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NCERT Class XII Chemistry
Chapter - General Principle of Process of Isolation of Elements
Questions with Solutions

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Question : 27
Total: 31
Predict conditions under which Al might be expected to reduce MgO.
Solution:  
The two equations are :
(a)
4
3
 Al+ O2 ––––––▶
2
3
 Al2O3
(b) 2Mg+ O2 ––––––▶2MgO
At the point of intersection of Al2O3 and MgO curves the ∆G° becomeszero for the reaction.
For the reduction of MgO by Al consider the reaction :
3MgO( s)+2Al( s)→3Mg(s,l,g)+Al2O3(s)
For this reaction the temperature should be chosen so that,
Δ,G0 ( Al2O3 )<ΔfG0( MgO)
Around 2000 K the line for Al2O3 lies below that of MgO, therefore, Al canreduce MgO. However, at a temperature below the intersection point oftwo lines MgO is more stable and cannot be reduced by Al.
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