Please refer to General Principles and Processes of Isolation of Elements Class 12 Chemistry Important Questions with solutions provided below. These questions and answers have been provided for Class 12 Chemistry based on the latest syllabus and examination guidelines issued by CBSE, NCERT, and KVS. Students should learn these problem solutions as it will help them to gain more marks in examinations. We have provided Important Questions for Class 12 Chemistry for all chapters in your book. These Board exam questions have been designed by expert teachers of Standard 12.
Class 12 Chemistry Important Questions General Principles and Processes of Isolation of Elements
Very Short Answer Questions
Question. Name the chief ores of aluminium and zinc.
Answer. The chief ore of aluminium is bauxite and that of zinc is zinc blende or sphalerite.
Question. 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. Those minerals from which metals can be extracted economically are called ores.
Question. In the extraction of Al, impure Al2O3 is dissolved
in conc. NaOH to form sodium aluminate and leaving impurities behind. What is the name of this process?
Question. Which solution is used for the leaching of silver metal in the presence of air in the metallurgy of silver?
Answer. Dilute solution of NaCN or KCN in the presence of air (for O2).
Question. Which of the following ores can be concentrated by froth floatation method and why?
Fe2O3, ZnS, Al2O3
Answer. ZnS (Froth oatation method is used to remove gangue from sulphide ores).
Question. Name the method used for removing gangue from sulphide ores.
Answer. Froth oatation method.
Question. Why is it that only sulphide ores are concentrated by froth floatation process?
Answer. This is because the sulphide ore particles becomes lighter when preferentially wetted by oil and rise to surface along with the froth, whereas gangue particles are preferentially wetted by water become heavier and settle down.
Question. What type of ores can be concentrated by magnetic separation method?
Answer. If either the ore or the gangue particles are capable of being attracted by magnetic field, then such ore is concentrated by magnetic separation method.
Question. What is the role of zinc metal in the extraction of silver?
Answer. (ii) Zinc in the extraction of silver : Role of zinc is to recover silver from the complex by displacement reaction. Silver ore is leached with dilute solution of NaCN in the presence of air or oxygen to form a soluble complex.
4Ag(s) + 8CN–(aq) + 2H2O(l) + O2(g) → 4[Ag(CN)2]– Soluble complex + 4OH–(aq)
2[Ag(CN)2]– + Zn → [Zn(CN)4]2– + 2Ag
Question. Write the role of the following :
Iodine in the refining of zirconium.
Answer. In this method, crude metal is heated in an evacuated vessel with iodine. The metal iodide formed is decomposed on a tungsten filament at high temperature (1800 K) to get pure metal.
Zr + 2I2 → ZrI4 → Zr + 2I2
Question. Describe the principle controlling the following process :
Preparation of pure alumina (Al2O3) from bauxite ore.
Answer. Al2O3 present in bauxite is soluble in concentrated NaOH solution whereas impurities are not, Al2O3 is reprecipitated from the solution.
Question. Write the reactions involved in the following process :
Leaching of bauxite ore to prepare pure alumina.
Answer. (i) Leaching of bauxite ore to prepare pure alumina :
Question. Write the principle of the following method.
Froth floatation method
Answer. The principle behind this method is that the sulphide ore is preferentially wetted by oils while gangue is preferentially wetted by water. Then on agitation froth is formed that carries mineral particles and can be skimmed off.
Question. Describe the role of the following :
NaCN in froth floatation process
Answer. NaCN is used as a depressant in froth floatation process which selectively prevents ZnS from coming to the froth but allows PbS to come with the froth.
Question. Write the reaction involved in the following process :
Recovery of gold after gold ore has been leached with NaCN solution.
Answer. Gold is recovered from [Au(CN)2]– complex by displacement method using a more electropositive metal zinc.
2[Au(CN)2]–(aq) + Zn(s) → 2Au(s) + [Zn(CN)4]2–(aq)
Question. Why is electrolytic reduction preferred over chemical reduction for the isolation of certain metals?
Answer. Electropositive metals which cannot be reduced by chemical reduction are easily reduced electrolytically.
Question. Name the method of refining of metals such as germanium.
Answer. Zone refining is used for refining of semiconductors or other metals of very high purity.
Question. Name the method of refining of nickel.
Answer. Mond’s process : Impure nickel is treated with carbon monoxide at 60-80°C to form volatile nickel carbonyl which decomposes to give pure nickel at 180°C.
Ni + 4CO → Ni(CO)4 → Ni + 4CO
Question. Indicate the principle behind the method used for the refining of zinc.
