Bihar Board - Class 12 Chemistry - Chapter 6: General Principles and Processes of Isolation of Elements Short Answer Question

                            Short Question Answer

Q. 1. Differentiate between minerals and ores.

Ans⇒  Differentiate between minerals and ores are as follows :

Q. 2. Write chemical reactions taking place in the extraction of zinc from zinc blende.

Ans⇒  (a) Roasting : The concentrated ore is heated with oxygen at 900C in a reverberatory furnace to convert zinc sulfide to zinc oxide. At 900C zinc sulfate which is formed due to the oxidation of zinc sulfide also decomposes to zinc oxide.

2 ZnS +3O22ZnO+2SO2

ZnS+2O2 ZnSO4

2ZnSO42ZnO+2SO2+O2

(b) Reduction : Zinc oxide is reduced to zinc with the help of carbon.

ZnO+CZn +CO

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Q. 3. Copper can be extracted by hydrometallurgy but no zinc. Explain.

Ans⇒  Copper can be extracted by hydrometallurgy. The solution is leached out using acid. The solution containing Cu2+ is treated with H2.

Cu2+ (aq)+H2 (g) Cu (S) + 2H+ (aq) 

Zinc cannot be extracted by hydrometallurgy as zinc is a highly reactive metal. It may not be possible to replace it with a solution.

Q. 4. What is the role of depressant in the froth floatation process ?

Ans⇒  If a mixture of two sulfides (say, ZnS and PbS) are to be separated and concentrated by the Froth Floatation process, a substance called depressant (in this case NaCN ) is used. It selectively prevents ZnS from coming to the froth but allows PbS to come up with the froth.

Q. 5. Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction ?

Ans⇒  The Gibbs energies of formation of most sulfides are greater than for Cu2O. In fact Cu2O is an endothermic compound. Hence it is a common practice to roast sulfide ores to corresponding oxides prior to reduction.

Q. 6. State the role of silica in the metallurgy of copper.

Ans⇒  Silica acts as a flux to remove the impurities oxides of iron remaining in the matter by forming silicate, FeSiO3 which is a fusible slag.

SiO2+FeOFeSiO3
Silica                Slag

Q. 7. What is meant by the term chromatography ?

Ans⇒  Chromatography : It is defined as a technique of separating the components of a mixture in which separation is achieved by the differential movement of individual components through a stationary phase under the influence of a mobile phase.

Q. 8. What criterion is followed for the selection of the stationary phase in chromatography ?

Ans⇒ Different components to be separated from the given mixture should be able to get differently adsorbed on the adsorben like silica or alumina gel (SiO2 or Al2O3). The stationary phase should be immobile and immiscible.

Q. 9. Describe a method for refining nickel.

Ans⇒ Nickel is refined by the MOND process. W impure nickel is heated in a current of CO at 330-350 k It forms volatile nickel carbonyl leaving behind the impurities. The nickel carbonyl [Ni(CO)4] thus obtained is then heated to a higher temperature (450-470K) when it undergoes thermal decomposition giving pure metal.

Ni+4CO Ni(CO)4
                                        Nickel tetracarbonyl ]

Ni(CO)4 Ni+4CO
.                                           Pure

Q. 10. How is ‘cast iron’ different from pig iron ?

Ans⇒ Pig Iron : The iron obtained from Blast Furnace contains about 4 % Carbon and many impurities like S , P , Si and Mn is smaller. This is known as pig iron and cast into a variety of shapes.
Cast Iron : It is different from pig iron and is made by melting pig iron with scrap iron and coke using hot air blast. It has slightly lower carbon content (about 3 %) and is extremely hard and brittle.

Q. 11. What is the role of cryolite in the metallurgy of aluminum ?

Ans⇒ Cryolite (Na3AIF6) is added to the electrolyte to lower the melting point and to make the melt more conducting.

Q. 12. How is leaching carried out in case of low grade copper ores ?

Ans⇒ Copper is extracted from low grade copper ores by hydrometallurgy. Copper is leached out using acid or bacteria. The solution containing Cu2+ ions is treated with scrap iron or H2.

Cu2+ (aq)+H2 (g) Cu (s)+2H+ (aq)

Cu2+ (aq)+Fe Fe2 (aq)+Cu(s)

Q.13 . Name the common elements present in the anode mud in the electrolytic refining of copper. Why are they so present?

