Ellingham Diagram

Ellingham diagram is a plot between Gibbs free energy and temperature. It helps us to know the best reducing agent for a particular metal oxide and sulfides. As we know – 

G° = H°-T.∆S°

If  G°<0, then reaction will be spontaneous.

If G°=0, the reaction will be at equilibrium 

If G°>0, then the reaction will be non-spontaneous. 

In metallurgy, we want metal oxides and sulfides to get reduced to get pure metal at low cost. For this purpose, metal oxides and sulfides are reduced by best suitable reducing agents through spontaneous reactions. In this Ellingham diagram plays an important role to select a best suitable reducing agent. Thus, in a way Ellingham diagram relates thermodynamics and metallurgy. 

Suppose for a reaction A + B 🡪 C, (image will be updated soon)

Values above the G°=0 will be positive while below the G°=0 will be negative. General representation is shown below for your better understanding – (image will be updated soon)

If G°<0 for A + B 🡪 C then this reaction will be spontaneous. 

If we reverse the reaction –

C 🡪 A + B, then G°>0 so the reaction will be non-spontaneous. 

Now if G°>0 for A + B 🡪 C then this reaction will be non – spontaneous.

If we reverse the reaction –

C 🡪 A + B, then G°<0 so the reaction will be spontaneous. 

For example, suppose a graph is given for following reactions of metal oxides –

2Ag2O ∆→ 4Ag + O2

2HgO ∆→ 2Hg + O2

2ZnO ∆→ 2Zn + O(image will be updated soon)

As you can see in the graph that silver oxide and mercuric oxide need lower temperature to get reduced while zinc oxide require higher temperature. Providing higher temperatures for reduction of metal oxides is an expensive technique for industries to get metals from their oxides. So, for those metal oxides which require high temperature for reduction are reduced by suitable reducing agents. Now here Ellingham diagram comes in picture as it gives the information about the best suitable reducing agent for a specific metal oxide. Zinc oxide is reduced by carbon. (image will be updated soon)

For C🡪 CO2, G°=0 so reaction will be at equilibrium. 

For CO 🡪 CO2, G°> 0 so the reaction will be non-spontaneous. 

For C🡪 CO, G°<0 so the reaction will be spontaneous. 

So, we use carbon as a reducing agent for zinc oxide. Reaction is given below –

ZnO + C 🡪 Zn + CO

Thus, we can say for spontaneous reaction graph between G° and T will be – (image will be updated soon)

For non-spontaneous reaction graph between G° and T will be – (image will be updated soon)

Salient features of Ellingham Diagram 

Ellingham diagrams were 1st constructed by Harold Ellingham in 1944. (image will be updated soon)

 Following are the Salient Features of it –

  • It is a plot of G°in kJ/mol of oxygen and temperature of formation of oxide.

  • As G° becomes less negative at high temperatures so each line of converting metals into metal oxide slope upwards. 

  • Each plot line is straight line except some lines where the change in phase such as solid 🡪 liquid or Liquid 🡪 Gas etc. takes place. 

  • With increase in temperature slope lines cross G°= 0 which means for them G°>0. Theoretically this happens in case of mercury, silver and gold. 

Applications of Ellingham Diagram

Few applications of Ellingham diagram are listed below –

  • It is used to evaluate the ease of reduction of metal oxides and sulfides. 

  • In metallurgy it is used to predict the equilibrium temperature between metal, oxide and oxygen. It also predicts the reaction of metals with nitrogen, sulfur and nonmetals.

  • By Ellingham diagram we can predict the condition under which an ore can be reduced to its metal.

  • It is used for finding the best suitable reducing agent for reduction of metal oxides.

  • It is used to find out the feasibility of thermal reduction of an ore.

  • As we know, the Ellingham curve for aluminium lies below most metals such as Fe, Cr etc. which indicates that Al can be used as the reducing agent for oxides of all these metals. 

Limitations of Ellingham Diagram 

It has few limitations as well which are listed below –

  • It ignores the reaction kinetics means it does not provide any information about kinetics of the reduction reaction.

  • The analysis is thermodynamic in nature, it means the reactions which are predicted by the Ellingham diagram can be very slow. 

  • It assumes that the reactants and products are in equilibrium, but it is not always the case.

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