Explain how thermodynamics is helfpul in selection of reducing agent for metallurgical operations.   

Explanation

To understand the theory of metallurgical transformations, the Gibb.s free energy changes is the most significant term. For any reaction, the Gibb.s free energy change is given by :
`DeltaG = DeltaH - TDeltaS`
Where, `DeltaH` = Enthalpy change
T = Temperature in kelvin.
`DeltaS` = Entropy change for the process.
The criterion for the feasibility of a thermal reduction is that at a given temperature the Gibb.s free energy change for the reaction must be negative. When the value of `DeltaG` is negative only then the reaction will proceed. Under following conditions the value of `DeltaG` is negative.
(i) If `DeltaS` is positive on increasing the temperature (T) the value of `TDeltaS` increases so that `DeltaH < TDeltaS`. In this situation `DeltaG` will become negative on increasing temperature.
(ii) If coupling of the two reactions, i.e. reduction and oxidation results in negative value of `DeltaG` for overall reaction, the final reaction becomes feasible. Such coupling can be understood by studying plots of `(Delta_rG^(Theta))` v/s T for the formation of oxides. These plots are drawn for free energy changes when one gram mole of oxygen is consumed.
The graphical representation of Gibb.s free energy versus temperature was first used by H.J.T. Ellingham which provides the basis for considering choice of reducing agent in the reduction of oxides. This is known as Ellingham diagram. Such diagrams helps in predicting the feasibility of thermal reduction of an ore.