What chemical principle is involved in choosing a reducing agent for getting the metal from its oxide ore? Consider the metal oxides `Al_(2)O_(3) and Fe_(2)O_(3)` and justify and choice of reducing agent in each case.

Explanation

The chemical principle involved in the reducing of metal oxides to metals is that at any given temperature, any metal will reduce the oxide of other metals which lie above it in the Ellingham diagram. This is becuase the standard free energy change (`Delta_rG^(@)` ) of the combined redox reaction will be - ve by an amount equal to the difference in ` Delta_f G^(@) ` of the two metal oxides at that temperature. It is evident that the `Delta_fG^(@)`curve for the formation FeO `(or Fe_2O_3` not shown in the diagram ) lies above the ` Delta _fG^(@)` curve for the formation of `Al_2O_3`. Therefore, Al can reduce ` FeO or Fe_ 2 O _ 3 ` to
` 2Al + 2 FeO overset ( Delta ) to Al_2O_3+ 3Fe or " " 2 Al + Fe_2O_3 overset (Delta ) to Al_2 O_3 + 2 Fe `
Fe but the reverse is not true, i.e., Fe cannot reduce `Al_2 O_3 ` to ` Al`
` " "2 Fe + Al_2 O_3 to Fe_2O_3 + 2Al `
Above 1073 K, the C, CO curve lies below Fe, FeO curve, therefore, above 1073, C can reduces FeO to Fe.
` FeO + C overset ( gt 1073K) to Fe + CO `
In contrast, below 1073 K, the CO, `CO_2 ` cuve lies below Fe, FeO curve, therefore, below 1073 K, CO can reduce FeO to Fe.
` " " FeO + CO overset ( lt 1073 K ) to Fe + CO _ 2 `