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Question
The value of `triangle_tG^theta` for formation of Cr2O3 is − 540 kJmol−1 and that of Al2 O3 is − 827 kJmol−1. Is the reduction of Cr2O3 possible with Al?
Solution
The value of `triangle_tG^theta` for the formation of Cr2O3 from Cr (−540 kJmol−1) is higher than that of Al2O3 from Al (−827 kJmol−1). Therefore, Al can reduce Cr2O3 to Cr. Hence, the reduction of Cr2O3 with Al is possible.
Alternatively,
`2Al +3/2 O_2 -> Al_2O_3 triangle_rG^theta = - 827 kJmol^(-)1`
`2Cr + 3/2O_2 ->Cr_2O_3 triangle_rG^theta = -540 kJmol^(-)1`
Subtracting equation (ii) from (i), we have
`2Al + Cr_2O_3 -> Al_2O_3 + 2Cr`
`triangle_rG^theta = -827 - (-540)`
`= -287 "kJ mol"^(-)1`
As `triangle_tG^theta` for the reduction reaction of Cr2O3 by Al is negative, this reaction is possible.
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