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Class 12 Chemistry
Chapter - 6 Important Questions
General Principles & Process of Isolation of Elements Important Points
1. What is slag?
Ans. It is easily fusible material fusible material, which is formed when gangue still
present in roasted ore combines with the flux.
e.g. CaO (flux) + SiO2 (gangue) →CaSiO3 (slag)
time to time.
5. What is the role of cryolite in electrometallurgy of aluminum?
Ans. alumina cannot be fused easily because of high melting point. Dissolving of alumina
Ans. The E0 of Zn is lower than that of Cu thus Zn can displace Cu2+ ion from its solution.
On other hand side to displace Zn from Zn2+ ion, we need a more reactive metal than it.
8. Give name and formula of important ore of iron.
Ans. Haematite - Fe2O3, Magnetite -Fe3O4, Iron pyrites FeS2.
At 450-470K Ni (CO)4 → Ni + 4 CO
At Cathode: - Cu2+ + 2e → Cu
At Anode: - Cu → Cu2+ + 2e
4. Write difference between calcinations and roasting.
Ans. Roasting: It is used to convert sulphide ores into oxides. Roasting involves strong
heating of iron ore in the presence of excess air. For example, copper sulphide in copper
glance ore is converted into copper (I) oxide by heating it in the presence of oxygen.
Calcination: It is used to convert carbonate ores into oxides. Calcination involves strong
heating of the ore in the absence of air. For example, calamine ore, which is chemically
zinc carbonate, is converted into zinc oxide by heating it in the absence of air.
ZnO. However, the free energy of formation of CO2 from CO is always higher than that of
7. The value of ∆fG0 for Cr2O3 is -540kJ/mole & that of Al2O3 is -827kJ/mole. Is the
matte in silica- lined converter, FeS present in matte is oxidized to FeO, which combines
with silica to form slag.
I. 2FeS + 3O2→2FeO +2 SO2,
2Cu2O+2Cu2S→6Cu + SO2.
c. 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.
2. Explain the method for obtaining pig iron from magnetite.
Ans. 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.
Fe2O3.xH2O→ Fe2O3 + xH2O.
III. Roasting: - It involves heating of ore in presence of air, thus moisture, CO2,SO2, As2O3
IV. Smelting of roasted ore: - A mixture of ore, coke & CaCO3 is smelted in long BLAST
removed . Na [Al(OH)4] ,then reacts with CO2 then pure Alumina is obtained.
Step:-2 Electrolytic reduction of pure alumina takes place in iron box (cathode) with
cryolite (Na3AlF6) & fluorspar CaF2.Graphite rods act as anode. Following reactions
take place:-
At cathode:- Al3+ + 3e→ Al, At Anode:- 2O2-→O2 + By this process 98.8% pure
Aluminum is obtained.
4. Describe the principles of extraction of Zinc from zinc blende.
Ans. Important ores of Zn:-Zinc blende - ZnS, Calamine- ZnCO3, and Zincite - ZnO. ZnS is
The mass obtained during roasting is porous and is called porous clinker.
III. Reduction of ZnO to Zn: - ZnO is made into bricketts with coke and clay and heated at
1163K. Zn formed distills off and is collected by rapid cooling of zinc vapours.
ZnO + C → Zn + CO