Question

The value of Δ_fG° for formation of Cr₂O₃ is −540 kJ mol⁻¹ and that of Al₂O₃ is −827 kJ mol⁻¹. Is the reduction of Cr₂O₃ possible?

Answer 1

It is possible to reduce Cr₂O₃ using Al. The following explanation applies to this. The involved reactions can be written as follows.

\[\ce{2Al_{(s)} + \frac{3}{2} O2_{(g)} -> Al2O3_{(s)}}\]; Δ_fG° = −827 kJ mol⁻¹    ...(i)

\[\ce{2Cr_{(s)} + \frac{3}{2} O2_{(g)} -> Cr2O3_{(s)}}\]; Δ_fG° = −540 kJ mol⁻¹    ...(ii)

Subtracting eq. (ii) from eq. (i), we get

\[\ce{2Al_{(s)} + Cr2O3_{(s)} -> Al2O3_{(s)} + 2Cr_{(s)}}\];

Δ_fG° = −827 − (−540) = −287 kJ mol⁻¹

The process is feasible, meaning that Cr₂O₃ can be reduced with Al, because the Δ_fG° for the combined redox reaction is negative.