THERMODYNAMICS OF METALLURGY
The method employed for extracting a metal from its ores depends on the nature of the metal and that of the ore and may be related to the position of the metal in the electrochemical series. In general, metals with Eo < – 1.5 volt yield compounds which are very difficult to reduce and electricity is usually used for the isolation of such metals. One the other hand, noble metals with Eo > + 0.5 volt form easily reducible compound. A metal higher up in the electrochemical series should be more difficult to reduce to metallic form. As we move down, the reduction becomes more and more easy.
Standard electrode potential of a metal provides some idea regarding the selection of an appropriate method for extracting the metal from its compounds. However, the free energy changes (ΔG) occurring during the reduction processes are of more importance and help in deciding the suitable method.
In order that the reduction of an oxide, halide or sulphide or by an element may take place spontaneously at a given temperature and pressure, it is essential that there is a decrease in the free energy of the system (negative ΔG). As a matter of fact, the more the negative value of ΔG, the higher is the reducing power of an element.
The free energy change (ΔG) is related to the heat change (Δ) as well as to the product of temperature (T) and the entropy change by an expression
ΔG = ΔH – TΔS …… (1)
When all the reacting substances are at unit activity, ΔG = ΔGo (standard free energy change).
For a reaction such as the formation of an oxide,
2M + O2 → 2MO ……. (2)
ΔG becomes smaller with the increase in temperature. This is because the gaseous reactant oxygen is consumed in the reaction leading to the decrease in randomness or entropy (S) of the system and consequently ΔS becomes negative. With further increase in temperature, TΔS acquires more negative value. Since the term TΔS is subtracted from ΔH, ΔG will become increasingly less negative with increase in temperature.