Question
Thu October 11, 2012 By: Drisya Ss

What is Ellingham diagram?Explain briefly.

Expert Reply
Fri October 12, 2012
An Ellingham diagram is a graph showing the temperature dependence of the stability for compounds.
Ellingham plotted the experimentally determined standard free energy of
formation, ?G , of various oxides using one mole of oxygen with temperature.
(2x/y) M(s) + O (g) ------> (2/y) M (s) ; ?G =?H -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 =?H 289 -T?S 298
Thus, the general forms of ?G –T ?S relationships could be approximated to straight lines over temperature ranges
?G = c + mx ; c =?H 289 m=-?S 298
The slope of free energy vs temperature plot would thus be (-?S ) 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.
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