General Principles and Processes of Isolation of Elements Class 12 Chemistry Important Questions

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Important Questions of General Principles and Processes of Isolation of Elements  Class 12

Very Short Answer Questions

Question. Describe the underlying principle of recovery of silver from the solution obtained by leaching silver ore with a solution of NaCN.
Answer. During leaching Ag is oxidised to Ag⁺ which then combines with CN^– ions (from NaCN) to form soluble complex [Ag(CN)₂]^–. Silver is then recovered from this complex by displacement method using more electropositive zinc metal.
2[Ag(CN)₂]^–_(aq) + Zn_(s) → 2Ag_(s) + [Zn(CN)4]^2–_(aq)

Question. Answer the following :
Differentiate between roasting and calcination.
Answer. 

Question. What is meant by the term ‘pyrometallurgy’?
Answer. The process of extraction of metal from its ore by heating the ore with a suitable reducing agent is known as pyrometallurgy.

Question. What is the role of coke in the extraction of iron from its oxides?
Answer. Coke reduces iron oxide to iron.
FeO_(s) + C_(s) → Fe_(s/l) + CO_(g)

Question. What is the role of limestone in the extraction of iron from its oxides?
Answer. Limestone decomposes to form CaO and CO₂. CaO thus formed acts as a flux and combines with silica (present as impurity) to form fusible calcium silicate slag.
CaCO₃ → CaO + CO₂
CaO + SiO₂ → CaSiO₃
           (Silica)     (slag)

Question. What is the role of silica in the extraction of copper?
Answer. During the metallurgy of copper from copper pyrites, CuFeS₂, its roasting gives FeO besides Cu₂O and SO₂.

Question. Which form of the iron is the purest form of commercial iron?
Answer. Wrought iron is the purest form of commercial iron.

Question. What is ‘copper matte’?
Answer. Copper matte is mixture of Cu₂S and FeS.

Question. Out of C and CO, which is a better reducing agent at the lower temperature range in the blast furnace to extract iron from the oxide ore?
Answer. Ellingham diagram for oxides of metals and
carbon shows
C_(s) + O_2(g) → CO_2(g)                        (i)
2C_(s) + O_2(g) → 2CO(g)                     (ii)
2CO_(g) + O_2(g) → 2CO_2(g)               (iii)
that at 673 K, Δ_fG°(CO₂) < Δ_fG°(CO) hence
formation of CO₂ gas is more favourable than CO gas.
Hence, out of C and CO, CO is a better reducing agent.

Question. What is the composition of ‘copper matte’?
Answer. Cu₂S and FeS.

Question. Give reason for the following :
Zinc oxide can be reduced to metal by heating with carbon but Cr₂O₃ cannot be reduced by heating with carbon.
Answer. Carbon is suitable reducing agent for reduction of zinc oxide. Reduction of Cr₂O₃ by carbon is not thermodynamically favourable.

Question. Although thermodynamically feasible, in practice, magnesium metal is not used for reduction of alumina in the metallurgy of aluminium. Why?
Answer. Below the temperature (1623K) corresponding to the point of intersection of Al₂O₃ and MgO curves in ellingham diagram magnesium can reduce alumina. But magnesium is a much costlier metal than aluminium and hence the process will be uneconomical.

Question. Describe the principle controlling the following process :
Preparation of cast iron from pig iron.
Answer.  Pig iron is melted with scrap iron and coke using hot air blast. Due to this, impurities such as C, S and P present in the pig iron are removed as CO₂, SO₂ and P₂O₅ and carbon content is reduced to about
3%.

Question. Copper matte is charged into a silica lined converter in extraction of copper. What is the role of silica lining here? 
Answer. The copper matte containing Cu₂S and FeS is put in silica lined converter. Some silica is also added and hot air blast is blown to convert remaining FeS to FeO, which is removed as slag with silica.

Question. The reaction Cr₂O₃ + 2Al → Al₂O₃ + 2Cr ;
ΔG° = – 421 kJ is thermodynamically feasible as is apparant from the Gibbs energy value. Why does it not take place at room temperature?
Answer. In solid state chance of contact between reactants is negligible.
Some thermodynamically feasible reactions also require some activation energy for initiation.

Question. What is the role of cryolite in the extraction of aluminium?
Answer. Purified alumina (Al₂O₃) is mixed with cryolite (Na₃AlF₆) which lowers the melting temperature of electrolyte and also increase its conductivity.

Question. Which reducing agent is employed to get copper from the leached low grade copper ore?
Answer. Scrap iron or H₂.

Question. Write the role of graphite rod in the electrometallurgy of aluminium.
Answer. Graphite anode is used in the electrometallurgy of aluminium from alumina, Al₂O₃. Oxygen liberated at high temperature reacts with graphite to form both CO₂ and CO gases and prevent the liberation of O₂ gas at the anode which may react with Al metal to give Al₂O₃ again.

Question. How is copper extracted from a low grade ore of it?
Answer. 

Copper is extracted by hydrometallurgy from
low grade ores. It is leached out using acid or bacteria.
e solution containing Cu2+ is treated with scrap
iron or H2.
Cu²⁺_(aq) + H_2(g) → Cu^(s) + 2H⁺_(aq)

Question. Which of the two scraps, zinc or iron would be preferred for the recovery of copper from the leached copper ore and why?
Answer. Zinc scrap should be preferred because zinc is more electropositive than iron.
But zinc is costlier metal than iron so using iron scraps will be preferred.

