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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 : 21
Total: 31
The value of ∆fG° for formation of Cr2O3 is – 540 kJ mol‌–1 and that of Al2O3 is – 827 kJ mol‌–1. Is the reduction of Cr2O3 possible with Al?
Solution:  
The two equations are :
4
3
 Al(s)+O2( g) ––––––▶
2
3
Al2O3(s); ΔfG Al,A2O30=−827 kJmol−1
4
3
 Cr(s)+O2( g)––––––▶
2
3
 Cr2O3(s);ΔfG0=−540 kJ mol−1
Subtracting equation (ii) from equation (i), we get
4
3
 Al( s)+
2
3
 Cr2O3( g)––––––▶
2
3
Al2O3(s)+
4
3
Cr(s);Δ,G0=−287 kJ mol−1
Since ∆fG° of the combined redox reaction is negative, therefore, reductionof Cr2O3 by Al is possible.
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