The decomposition of Fe₂O₃ is a non-spontaneous process

2 Fe₂O_{3 (S)} → 4 Fe _(S) + 3 O_{2 (g)} ∆_r GÙ’ = +1487 kJ mol^-1

Show that the reduction of Fe₂O₃ by CO can be made spontaneous by coupling with the reaction :

2CO_(g) + O_{2 (g)} → 2 CO_{2 (g)} ∆_r GÙ’ = -514 kJ mol^-1

Positive delta G of equation 1 means that it is non spontaneous

In equation 1 3 moles of oxygen is produced, therefore equation 2 can be modified as:

6CO + 3O2 → 6CO2 deltaG = -1543.2 KJ/mol

Now on adding equation 1 and 2 we get:

2Fe2O3 + 6CO → 4Fe + 3CO2 deltaG = -56.2 KJ/mol

Negative deltaG means that the reaction is now spontaneous.