25.10: Reducing Agents
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A major application of the Ellingham diagram is the determination of the conditions required to reduce metal compounds, such as oxides or sulphides, to obtain pure metal. This is often the basis of extraction metallurgy, the extraction of metals from their ores. It is also very important in the recycling of metals. See the Ellingham Diagram section of Recycling of Metals TLP for more information.
A chemical is a reducing agent with respect to a particular metal when the free energy change for its oxidation is more negative than the free energy change of oxidation of the pure metal. This means that when the reducing agent is placed in a closed system containing the metal ore there is a driving force for the dissociation of metal and oxidiser (ie. oxygen or sulphur gas, for example).
Consider the two oxidation reactions below, whose lines on the Ellingham diagram cross each other:
2A + O2 = 2AO (a)
B + O2 = BO2. (b)
As we can see, the y-intercept, ΔH°(a) , of the first reaction is greater than the y-intercept of the second reaction, ΔH°(b) . Since the lines cross, the gradient of the second line, ΔS°(b) , is greater than that of the first.
At the point that the lines cross the standard free energy changes for two reactions are equal. This means that a closed system containing the metals A and B will be at equilibrium. This can be shown by considering the reaction below, obtained by subtracting reaction (a) from reaction (b):
B + 2AO = 2A + BO2
At T = TE , ΔG for this reaction is zero, and no reaction occurs. However above this temperature A is reduced by B, and below it B is reduced by A.
Drawing a line from the origin through the point at which the lines cross and extending it to the nomographic scale for oxygen pressure gives the partial pressure of oxygen at equilibrium.
Consider the reaction
3C + 2Fe2O3 = 3CO2 + 4Fe
ΔG is only negative for this reaction above T=1020K. Hence, steel furnaces operate above 1020K.