6.21 The value of fGƟ for formation of Cr2 O3 is – 540 kJmol−1and that of Al2 O3 is – 827 kJmol−1. Is the reduction of Cr2 O3 possible with Al ?

The value of tGθ for the formation of Cr2O3 from Cr (−540 kJmol−1) is higher than that of AI2O3 from Al (−827 kJmol−1). Therefore, Al can reduce Cr2O3 to Cr. Hence, the reduction of Cr2O3 with Al is possible.


2AI + 32O2  AI2O3    tGθ = -827 kJmol-12Cr + 32O2  Cr2O3    tGθ = -540 kJmol-1

Subtracting equation (ii) from (i), we have

2AI + Cr2O3  AI2O3 + 2 Cr     tGθ = -827 - (-540)                                                                       = -287 kJmol-1

As tGθ for the reduction reaction of Cr2O3 by Al is negative, this reaction is possible.