An ellingham diagram is a graph displaying the temperature dependence of the stability for compounds. this analysis is normally used to evaluate the benefit of reduction of metallic oxides and sulfides. those diagrams had been first built by way of harold ellingham in 1944. in metallurgy, the ellingham diagram is used to expect the equilibrium temperature among a metal, its oxide, and oxygen — and with the aid of extension, reactions of a metal with sulfur, nitrogen, and other non-metals. the diagrams are beneficial in predicting the conditions under which an ore could be reduced to its steel. the evaluation is thermodynamic in nature and ignores response kinetics. for this reason, strategies which might be predicted to be beneficial by way of the ellingham diagram can nevertheless be sluggish.
Ellingham diagrams are a selected graphical form of the precept that the thermodynamic feasibility of a reaction depends at the sign of δg, the gibbs free strength trade, that’s equal to δh − tδs, where δh is the enthalpy change and δs is the entropy change.
The ellingham diagram plots the gibbs unfastened power alternate (δg) for every oxidation reaction as a feature of temperature. for evaluation of different reactions, all values of δg refer to the reaction of the equal amount of oxygen, selected as one mole o (1⁄2 mol o
2) by means of some authors and one mole o
2 by using others. the diagram shown refers to one mole o
2, so that as an instance the line for the oxidation of chromium suggests δg for the reaction four⁄three cr(s) + o
2(g) → 2⁄three cr
Three(s), which is 2⁄3 of the molar gibbs electricity of formation δgf°(cr
Inside the temperature levels generally used, the metallic and the oxide are in a condensed state (solid or liquid), and oxygen is a gas with a far larger molar entropy. for the oxidation of every metallic, the dominant contribution to the entropy trade (δs) is the removal of 1⁄2 mol o
2, so that δs is terrible and more or less identical for all metals. the slope of the plots displaystyle ddelta g/dt=-delta s is consequently fantastic for all metals, with δg always turning into greater negative with decrease temperature, and the lines for all the steel oxides are approximately parallel. in view that those reactions are exothermic, they continually become feasible at decrease temperatures. at a sufficiently high temperature, the sign of δg may also invert (turning into nice) and the oxide can spontaneously reduce to the steel, as shown for Ag and cu.
For oxidation of carbon, the pink line is for the formation of co: c(s) + 1⁄2 o
2(g) → co(g) with an increase in the quantity of moles of fuel, leading to a tremendous δs and a terrible slope. the blue line for the formation of co
2 is about horizontal, for the reason that reaction c(s) + o
2(g) → co
2(g) leaves the number of moles of gas unchanged so that δs is small.
As with all chemical response prediction based totally on simply thermodynamic grounds, a spontaneous response may be very sluggish if one or greater stages inside the reaction pathway have very high activation energies ea.
If two metals are gift, two equilibria should be considered. the oxide with the greater terrible δg may be fashioned and the alternative oxide could be decreased.
Curves within the ellingham diagrams for the formation of steel oxides are basically immediately lines with a superb slope. the slope is proportional to δs, which within reason constant with temperature.
the lower the placement of a metal’s line within the ellingham diagram, the extra is the stableness of its oxide. for example, the line for al (oxidation of aluminium) is determined to be below that for fe (formation of fe
stability of steel oxides decreases with growth in temperature. highly volatile oxides like Ag
2o and hgo effortlessly undergo thermal decomposition.
the formation loose electricity of carbon dioxide (co
2) is sort of impartial of temperature, at the same time as that of carbon monoxide (co) has poor slope and crosses the co
2 line near seven hundred °c. in step with the boudouard reaction, carbon monoxide is the dominant oxide of carbon at higher temperatures (above about seven-hundred °c), and the higher the temperature (above seven-hundred °c) the more powerful a reductant (reducing agent) carbon is.
if the curves for two metals at a given temperature are as compared, the metal with the decrease gibbs unfastened electricity of oxidation on the diagram will lessen the oxide with the higher gibbs unfastened power of formation. as an instance, steel aluminium can lessen iron oxide to metal iron, the aluminium itself being oxidized to aluminium oxide. (this response is hired in thermite.)
the extra the space among any lines, the more the effectiveness of the reducing agent similar to the lower line.
the intersection of lines implies an oxidation-discount equilibrium. reduction the usage of a given reductant is viable at temperatures above the intersection point in which the δg line of that reductant is lower at the diagram than that of the metallic oxide to be decreased. at the point of intersection the free electricity alternate for the response is 0, beneath this temperature it is positive and the steel oxide is strong in the presence of the reductant, at the same time as above the point of intersection the gibbs electricity is poor and the oxide can be decreased.
In commercial tactics, the reduction of metal oxides is frequently effected by a carbothermic response, using carbon as a reducing agent. carbon is to be had cheaply as coal, which can be rendered to coke. whilst carbon reacts with oxygen it paperwork the gaseous oxides carbon monoxide and carbon dioxide, so the thermodynamics of its oxidation isn’t the same as that for metals: its oxidation has a extra bad δg with better temperatures (above seven-hundred °c). carbon can consequently serve as reducing agent. using this property, reduction of metals may be executed as a double redox reaction at rather low temperature.
Use of Ellingham diagrams
The principle software of ellingham diagrams is inside the extractive metallurgy enterprise, where it helps to pick out the satisfactory decreasing agent for diverse ores inside the extraction process, purification and grade placing for metallic manufacturing. it also enables to guide the purification of metals, mainly the removal of hint elements. the direct discount technique for making iron rests firmly at the steering of ellingham diagrams, which display that hydrogen can by myself reduce iron oxides to the steel.
Reducing agent for haematite
In iron ore smelting, haematite receives reduced on the top of the furnace, where temperature is inside the range 600 – 700 °c. the ellingham diagram indicates that in this variety carbon monoxide acts as a more potent decreasing agent than carbon for the reason that method
2 co + o
2 → 2 co
has a greater-bad unfastened strength exchange than the manner:
2 c + o
2 → 2 co.
In the higher a part of the blast furnace, haematite is reduced by using co (produced by way of oxidation of coke lower down, at higher temperatures) even within the presence of carbon – although that is in particular because the kinetics for gaseous co reacting with the ore are better.