THE ELLINGHAM DIAGRAM

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Introduction : -

In 1994 H.G.T Ellingham proposed the Ellingham diagram to predict the spontaneity of reduction of various metal oxides. The Ellingham diagram is basically a curve which relates the Gibbs energy value with the temperature. Gibbs equation also known as free energy equation helps us to predict the spontaneity of a reaction on the basis of enthalpy and entropy values. It helps us to know the feasibility of a reaction under usual conditions. Gibbs energy is given as:

ΔG = ΔH – TΔS

Where ΔH is the change in enthalpy and ΔS is the change in entropy.

With respect to a reaction, Gibbs energy can be related to the equilibrium constant as:

ΔGo= – RTlnK

Where K is the equilibrium constant.

Gibbs free energy can also be considered as the energy left in the system to do work. If we freeze a reaction then, with the knowledge of gibbs energy we can predict the equilibrium status of the reaction. It also helps us to check whether an equilibrium shift in case of physical changes. 

Thus, when the reaction is exothermic and enthalpy of the system is negative, Gibbs free energy  becomes negative. Hence, we can say that the reaction shall proceed in the forward direction due to a positive value of the equilibrium constant. This law can be scaled for two different reactions taking place in a system too.  The overall reaction (combination of two reactions) will occur if and only if net ΔG (sum of ΔG’s of both the reactions) of the two possible reactions is negative.

 Principle : - 

The Ellingham diagram is a plot between ΔfGo and T for the formation of oxides of metals. A general reaction expressing oxidation is given by:

2xM(s) + O2(g) → 2MxO(s)

 

The Ellingham Diagram

As is evident from the reaction, the gaseous amount of reactant is decreasing from left to right as the product formed is solid metal oxide on the right side. Hence, we can say that molecular randomness is also decreasing from left to right. Thus, ΔS is negative and ΔG shifts towards the higher side despite rising T. Hence, for most of the reactions shown above for the formation of MxO (s), the curve is positive.

Except for the processes in which change of phase takes place, each plot is a straight line. This temperature at which change of phase takes place is indicated by a positive increase in the slope. For example, the melting is indicated by an abrupt change in the curve in Zn, ZnO plot.

The metal oxide (MxO) is stable at the point in a curve below which ΔG is negative. Above this point, the metal oxide is unstable and decomposes on its own.

Feasibility of reductions of the oxide of the upper line by the element represented by the lower line is determined by the difference in the two ΔrG0 values after the point of intersection in the Ellingham diagram.

Applications of Ellingham Diagram : -

The most important application of the Ellingham diagrams is in the extractive metallurgy industry. It helps to select the best reducing agent for various ores in the extraction process of various metals. It aids us to choose various feasible and inexpensive methods for isolation of the metals from ore. It also helps to guide the purification of metals, especially the removal of trace elements. The direct reduction process for making iron rests firmly on the guidance of Ellingham diagrams, which show that hydrogen by itself can reduce iron oxides to the metal.

Advantages and Disadvantages : -

By the use of Ellingham diagrams the commercial production of metals has increased drastically. Besides its metallurgical applications it is used for reference for displacement reactions in the lab. Irrespective of its numerous applications and advantages, its disadvantages count a little.

 Major disadvantage is that it is complex and congested for lab use. It ignores the reaction kinetics. It does not provide any information about the 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.


Blog Credits : - OJAS TUMBDE (SY Metallurgy and Materials Technology) MIS :- 112011069


References : -

  1. https://web.mit.edu/2.813/www/readings/Ellingham_diagrams.pdf

  2. https://www.researchgate.net/publication/47393782_Generalized_Ellingham_diagrams_for_utilization_in_solid_oxide_fuel_cells

  3. chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/viewer.html?pdfurl=https%3A%2F%2Fweb.mit.edu%2F2.813%2Fwww%2Freadings%2FEllingham_diagrams.pdf&clen=167568&chunk=true


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