6.21 The value of
\Delta _ f G ^ \thetafor formation of Cr _ 2 O _3 is - 540 KJ mol ^{-1}and that of Al_2 O_3 \: \: is \: \: -827 KJ mol^{-1}. Is the reduction of Cr _ 2 O _3 possible with Al?

Answers (1)
M manish

 

2Al+3/2O_{2}\rightarrow Al_{2}O_{3}......................... (-827 KJ/mol)
2Cr+3/2O_{2}\rightarrow Cr_{2}O_{3}.........................  (-540 KJ/mol)
_________________________________          (On substracting )
2Al+Cr_{2}O_{3}\rightarrow Al_{2}O_{3}+2Cr      ...............(-287 KJ/mol)                                                       

In the above reaction, we see that \Delta_f G^0 of formation of Cr is negative (-287 KJ/mol) Hence the reduction of  Cr _ 2 O _3 with aluminium is possible.

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