The selection of reducing agent depends on the thermodynamic factor.Explain with an example.

Explanation

From the Ellingham diagram ,it is clear that metals for which the standard free energy of formation `(Delta_(f)G^(@))` of their oxides is more negative can reduce the metal for which the standard free energy of formation `(Delta_(f)G^(@))` of oxides is less negative.Thermodynamic factor has a major role in selecting the reducing agent for a particular reaction.Only that reagent will be preferred which will lead to decrease in the energy `(DeltaG^(@))` at a certain specific temprature.
E.g. Carbon reduce ZnO to Zn but not CO.
`ZnO+CtoZn+CO`
`ZnO+COtoZn+CO_(2)`
In the first case,there is increase in the magnitude of `DeltaS^(@)` while in the second case,it almost remains the same.In other words,`DeltaG^(@)` will have more negative value in the first case,when C is the reducing agent then in the second case when CO acts as the reducing agent.Therfore ,C is a better reducing agent.