Thermodynamic principle of metallurgy: The extraction of metals from their oxides can be carried out by using different reducing agents. For example, consider the reduction of a metal oxide MxOy .
\(\frac{2}{y}\)MxOy(S) \(\frac{2x}{y}\)M(s) + O2(g) ......(1)
The above reduction may be carried out with carbon. In this case, the reducing agent carbon may be oxidised to either CO or CO2 .
- C + O2 → CO2(g) ……….(2)
- 2C + O2 → 2CO(g) …………(3)
If carbon monoxide is used as a reducing agent, it is oxidised to CO2 as follows,
2CO + O2 → 2CO2(g) ……………(4)
A suitable reducing agent is selected based on the thermodynamic considerations. We know that for a spontaneous reaction, the change in free energy (AG) should be negative. Therefore, thermodynamically, the reduction of metal oxide [equation (1)] with a given reducing agent [Equation (2), (3) or (4)] can occur if the free energy change for the coupled reaction. [Equations (1) & (2), (1) & (3) or (1) & (4)] is negative. Hence, the reducing agent is selected in such a way that it provides a large negative AG value for the coupled reaction.
Ellingham diagram:
The change in Gibbs free energy (∆G) for a reaction is given by the expression.
∆G = ∆H – T∆S ……….(1)
where, ∆H is the enthalpy change , T the temperature in kelvin and ∆S the entropy change. For an equilibrium process, ∆G° can be calculated using the equilibrium constant by the following expression ∆G° = – RT lnKp
Harold Ellingham used the above relationship to calculate the ∆G° values at various temperatures for the reduction of metal oxides by treating the reduction as an equilibrium process. He has drawn a plot by considering the temperature in the x-axis and the standard free energy change for the formation of metal oxide in y-axis. The resultant plot is a straight line with ∆S as slope and ∆H as y-intercept. The graphical representation of variation of the standard Gibbs free energy of reaction for the formation of various metal oxides with temperature is called Ellingham diagram.
