General Principles And Processes of Isolation Of Elements

Question 1
CBSEENCH12009895

Which of the ores mentioned in Table 6.1. could be concentrated by magnetic separation method?
Table 6.1 Principal ores of some important metal.





Solution
The ore which can be attracted by the magnetic field can be concentrated by the process of magnetic separation. Among the ores mentioned in table 6.1, the ores of iron such as haematite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3), and iron pyrites (FeS2) can be separated by the process of magnetic separation. 
Question 2
CBSEENCH12009896

What is the significance of leaching in the extraction of aluminium?

Solution
Leaching is significant as it helps in removing the impurities like SiO2, Fe2O3 etc. from the bauxite ore to get concentrated pure alumina (Al2O3)
Question 3
CBSEENCH12009897

The reaction.
Cr2O3 + 2Al → Al2O3 + 2Cr                  (ΔG° = – 421 kJ)
is thermodyanmically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature?

Solution
The change in Gibbs energy is related to the equilibrium constant, K as
 
G = -RT lnK

At room temperature, all reactants and products of the given reaction are in the solid state. As a result, equilibrium does not exist between the reactants and the products. Hence, the reaction does not take place at room temperature.Certain amount of activation energy is essential even for such reactions which are thermodynamically feasible. 



Question 4
CBSEENCH12009898

Is it true that under certain conditions. Mg can reduce Al2O3 and Al can reduce MgO? What are those conditions?

Solution
Yes, below 1350°C Mg can reduce Al2Oand above 1350°C. Al can reduce MgO. This can be inferred from ΔG° Vs T plots.




Fig.  Gibbs energy (ΔG°) V T plots for formation of some oxides.

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