Ncert Exercises & Solutions

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^{θ}

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,

$2 A I + \frac{3}{2} O_{2} \to A I_{2} O_{3} ∆_{t} G^{θ} = - 827 k J m o l^{- 1}$
$2 C r + \frac{3}{2} O_{2} \to C r_{2} O_{3} ∆_{t} G^{θ} = - 540 k J m o l^{- 1}$

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

$2 A I + C r_{2} O_{3} \to A I_{2} O_{3} + 2 C r ∆_{t} G^{θ} = - 827 - (- 540)$
$= - 287 k J m o l^{- 1}$

As $∆_{t} G^{θ}$ for the reduction reaction of $C r_{2} O_{3}$ by Al is negative, this reaction is possible.