Question. What is the role of dilute NaCN in the extraction of gold?
Answer. NaCN is used for leaching of gold ore in the presence of air to form soluble gold complex from which metal is displaced by adding more reactive metal.
4Au_(s) + 8CN^–_(aq) + 2H₂O_(l) + O_2(g) → 4[Au(CN)₂]^–_(aq)+ 4OH^–_(aq)

Question. Describe the underlying principle of the following process :
Vapour phase refining of metals.
Answer. Vapour phase refining : In this method, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal. So, the two requirements are :
(a) The metal should form a volatile compound with an available reagent.
(b) The volatile compound should be easily decomposable, so that the recovery is easy.

Question. Name the methods used for the vapour phase
Answer. Titanium is refined by van Arkel method and nickel is refined by Mond’s process.

Question. Explain the role of the following :
Iodine in the refining of titanium.
Answer. Iodine forms a volatile compound with titanium which on further heating decomposes to give pure titanium.

Question. Write the chemical reaction which takes place in
Answer. 

Question. What types of metals are usually purified by the
Answer. The metals such as germanium, silicon, gallium, etc. which are used as semiconductors are purified by zone refining which is based upon the principle that impurities are more soluble in the melt than in the solid state of metals.

Question. Describe the principle controlling the following process :
Vapour phase refining of titanium metal.
Answer. In this method, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal.

Question. Write the reactions involved in the following process :
refining of zirconium by van Arkel method.
Answer. Refining of zirconium by van Arkel method.

Short Answer Questions

Question. Write all the reactions involved in the extraction of aluminium from bauxite ore.
Answer. Aluminium metal is extracted from bauxite
(Al₂O₃·2H₂O) in two steps.
Stage I (Leaching) : The ore is treated with sodium hydroxide solution. Aluminium oxide and silica dissolve to form sodium aluminate and sodium silicate respectively. Iron oxide and TiO₂ is filtered off.

Stage II (Electrolysis) : the alumina is dissolved in molten cryolite Na₃[AlF₆] and then electrolysed in a large steel tank lined with graphite which acts as cathode. the anodes are made of carbon. On passing current, molten aluminium is produced at cathode and oxygen gas is evolved at the anode which reacts with carbon anode producing CO and CO₂. The electrolytic reactions may be written as :
Cathode : 2Al³⁺ (melt) + 6e^– → 2Al(l)
Anode : C_(s) + O^2– → CO_(g) + 2e^–
C_(s) + 2O^2– → CO_2(g) + 4e^–
The anode burns away. therefore the graphite rods must be replaced from time to time.

Question. The extraction of gold by leaching with NaCN involves both oxidation and reduction. Justify giving chemical equations.
Answer.  During the leaching process, Au is first oxidised to Au⁺ by O₂ of the air which then combines with CN^– ions to form the soluble complex, sodium dicyanoaurate (I).

Gold is then extracted from this complex, by displacement method using a more electropositive zinc metal. In this reaction,
Zn acts as a reducing agent. It reduces Au+ to Au while itself gets oxidized to Zn²⁺ which combines with CN^–ions to form soluble complex, sodium tetracyanozincate (II).

Thus, extraction of Au by leaching with NaCN involves both oxidation and reduction.

Question. How is chemical reduction different from electrolytic reduction? Name a metal each which is obtained by
(i) electrolytic reduction,
(ii) chemical reduction.
Answer. In chemical reduction, metal is obtained by reduction of its ore by a suitable reducing agent, whereas in electrolytic reduction, reduction is carried out by passing electric current through molten ore.
(i) Aluminium is obtained by electrolytic reduction.
(ii) Iron is obtained by chemical reduction.

Long Answer Questions

Question. Write the principle behind the froth floatation process. What is the role of collectors in this process?
Answer. Froth oatation method : The method has been in use for removing gangue from sulphide ores. In this process, a suspension of the powdered ore is made with water. To it collectors and froth stabilisers are added.
Collectors (e.g., pine oils, fatty acids, xanthates, etc.) enhance non wettability of the mineral particles and froth stabilisers (e.g., cresols, aniline) stabilise the froth.
The mineral particles become wet by oils while the gangue particles by water. A rotating paddle agitates the mixture and draws air in it. As a result, froth is formed which carries the mineral particles. The froth is light and skimmed off.

Question. What is meant by the term, ‘Chromatography What criterion is followed for the selection of the stationary phase in chromatography?
Answer. The term chromatography was derived from the Greek word “chroma”, meaning colour and “graphy”, for writing. It was used for the first time to separate the colour pigments of plants. Chromatography is a technique for analysing or separating mixtures of gases, liquids or dissolved substances. The criterion of selecting, the stationary phase depends on the different adsorbing powers of the components of a mixture to be separated. Generally used adsorbent material is silica gel or alumina. The adsorbent can be spread over a flat glass plate (thin – layer chromatography) or taken in a glass tube (column chromatography). In paper chromatography, the stationary phase is water held in the pores of the paper. Here, the separation depends upon the different distribution of the components between water in stationary phase and the element.