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Question

The standard Gibbs free energies for the reactions at 1773K are given below:
C(s)+O2(g)CO2(g);ΔGo=380 kJmol1
C(s)+O2(g)2CO(g);ΔGo=500 kJmol1
Discuss the probability of reducing Al2O3 and PbO with carbon at this temperature,
4Al+3O2(g)2Al2O3(s);ΔGo=22500 kJmol1
2Pb+O2(g)2PbO(s);ΔGo=120 kJmol1

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Solution

Let us consider the reduction of Al2O3 by carbon:
2Al2O3(s)+3C(s)4Al(s)+3CO2(g);ΔGo=380×3+22500=+21360kJ
2Al2O3(s)+6C(s)4Al(s)+6CO(g);ΔGo=500×3+22500=+21000kJ
Positive values of ΔGo show that the reduction of Al2O3 is not possible by any of the above methods.
Now let us consider the reduction of PbO
PbO(s)+C2Pb+CO2(g);ΔGo=+120+(380)=260kJ
2PbO(s)+C2Pb+CO(g);ΔGo=+120+(500)=380kJ
Negative value of ΔGo shows that the process is spontaneous and PbO can be reduced by carbon.

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