryolite
Step-1: Bauxite is treated with \( \text{NaOH} \). Following reaction takes place:-
\( \text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{Na}[\text{Al(OH)}_4] \) and impurities of \( \text{Fe}_2\text{O}_3 \), \( \text{TiO}_2 \) & \( \text{SiO}_2 \) are removed. \( \text{Na}[\text{Al(OH)}_4] \) then reacts with \( \text{CO}_2 \) then pure Alumina is obtained.
\( \text{Na}[\text{Al(OH)}_4] + 2\text{CO}_2 \rightarrow \text{Al}_2\text{O}_3 \cdot x\text{H}_2\text{O} + 2\text{NaHCO}_3 \)
Step-2: Electrolytic reduction of pure alumina takes place in iron box (cathode) with cryolite (\( \text{Na}_3\text{AlF}_6 \)) & fluorspar \( \text{CaF}_2 \). Graphite rods act as anode. Following reactions take place:-
At cathode:- \( \text{Al}^{3+} + 3\text{e}^- \rightarrow \text{Al} \)
At Anode:- \( 2\text{O}^{2-} \rightarrow \text{O}_2 + 4\text{e}^- \)
By this process 98.8% pure Aluminum is obtained.

Question. Describe the principles of extraction of Zinc from zinc blende.
Answer: Important ores of Zn:- Zinc blende - \( \text{ZnS} \custom, Calamine - \( \text{ZnCO}_3 \), and Zincite – \( \text{ZnO} \). \( \text{ZnS} \) is commercially important ore of Zn. Various stages involved in the extraction of Zn from \( \text{ZnS} \) are as following:-
(i) Concentration of ore: It is concentrated by Froth flotation process followed by gravity separation process.
(ii) Roasting: The concentrated ore is roasted in presence of air. Following reactions take place:-
\( 2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2 \)
The mass obtained during roasting is porous and is called porous clinker.
(iii) Reduction of ZnO to Zn: \( \text{ZnO} \) is made into briquettes with coke and clay and heated at \( 1163\text{K} \). Zn formed distills off and is collected by rapid cooling of zinc vapours.
\( \text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO} \)