Ellingham diagram helps us to select a suitable reducing agent and appropriate temperature range for reduction. The reduction of a metal oxide to its metal can be considered as a competition between the element used for reduction and the metal to combine with oxygen.
If the metal oxide is more stable, then oxygen remains with the metal and if the oxide of element used for reduction is more stable, then the oxygen from the metal oxide combines with elements used for the reduction. From the Ellingham diagram, we can infer the relative stability of different metal oxides at a given temperature.
1. Ellingham diagram for the formation of Ag2O and HgO is at upper part of the diagram and their decomposition temperatures are 600 and 700 K respectively. It indicates that these oxides are unstable at moderate temperatures and will decompose on heating even in the absence of a reducing agent.
2. 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 diagram. For example, in the Ellingham diagram, for the formation of chromium oxide lies above that of the aluminium, meaning that Al2O3 is more stable than Cr2O3 . Hence aluminium can be used as a reducing agent for the reduction of chromic oxide. However, it cannot be used to reduce the oxides of magnesium and calcium which occupy lower position than aluminium oxide.
3. The carbon line cuts across the lines of many metal oxides and hence it can reduce all those metal oxides at sufficiently high temperature. Let us analyse the thermodynamically favourable conditions for the reduction of iron oxide by carbon. Ellingham diagram. for the formation of FeO and CO intersects around 1000 K.
Below this temperature the carbon line lies above the iron line which indicates that FeO 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 reducing agent above this temperature. The following free energy calculation also confirm that the reduction is thermodynamically favoured.
From the Ellingham Diagram at 1500 K:
- 2Fe(s) + O2(g) → 2FeO(g)
- 2C(s) + O2(g) → 2CO(g)
- ∆G1 = – 350 kJ mol-1 …………(5)
- ∆G2 = – 480 kJ mol-1 ………….(6)
Reverse the reaction (1)
- 2FeO(s) → 2Fe(s) + O2(g)
- – ∆G1 = +350 kJ mol-1 ………. (7)
Now couple the reactions (2) and (3)
- 2FeO(s) + 2C 2Fe (l,s) + 2CO(g)
- ∆G3 = -130 kJ mol-1 ……………. (8)
The standard free energy change for the reduction of one mole of FeO is:
∆G3/2 = -65 kJ mol-1.
