1. The conditions under which:
(a) Ellingham diagram is used to predict thermodynamic feasibility of reduction of oxides of one metal by another metal. Any metal can reduce the oxides of other metals that are located above it in the Ellingham diagram. In the Ellingham diagram, for the formation of magnesia (magnesium oxide) occupy lower position than aluminium oxide. Therefore aluminium cannot be used to reduce the oxides of magnesium (magnesia). Above 1623K, Al can reduce MgO to Mg, so that ArG° becomes negative and the process becomes thermodynamically feasible.
(b)
At the point of intersection of the Al2O3 and MgO curves in Ellingham diagram. ∆G°
becomes zero for the reaction:
Below that point magnesium can reduce alumina.
2. From the Ellingham diagram, we find that at 983 K, the curves intersect.
The value of ∆G° for change of C to CO2 is less than the value of ∆G° for change of CO to CO2 . Therefore, coke (C) is a better reducing agent than CO at 983 K or above temperature. However below this temperature. (e.g. at 673K), CO is more effective reducing agent than C
3. Yes, it is possible to reduce Fe2O3 by coke at a temperature around 1200 K. In the Ellingham diagram, carbon line cuts across the lines of many metal oxides and hence it can reduce all those metal oxides at sufficiently high temperature. Ellingham diagram for the formation of Fe2O3 and CO intersects around 1000 K.
Below this temperature, the carbon line lies above the iron line which indicates that Fe2O3 is more stable than CO and hence at this temperature range the reduction is not thermodynamically feasible. However above 1000 K carbon line lies below the iron line and hence we can use coke as a reducing agent around 1200 K. Around 1200 K, coke is better reducing agent because above 1000 K, Gibb’s free energy for the formation of Fe2O3 is more than the formation of CO2 from C.