Ans⇒ Selenium, silver, tellurium, platinum, antimony and gold are the common elements present in the anode mud during the electrolytic refining of copper.
As these metals are almost unreactive, they are not affected during the purification method. Thus, they are present in the anode mud.

Q.14. “Chromatography.” What do you understand by this term?

Ans⇒ Chromatography is a collective term used for a range of laboratory techniques for the purification, separation, characterization and identification of the species of a mixture, whether coloured or colorless. The word has been obtained from two Greek words; ‘graphy’, meaning ‘to write’ and ‘chroma’, meaning ‘color’. There are numerous chromatographic techniques like column chromatography, paper chromatography, gas chromatography, etc.

Q.15 . What is the criterion followed while selecting the stationary phase of chromatography?

Ans⇒ The choice of stationary phase is done in a way that the elements of the mixture have different solubility in the phase. Thereby, different elements have different movement speeds through the phase, because of which they can be separated from each other.

Q.16. Differentiate between “minerals” and “ores”.

Ans⇒ Minerals are naturally available substances having metals or their compounds.
Ores are rocks and minerals from which metals are economically and conveniently obtained.
For e.g., there are a lot of minerals that contain, but zinc cannot be conveniently and economically extracted from all of them. Zinc is extracted from calamine (ZnCO3) , zinc blende (ZnS) , etc. Hence, these minerals qualify as ores of zinc.

Q.17. Why is zinc not extracted from zinc oxide through reduction using CO?

Ans⇒ As the standard Gibbs free energy of the formation of CO to CO2  is higher than that of Zn  to ZnO , CO  will not be able to reduce ZnO to Zn . Thereby, zinc oxide reduction using CO  isn’t used to extract zinc.

Q.18. Which of the ores mentioned  can be concentrated by magnetic separation method?

Ans⇒ Ores Which are magnetic in nature can be separated from non-magnetic gangue particles by magnetic separation method. For ex: ores of iron such as hematite (Fe2O3) ,  Magnetite (Fe3O4),  siderite (FeCO3) and iron pyrites (FeS2) being magnetic can be separated from non-magnetic silica and other impurities by magnetic separation method.

Q.19.  Name the method used for refining copper metal.
Ans⇒ Electrolysis is the method used for refining the copper metal. In this method, a block of impure metal is used as anode and a thin strip of pure metal as cathode. A solution of copper sulfate acts as an electrolyte. 

Q.20. What are the collectors used in the froth floatation process? Name a substance that can be used as such. 

Ans⇒ Collectors are reagents that are used to selectively adsorb onto the surface of particles. A monolayer is formed by the collectors on the particle's surface that essentially makes a thin film of non-polar hydrophobic hydrocarbons.

Q.21.  Define the following terms:
1. Roasting
2. Calcination
Ans⇒

(1) Roasting: The process of heating metal ore below its melting point in the presence of air is called roasting. 

(2) Calcination: The process of heating metal ore in the absence of air is called calcination.

Q.22.  (i) Give an example of zone refining of all metals. 

(ii) What is the role of cryolite in the metallurgy of aluminum? 

Ans⇒

(i) Zone-refining method is used for the production of  semiconductors and various other metals of high purity. e.g. silicon, boron, germanium, gallium and indium. 

(ii) Cryolite is used as a solvent for bauxite in the electrolytic production of aluminum and has various other metallurgical applications, and it is used in the glass and enamel industries, in bonded abrasives as a filler, and in the manufacture of insecticides. A large amount of synthetic cryolite is made from fluorite.

Q.23. 1. Name the method used for removing gangue from  sulfide ores.
2. How is wrought iron different from steel?
Ans⇒

(1) Froth floatation method is used to remove gangue from sulfide ores because of the high wettability of ore in pine oil as compared to gangue. 

(2) Wrought iron is almost pure iron with around 0.02 -0.03 % carbon whereas steel contains 0.15 % -0.25 % of carbon. Wrought iron is ductile and resistant to corrosion whereas steel has more strength due to more percent of carbon in it. 

Q.24.  Name the methods used for the vapor phase  refining of impure titanium and nickel metals. 

Ans⇒ Van-Arkel method for titanium and Mond's process  for nickel. 

Q.25.  What types of ores can be concentrated by magnetic  separation method? 

Ans⇒ Those ores which are magnetic in nature and associated impurities are non-magnetic in nature or vice-versa, are concentrated by magnetic separation method.