CBSE Class 12 Chemistry General Principles Process Of Isolation Of Elements Chapter Notes

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                                                           Chemistry Ch 6: General Principles & Processes  Of Isolation of Elements Top concepts:

1. Minerals: The naturally occurring chemical substances in the earth’s crust which are obtained by mining are known as minerals. Metals may or may not be extracted profitably from them.

2. Ores: The rocky materials which contain sufficient quantity of mineral so that the metal can be extracted profitably or economically are known as ores.

3. Gangue: The earthy or undesirable materials present in ore are known as gangue.

4. Metallurgy: The entire scientific and technological process used for isolation of the metal from its ores is known as metallurgy.

5. Chief Ores and Methods of Extraction of Some Common Metals:                          

class_12-chemistry_concept_101

class_12-chemistry_concept_102

6. Steps of metallurgy:

   a. Concentration of ore

   b. Conversion of concentrated ore to oxide

  c. Reduction of oxide to metal

   d. Refining of metal

7. Concentration of ore: The process of removal unwanted materials like sand, clay, rocks etc from the ore is known as concentration, ore – dressing or benefaction. It involves several steps which depend upon physical properties of metal compound and impurity (gangue). The type of metal, available facilities and environmental factors are also taken into consideration.

8. Hydraulic washing (or gravity separation): It is based on difference in densities of ore and gangue particles. Ore is washed with a stream of water under pressure so that lighter impurities are washed away whereas heavy ores are left behind.

9. Magnetic separation: This method is based on the difference in magnetic and non – magnetic properties of two components of ore (pure and impure). This method is used to remove tungsten ore particles from cassiterite (SnO2). It is also used to concentrate magnetite (Fe3O4), chromite (FeCr2O4) and pyrolusite (MnO2) from unwanted gangue.

10. Froth Floatation Process: The principle of froth floatation process is that sulphide ores are preferentially wetted by the pine oil, whereas the gangue particles are wetted by the water. Collectors are added to enhance the non-wettability of the mineral particles. Examples are pine oil, fatty acids and xanthates are added to it. Froth stabilizers are added to stabilize the froth. Examples are cresols, aniline. If two sulphide ores are present, it is possible to separate the two sulphide ores by adjusting proportion of oil to water or by adding depressants.For example- In the case of an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to froth but allows PbS to come with the froth.

11. Leaching (Chemical separation): It is a process in which ore is treated with suitable solvent which dissolves the ore but not the impurities.

12. Purification of Bauxite by Leaching: Baeyer’s process:

CBSE Class 12 Chemistry - General Principles _ Process of Isolation of Elements Chapter Notes 1

14. Conversion of ore into oxide: It is easier to reduce oxide than sulphide or carbonate ore. Therefore, the given ore should be converted into oxide by suitable method:

CBSE Class 12 Chemistry - General Principles _ Process of Isolation of Elements Chapter Notes 2

CBSE Class 12 Chemistry - General Principles _ Process of Isolation of Elements Chapter Notes 3

 

15. Reduction of oxide to metal: The process of converting metal oxide into metal is called reduction. It needs a suitable reducing agent depending upon the reactivity or reducing power of metal. The common reducing agents used are carbon or carbon monoxide or any other metals like Al, Mg etc. 
 
16. Thermodynamic principles of metallurgy: Some basic concepts of thermodynamics help in understanding the conditions of temperature and selecting suitable reducing agent in metallurgical processes: 
 
a. Gibbs free energy change at any temperature is given by ΔG =ΔH – TΔS where ΔG is free energy change, ΔH is enthalpy change and ΔS is entropy change.
b. The relationship between ΔGӨ and K is ΔGӨ = –2.303 RT log K where K is equilibrium constant. R = 8.314 JK-¹ mol-1, T is temperature in Kelvin. 
c. A negative ΔG means +ve value of K i.e., products are formed more than the reactants. The reaction will proceed in forward direction. 
d. If ΔS is +ve, on increasing temperature the value of TΔS increases so that TΔS > ΔH and ΔG will become negative. 
 
17. Coupled reactions: If reactants and products of two reactions are put together in a system and the net ΔG of two possible reactions is –ve the overall reaction will take place. These reactions are called coupled reactions.
 
18. Ellingham diagrams: The plots between ΔfGӨ of formation of oxides of elements vs. temperature are called Ellingham diagrams. It provides a sound idea about selecting a reducing agent in reduction of oxides. Such diagrams help in predicting the feasibility of a thermal reduction of an ore. ΔG must be negative at a given temperature for a reaction to be feasible.
 
19. Limitations of Ellingham Diagrams: It does not take kinetics of reduction into consideration, i.e., how fast reduction will take place cannot be determined.

 

 
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