Application of Ellingham Diagram Chemistry Notes
Application of Ellingham Diagram :
Reduction of Haematite : Ellingham diagram can easily explain the reduction of haematite. In Ellingham diagram, there are three curves which illustrate the following reactions :
- Formation of ferric oxide from iron.
- Formation of CO from carbon.
- Formation of CO, from CO.
These three curves are represented in figure 6.6.
→ Three curves cross each other at 1073 K. Above 1073K temperature, the value of ∆G° for the formation of Fe, og is less negative than the value of ∆G° for the formation of CO from C. Thus above 1073 K temperature, carbon or coke can reduce the haematite because the (∆rG°) for the reaction becomes more negative.
→ Below 1073 K temperature, the value of ∆G° for the formation of CO2 is less negative than the value of ∆G° for the formation of Fe0g. Hence, the ∆G° for the reaction will become positive so below 1073 K temperature, the reduction of Fe2O3 by Cis not possible.
→ But in Ellingham diagram, we can observe that below 1073 K, the value of ∆G° for the formation of CO, from CO is more negative than the value of ∆G° for the formation of Fe2 O3 ∆G° (Fe → Fe2O3) > ∆G° (CO – CO. It means below 1073 K temperature, Fe 03 can be reduced by CO as(4,6°) will become negative for the reaction.
→ Thus, from the above disucssion it is clear that at the bottom of the furnace where the temperature is above 1073 K, the reduction of Fe2O3 is done by carbon or coke i.e., carbon will behave as reducing agent while at the top part of the furnace where the temperature is below 1073 K, the reduction of Fe2O3 is done by carbon mono oxide i.e., at the top CO will behave as reducing agent.
→ Reducing nature of Coke (Carbon) and Carbon monooxide
→ Reducing nature of Carbon (Coke): As it is well know that carbon acts as reducing agent. When it shows reduction then following reactions take place:
- C(s) + O2(g) → CO2(g)
- 2C(s) + O2(g) → 2CO(g)
- 2CO(g) + O2(g)→ 2CO2(g)
→ From above given reactions, it is clear that in case (a), there is no change in entropy i.e., ∆S° = 0 because the volume of O2 and CO2 gases are same. Therefore change in Gibb’s free energy (AGR) is also zero ie, it remains constant during the reaction. Now in case (b) there is increase in entropy i… ∆S° = +ve.
→ It means ∆G° also becomes more negative as carbon is involved in the reduction of metal oxide. But in case (c) the value of ∆S° becomes -ve as number of gaseous moles in reactants are larger than the number of gaseous moles in products. So, in this case, ∆G° becomes +ve. Thus from this discussion we can conclude that carbon will behave as reducing agent only when it changes into carbon monooxide during reduction process. It can also be understood with the help of Ellingham diagram given in fig. 6.7.
→ Thus, coke can act as reducing agent for the reduction of metal oxide into metal and CO gas is evolved. Some examples for the reduction of metal oxide by coke is as follows: