In this article, we have mentioned the important questions and answers for the students appearing for the CBSE Class 12th Chemistry Examination 2020. The list of important questions is prepared keeping in view the syllabus prescribed by the CBSE. The students will find the below-mentioned questions and answers helpful for the upcoming CBSE Board Class 12th Examinations 2020.
Key Points to be mentioned while writing the answers to the below mentioned important questions:
Question 1- Explain the phenomena of converting:
1- Pig iron into steel
2- Zinc oxide into metallic zinc.
3- Impure titanium into pure titanium.
Answer: 1- Pig iron is converted into steel by Basic Oxygen Process (BOP). The furnace is charged with molten pig iron and lime. After this pure O2 is blown over the surface of the metal at a high speed through water-cooled retractable lances. The O2 penetrates through the metal and oxidises the impurities rapidly
2C + O2 → 2CO,
2Fe +O2 → 2FeO,
2FeO + Si → 2Fe + SiO2,
CaO + SiO2 → CaSiO3 Slag
6CaO + P4O10 → 2Ca2 (P04)2 Slag
After the removal of the impurities, the steel of desirable properties is obtained by adding the required alloying elements (Cr, Ni, Mn etc).
2- At 673 K, Zinc oxide is converted into metallic zinc by reacting it with coke.
3- At 870 K, the impure titanium is heated in an evacuated vessel with iodine. After this process, the titanium iodide vapour is collected. It is then electrically heated up to 1800K and is then decomposed on a tungsten filament. The pure titanium is thus deposited on the filament.
Question 2- Explain the role of:
i) NaCN in the extraction of gold from gold ore.
ii) SiO2 in the extraction of copper from copper matte.
iii) Iodine in the refining of zirconium.
Answer: i) Metal present in the gold ore is leached with the help of NaCN by oxidising gold to Au+ which combines with CN- ions resulting in respective soluble complexes. The metal is recovered from this complex with the help of the Zinc.
4Au + 8CN- + 2H2O + O2→ 4[Au(CN2)]- + 4OH-
2[AU(CN)2]– + Zn → 2Au + [Zn(CN)4]2-
ii) SiO2 is used in the extraction of copper from copper matte as copper matte consists of Cu2S and FeS. In the converter, this is converted to FeO and Silica helps in the removal of FeO impurity as slag. SiO2 acts as a basic flux which is used in slag formation(FeO).
iii) Iodine is used in the refining of zirconium at 870 K to form a volatile Iodide which on decomposition gives pure metal.
Question 3- Describe:
i) Froth floatation method
ii) Electrolytic refining
iii) Zone refining
Answer: i) Froth floatation method: It is used 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 like pine oils, fatty acids, xanthates, etc. enhance non-wettability of the mineral particles and froth stabilisers like cresols, aniline, etc. 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 is skimmed off. It is then dried for recovery of the ore particles. Sometimes, it is possible to separate two sulphide ores by adjusting the proportion of oil to water or by using ‘depressants’. For example, in case of an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to the froth but allows PbS to come with the froth.
ii) Electrolytic refining: In electrolytic refining, the impure metal is made to act as an anode. A strip of the same metal in pure form is used as a 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. The reactions are:
Anode: M → Mn+ + ne–
Cathode: Mn+ + ne– → M
Copper and Zinc can be refined through electrolytic refining.
iii) Zone refining: Zone refining is based on the principle that the impurities are more soluble in the melt state than in the solid-state of the metal. A circular mobile heater is fixed at one end of a rod of the impure metal. The molten zone moves along with the heater which is moved forward. As the heater moves forward, the pure metal crystallises out of the melt and the impurities pass on into the adjacent molten zone. The process is repeated several times and the heater is moved in the same direction. At one end, impurities get concentrated. This end is cut off. This method is very useful for producing semiconductor and other metals of very high purity, e.g., germanium, silicon, boron, gallium and indium.
iv) Calcination: It involves heating when the volatile matter escapes leaving behind the metal oxide:
v) Roasting: In roasting, the ore is heated in a regular supply of air in a furnace at a temperature below the melting point of the metal. Some of the reactions involving sulphide ores are:
vi) Distillation: It is very useful for low boiling metals like zinc and mercury. The impure metal is evaporated to obtain the pure metal as distillate.
vii) Chromatography: ‘Chromatography’ is derived from the Greek word ‘chroma’ which means colour and ‘graphy’ which means writing. This method was first used for the separation of coloured substances into individual components. It is a widely used process for separation, purification, identification and characterization of the different components of a mixture which are differently adsorbed on a suitable adsorbent. There are several chromatographic techniques such as paper chromatography, column chromatography, gas chromatography, etc.
Question 4- Explain:
(a) the principle behind the method used for refining zinc
(b) the role of cryolite in the metallurgy of aluminium
(c) the role of coke in the extraction of iron from its oxides.
(d) the role of limestone in the extraction of iron from its oxides.
(a) Distillation method is used for refining Zinc as it has a low boiling point.
(b) Cryolite (Na3AlF6) lowers the melting point of the mixture, hence, bringing the conductivity.
(c) Coke acts as a reducing agent in the extraction of iron from its oxides in the zone of reduction in the blast furnace at a temperature that is 1123 K.
FeO + CO → Fe + CO2
(d) In the blast furnace, Limestone breaks into CaO and CO2. CaO acts as basic flux which combines with acidic impurities of the ore like silica to form calcium silicate in the form of slag.
Question 5- a) Which method is used for the refining of germanium.
(b) Which ore of lead is concentrated by the froth floatation process preferably, PbS or PbCO3?
(c) Mention the significance of leaching in the extraction of aluminium?
Answer: (a) Zone refining is used for the refining of germanium and is based on the principle that the impurities are more soluble in the melt than in the solid-state of the metal.
(b) PbS (Lead sulphide) is concentrated by the froth floatation process.
(c) Leaching process concentrates pure alumina from bauxite ore by digesting it with a hot concentrated solution of NaOH to form sodium meta aluminate leaving behind impurities. These impurities are then treated with CO2 forming hydrated alumina through precipitation.
Question 6- Explain.
(a) Copper obtained in the extraction from copper pyrites has a blistered appearance.
(b) NaCN in the extraction of silver from silver ore.
Answer: (a) The blistered appearance is due to the evolution of SO2 and therefore the copper obtained is called blister copper.
(b) NaCN acts as a leaching agent or oxidising agent, thus oxidises Ag to Ag+ which then combines with CN– ions to form respective soluble complexes.
Question 7- What is Vapour phase refining? Give Examples.
In the vapour phase refining, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal.
The requirements for vapour phase refining are:
(i) the metal should form a volatile compound with an available reagent
(ii) the volatile compound should be easily decomposable so that the recovery is easy. Examples:
1- Mond Process for Refining Nickel: In this process, nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl:
The carbonyl is subjected to a higher temperature so that it is decomposed giving the pure metal:
2- Van Arkel Method for Refining Zirconium or Titanium: It is very useful for removing all the oxygen and nitrogen present in the form of impurity in certain metals like Zr and Ti. The crude metal is heated in an evacuated vessel with iodine. The metal iodide being more covalent, volatilises:
The metal iodide is decomposed on a tungsten filament, electrically heated to about 1800K. The pure metal is thus deposited on the filament.
Question 8- Which methods are used for purifying the following metals :
(i) Nickel (ii) Germanium
Mention the principle behind each one of them.
Answer: (i) Nickel: Mond process. The metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal.
(ii) Germanium: Zone refining. It is based on the principle that the impurities are more soluble in the melt than in the solid-state of the metal.
Question 9- Why is copper matte put in silica lined converter?
Answer: Copper matte consists of Cu2S and FeS. When a hot air blast is blown through molten matte taken in a silica lined converter, FeS is oxidised to FeO. Silica (SiO2) present as lining acts as a flux and combines with FeO to form slag.
2FeS + 3O2 → 2FeO + 2SO2
FeO + SiO2 → FeSiO3 (slag)
Question 10- (a) Among the below-mentioned ores, which ore can be concentrated by the froth floatation method and why?
Fe2O3, ZnS, Al2O3
(b) How is wrought iron different from steel?
Answer: (a) ZnS. It can be concentrated by the froth floatation method as it is a sulphide ore.
(b) Wrought iron is the purest form of commercial iron and consists of about 0.2 – 0.5% carbon whereas steel contains about (0.5 – 1.5)% carbon.
The above-mentioned questions and answers are based on the latest syllabus as prescribed by the CBSE Board for Class 12th Chemistry Examination 2020. The students can go through the links mentioned below: