As already discussed, the reduction of the metallic oxide usually heating it with some reducing agent such as carbon, carbon monoxide, hydrogen or some other metal. The reducing agent joins with oxygen of the metal oxide.
M2O2 + yC xM +yCO Some metals oxides get reduced easily while others are reduced with difficulty. Some oxides are condensed at relatively low temperatures while others are reduced at relatively high temperatures. Thermodynamic considerations play an important role in deciding the temperature and the choice of reducing agent in the thermal reduction during metallurgy. For a spontaneous process, the changes in Gibbs energy ΔG must be negative. The changes in Gibbs energy ΔG for any process at any specific temperature, is described by the equation ΔG = ΔH - TΔS Where, ΔH is the enthalpy change T is the absolute temperature and ΔS is the entropy change for the process. For any reaction, this change could also be described from the equation: ΔG° = RTInK Where, K is the equilibrium constant of the 'reactant product' system at temperature T. a negative value of ΔG implies a positive value of K. Consider a reaction, the randomness of the system decreases because the gases have more randomness than solids. Hence, ΔS for this reaction is negative. Thus, if temperature is increased then TΔS becomes more negative. Since TΔS is subtracted in equation, ΔGbecomes less negative. On the other hand, if ΔS is positive, on increasing the temperature of ΔG decreased and becomes more negative. For example, in the reaction, 2C (s) + O2 (g) 2CO (g) ΔS is positive and ΔG decreases and becomes more negative as the T increases. If the reactants and products of two reactions are put together in a system and the net Gibbs free energy change, ΔG of the two possible reactions is negative, and then the overall reaction will occur. Thus, the process of interpretation of feasibility of a process involve coupling of the reactions, calculating the sum of their ΔG and then observing the magnitude and sign of ΔG. Such coupling is easily understood in the form of Ellingham diagram.