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Practice each question, then open the answer or worked solution.
In which of the following method of purification, metal is converted to its volatile
compound which is decomposed to give pure metal?
(i) heating with a stream of carbon monoxide.
(ii) heating with iodine.
(iii) liquation.
(iv) distillation.
Option (i) and (ii) are the answers.
Which of the following statements are correct?
(i) A depressant prevents a certain type of particle to come to the froth.
(ii) Copper matte contains Cu2S and ZnS.
(iii) The solidified copper obtained from the reverberatory furnace has blistered
appearance due to evolution of SO2
during the extraction.
(iv) Zinc can be extracted by self-reduction.
Option (i) and (iii) are the answers.
In the extraction of chlorine from brine _____________.
A. ΔG° for the overall reaction is negative.
B. ΔG° for the overall reaction is positive.
C. E° for the overall reaction has a negative value.
D. E° for the overall reaction has a positive value.
Option (ii) and (iii) are the answers.
III. Short Answer Type
Why is an external emf of more than 2.2V required for the extraction of Cl2 from brine?
2Cl– (aq) + 2H2O(l) → 2OH– (aq) + H2(g) + Cl2(g)
The ΔGᶱ for this reaction is +422kJmol-1.
Using the formula, ΔGᶱ = -nFE°
E° = 422 x 103/2×96500
we get = -2.2V
For the reaction to take place the external voltage should be greater than 2.2V otherwise the reaction will not happen.
At temperatures above 1073K coke can be used to reduce FeO to Fe. How can you justify this reduction with the Ellingham diagram?
The ∆G for the formation of FeO is less negative than ∆G for the formation of carbon monoxide from carbon. The summation of both the ∆G will be negative about 1073K. Above 1073K the line for the formation of FeO lies above the line for the oxidation of C to CO.
So, in this range, coke will be reducing the FeO and will itself be oxidised to CO.
FeO + C → Fe + CO
Wrought iron is the purest form of iron. Write a reaction used for the preparation of wrought iron from cast iron. How can the impurities of sulphur, silicon and phosphorus be removed from cast iron?
Fe2O3 + 3 C → 2 Fe + 3CO
Limestone is added as a flux to remove the impurities of sulphur, silicon and phosphorous. They form a slag which can be easily removed. The metal is removed from the slag by passing through rollers.
How is copper extracted from low-grade copper ores?
Copper is extracted by hydrometallurgy from low-grade copper ores. Cu is treated with scrap iron or H2. It is leached out with acid.
Cu2+(aq) + H2(g) → Cu(s) + 2H+(aq)
Write two basic requirements for refining of metal by Mond process and by Van Arkel Method.
(i) The metal should react easily with the reagent to form the complex.
(ii) The volatile complex should easily decompose and not give any side products and the recovery should also be easy.
Although carbon and hydrogen are better-reducing agents they are not used to reduce metallic oxides at high temperatures. Why?
Carbon and hydrogen will not reduce oxides to metals but they will instead form hydrides and carbides. Therefore, they are not used as reducing agents.
Solution coming soon.
How do we separate two sulphide ores by Froth Floatation Method? Explain with an example.
Two sulphide ores competing to come to the froth can be separated by adjusting the oil to water proportion or by adding a depressant like NaCN. If we have two sulphide ores, ZnS and PbS, NaCN will prevent ZnS from coming to the froth and instead PbS will form the froth. This way the two ores can be separated.