Why is zinc not extracted from zinc oxide through reduction using CO?

The standard Gibbs free energy formation of ZnO from Zn is lower than that of CO2 from CO. That is why CO cannot reduce ZnO to Zn.  thus, Zn is not extracted from ZnO through reduction using CO.

16.The value of DfG0 for the 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     Δ fG0 for the formation of Cr2O3 is – 540 kJ mol-1 which are higher than that of Al2O3 is – 827 Kjmol-1. Thus, Al can reduce Cr2O3 to Cr. Hence, the reduction of Cr2O3 with Al is possible.