What is Ellingham diagram?Explain briefly.
Ellingham plotted the experimentally determined standard free energy of
formation, ?G 0 , of various oxides using one mole of oxygen with temperature.
(2x/y) M(s) + O 2 (g) ------> (2/y) M x O y (s) ; ?G 0 =?H 0 -T?S 0
Ellingham pointed out that, the standard enthalpy and entropy of formation of a compound do not change significantly with the temperature as long as there is no change of state of product or reactant. ?G 0 T =?H 0 289 -T?S 0 298
Thus, the general forms of ?G 0 T ?S 0 relationships could be approximated to straight lines over temperature ranges
?G 0 = c + mx ; c =?H 0 289 m=-?S 0 298
The slope of free energy vs temperature plot would thus be (-?S 0 ) entropy change of the reaction and the intercept is the enthalpy value.
It provides a sound idea about selecting a reducing agent in the reduction of oxides. Such diagrams helps in predicting the feasibility of a thermal reduction of an ore. ?G must be negative at a given temperature for a reaction to be feasible.