Why suitable reducing agents are required for reduction of metal oxides? What are the guidelines to choose a particular reducing agent?
Reducing agents are necessary because only they can reduce the metal oxide. If any chemical is an electron donor reducing agent, another must be an electron recipient oxidizing agent (metals). A reducing agent is oxidized because it loses electrons in the redox reaction and gives that electrons to metal to fill its valence shell and makes it neutral.
The thermodynamic factor helps us in choosing a suitable reducing agent for the reduction of particular metal oxide to metal. The feasibility of thermal reduction can be predicted on the basis of ∆G0 Vs T plots for the formation of oxides, known as Ellingham diagram. From the diagram it can be predicted that metals for which the ΔfG0 (of their oxides) is more negative can reduce those metal oxides for which standard free energy of formation of their respective oxides is less negative .
In other words, a metal will reduce the oxides of the other metals which lie above it in the Ellingham Diagram because the ΔfG0 of the combined redox reaction will be negative by an amount equal to the difference in ΔfG0 of the two metal oxides.