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 ${∆}_{t}{G}^{\theta }$ for the formation of $C{r}_{2}{O}_{3}$ from Cr (−540 kJmol−1) is higher than that of $A{I}_{2}{O}_{3}$ from Al (−827 kJmol−1). Therefore, Al can reduce $C{r}_{2}{O}_{3}$ to Cr. Hence, the reduction of $C{r}_{2}{O}_{3}$ with Al is possible.

Alternatively,

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

As ${∆}_{t}{G}^{\theta }$ for the reduction reaction of $C{r}_{2}{O}_{3}$ by Al is negative, this reaction is possible.