For the reduction of given metal oxide to metal, consideration of thermodynamic factors helps in selecting a suitable reducing agent. Ellingham diagrams (plots of ΔfGo vs T) can predict the feasibility of thermal reduction. Metals with a more negative standard free energy of formation of oxides can reduce the metal oxides with a less negative standard free energy of formation of respective oxides. Metal will reduce the oxides of other metals which lie above it in Ellingham diagram as the standard free energy change for combined redox reaction will be negative by an amount equal to the difference in ΔfGo of two metal oxides. Thus, both Al and Zn can reduce FeO to Fe but Fe cannot reduce Al2O3 to Al or ZnO to Zn. Also, C can reduce ZnO to Zn but not CO.