Ellingam Diagrams. . . • An Ellingham diagram is a graph showing the temperature dependence of the stability for compounds. • This analysis is usually used to evaluate the ease of reduction of metal oxides and sulphides. • These diagrams were first constructed by Harold Ellingham in 1944. . which is fairly constant with temperature. Salient features of Ellingham Diagram Curves -metallic oxides are basically straight lines with a positive slope. the greater is the stability of its oxide Stability of metallic oxides decreases with increase in temperature. The slope is proportional to ΔS. . The lower the position of a metal's line in the Ellingham diagram. and the higher the temperature (above 700°C) the more effective a reductant (reducing agent) carbon is. carbon monoxide is the dominant oxide of carbon at higher temperatures (above about 700°C). (2CO=CO2+C) . while that of carbon monoxide (CO) has negative slope and crosses the CO2 line near 700°C. • According to the Boudouard reaction.• The formation free energy of carbon dioxide (CO2) is almost independent of temperature. . While the formation enthalpy of CO2 is higher than that of CO. the formation entropy is much lower. Consequently. the standard free energy of formation of CO2 from its component elements is almost constant and independent of the temperature. . while the free energy of formation of CO decreases with temperature. Al. (This reaction is employed in thermite. • For ex.) • The aluminium reduces the oxide of another metal. Al itself being oxidized to Al2O3. because aluminium forms stronger bonds with oxygen than iron: • Fe2O3 + 2 Al → 2 Fe + Al2O3(Metallothermic reduction reaction) • 2Fe2O3 + 3C → 4Fe + 3CO2(Carbothermic Reaction) .• A metal whose Gibbs free energy of formation is lower on the diagram at given “T”. can reduce FeO to Fe. most commonly iron oxide. will reduce an oxidized substance whose free energy of formation is higher on the diagram. . . The molten iron is the actual welding material. . The reactants are commonly powdered and mixed with a binder to keep the material solid and prevent separation. Commonly the reacting composition is five parts iron oxide red (rust) powder and three parts aluminium powder by weight. A strongly exothermic (heat-generating) reaction occurs that via reduction and oxidation produces a white hot mass of molten iron and a slag of refractory aluminium oxide. The products are aluminium oxide. so the set-up for welding must take into account that the actual molten metal is at the bottom of the crucible and covered by floating slag. ignited at high temperatures. free elemental iron.[3] and a large amount of heat. the aluminium oxide is much less dense than the liquid iron and so floats to the top of the reaction. At the point of intersection the free energy change for the reaction is zero. . below this “T”. while above the point of intersection the Gibbs energy is negative and the oxide can be reduced. The greater the gap between any two lines. the greater the effectiveness of the reducing agent corresponding to the lower line. it is positive and the metallic oxide is stable in the presence of the reductant. The intersection of two lines implies an oxidation- reduction equilibrium. at the required temperature. Richardson2 added a nomographic scale to the Ellingham diagram. ΔG = 0) through the point on the Ellingham line of interest. • The equilibrium partial pressure is found as follows: • A line is drawn from the origin of the graph (T = 0. The equilibrium partial pressure is read off at the point where the drawn line crosses the nomographic scale. Richardson Nomographic scale to Ellingham Diagram • To avoid calculating the equilibrium partial pressure for each value of ΔG°. . . • Use: At any temperature.Equilibrium Partial Pressure of Oxygen • The scale on the R.S is( Po2)is the equilibrium partial pressure of O2 that is equilibrium with the M and MO • Significance: • If the O2 pressure is higher than equilibrium O2 pressure then Metal will be oxidised.H. . EQM Partial- pressure can be found Directly. . Thus. . • Meatls and Mo only in Standard states. and ignores reaction kinetics. processes that are predicted to be favourable by the Ellingham diagram can still be slow. Disadvantages • The analysis is thermodynamic in nature.