Thermodynamic Principle in Extraction of Metals
The free energy change ( ΔG ) occurring during the reduction processes help in deciding the suitable method for reduction .
For the spontaneous reduction of an oxide , halide or sulphide by an element , the essential condition is that there is a decrease in the free energy of the system ( -ve ΔG ).
More the negative value of ΔG , the higher is the reducing power of an element . ΔG can be given as
ΔG = ΔH – TΔS
where , ΔH = enthalpy change ; ΔG = Gibbs free energy T = temperature ; ΔS = entropy change
For the reduction of a metal oxide with a reducing agent , the plot of ΔG ° against temperature is studied , which is called Ellingbam diagram .