Answer. Zinc is refined by electrolytic refining. In this method, the impure metal is made to act as anode. A strip of the same metal in pure form is used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the same metal. The more basic metal remains in the solution and the less basic
ones go to the anode mud.
At anode : Zn → Zn2+ + 2e–
At cathode : Zn2+ + 2e– → Zn
Question. What is the principle behind the zone refining of metals?
Answer. Zone refining is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal.
Question. What is meant by the term ‘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.
Question. Name the method used for refining of copper metal.
Answer. Copper is refined by electrolytic refining
Question. Write the role of the following :
CO in the purification of nickel
Answer. The role of CO is to convert impure nickel to volatile nickel tetracarbonyl whereas the impurities are left behind in the solid state. The vapour of nickel tetracarbonyl on heating at 450-470 K decomposes to give pure nickel metal and carbon monoxide.
Question. What is the function of collectors in the froth floatation process for the concentration of ores.
Answer. Collectors enhance non-wettability of the mineral particles. Pine oil can be used as collectors.
Question. Name the depressant which is used to separate ZnS and PbS ores in froth floatation process.
Answer. NaCN is used to separate ZnS and PbS ores infroth floatation process.
Question. What role is played by CO2 in getting pure alumina (Al2 O3) in the extraction of aluminium?
Answer. The aluminate in solution is neutralised by passing CO2 gas and hydrated Al2O3 is precipitated.
2Na[Al(OH)4](aq) + CO2(g) → Al2O3.xH2O(s)+ 2NaHCO3(aq).
Short Answer Questions
Question. Write the chemical reactions involved in the extraction of silver from silver ore.
Question. How is wrought iron different from steel?
Wrought iron is the purest form of iron. Steel is an alloy of iron which contains requisite amount of C, Mn, Ni, Cr, etc.
Question. Describe how the following changes are brought about :
(i) Pig iron into steel
(ii) Zinc oxide into metallic zinc.
Answer. (i) Pig iron is mixed with scrap iron and heated in a furnace. Aer removal of impurities required quantity of spiegel is added to make steel.
(ii) Zinc oxide is mixed with coke and clay and brickettes are made. The brickettes are heated at 1673 K to give metallic zinc.
Long Answer Questions
Question. Write down the reactions taking place in different zones in the blast furnace during the extraction of iron. How is pig iron different from cast iron?
Answer. Reduction of iron oxide in blast furnace :
(i) Lower zone of the blast furnace :
C + O2 → CO2 + heat
C + CO2 → 2CO
Coke is burnt to give temperature up to 2200 K at lower part of the blast furnace.
(ii) Middle zone of the blast furnace : CO and heat move up in the furnace. The temperature range in the middle zone of the blast furnace is 900-1500 K.
FeO + CO → Fe + CO2
Lime stone is also decomposed to CaO which removes silicate impurity of the ore as slag.
(iii) Upper zone of the blast furnace : Temperature range in this zone is 500-800 K.
3Fe2O3 + CO → 2Fe3O4 + CO2
Fe3O4 + 4CO → 3Fe + 4CO2
Fe2O3 + CO → 2FeO + CO2
Impure iron obtained from blast furnace is known as pig iron and cast into pigs (blocks). It contains 4-5 percent of carbon along with some phosphorus, silicon, manganese and sulphur.
Cast iron contains a less percentage of carbon (1.8 to 3 percent) made by melting pig iron with scrap iron and coke using hot air blast.
Question. What chemical principle is involved in choosing a reducing agent for getting the metal from its oxide ore? Consider the metal oxides, Al2O3 and Fe2O3, and justify the choice of reducing agent in each case.
Answer. Thermodynamic factor helps us in choosing a suitable reducing agent for the reduction of a particular metal oxide to the metallic state.
Any metal will reduce the oxides of other metals if ΔfG° of the oxide of the element is less than ΔfG° of
the metal oxide to be reduced.
Thus, both Al and Zn can reduce Fe2O3 and Mg can reduce Al2O3 to Al.
Question. What are the chief ores of zinc? Write chemical reactions taking place in the extraction of zinc from zinc blende.
Answer. The chief ores of zinc are
(i) Zinc blende, ZnS
(ii) Calamine, ZnCO3
(iii) Zincite, ZnO.
Extraction of zinc : From zinc blende zinc is extracted by roasting followed by reduction with coke.
(a) Roasting : The concentrated ore is heated with oxygen at 900°C in reverberatory furnace to convert zinc sulphide to zinc oxide.