Chemistry NCERT Exemplar Solutions Class 12th Chapter Six

14. The considerations during the extraction of metals by electrochemical method are :

a) Reactivity of metals: If the metals are reactive and are likely to react with water then the metals should be isolated by the electrolysis of their purified molten ore rather than their aqueous solution.

b) Suitability of electrodes:The electrode is carefully chosen so that they should not react with the product of electrolysis. If they react then the electrodes must be made of a material which is quite cheap so their replacement should not increase the cost of the process.

12. Sulfide ore of copper contains some types of impurities. Sulfide ores are not reduced easily, but oxide ores are easily reduced. It is necessary to make sulfide ores free from volatile impurities. This can be performed by a roasting method. In this moisture escape and impurities like sulfur, phosphorus, arsenic are oxidized to their volatile oxides. It is performed in a reverberatory furnace. After this process the mass becomes porous.

10. Sulfide ores are not reduced easily, but oxide ores are easily reduced. Sulfide ore of copper contains some types of impurities. This can cause by-product formation and degrade our final product. Iron oxide is present as a key impurity of concern in sulfide ore of copper. During the roasting process the temperature in the furnace is near about 1200°C. During such high temperatures iron oxide which is gangue (impurity) in ore forms slag. Slag is insoluble in molten metal and being lighter floats over the surface of molten metal. Here silica is used as flux.

FeO + SiO₂ →FeSiO₃ 

10. Column chromatography is a technique used to separate mixtures based on differences in their affinity towards the stationary phase. If one molecule in a solute has more affinity towards the stationary phase, it would form bonds with the stationary phase and hence remain in the column, Whereas, if the other one has less affinity, it would not form bonds and hence will travel down and come out of the column faster.

As compound A is eluted in preference to compound B, compound B is more readily adsorbed on the column of Al₂O₃.

8. Separation of two sulfide ores can be accomplished by adjusting the proportion of oil to water or by using depressants. In the case of an ore containing ZnS and PbS, for example, the depressant NaCN is used. It forms a complex with ZnS and prevents it from coming into contact with froth, but PbS remains in contact with froth.

7. The reduction is the removal of oxygen. Hydrogen and carbon both form oxides that are easy to remove both water and carbon dioxide which are produced as a gas at hot reactions. Although carbon and hydrogen are better reducing agents, they are not used to reduce metallic oxides at high temperatures because at high temperatures carbon and hydrogen react with metals to form carbide and hydrides respectively.

6. Both processes have two basic requirements:

(i) The metal should react with an available reagent to form a volatile compound.

(ii) The volatile compound should be easily decomposable, allowing for easy metal recovery.

5. Low-grade copper ores contain a small percentage of copper that is 0.27% copper. By hydrometallurgy, low-grade copper is leached by treating with acid when copper metal goes into solution as copper ions. The solution containing copper ions is treated with scrap iron or hydrogen gas. Since iron or hydrogen is more reactive than copper so they reduced copper ions from solution to copper metal.