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Ellingham Diagram is a diagram that is widely used in the field of metallurgy to understand the relation between Gibbs free energy and temperature for the formation of oxides of metals. It is a plot between ΔfGo and T. This diagram was brought up by Harold Ellingham in the year 1944, hence its name. It represents the change in Gibbs energy when 1 gram molecule of sulphur, oxygen or halogen is used in the formation of sulphides, oxides or halides of the metals against the temperature. This is helpful in the understanding of the ease with which a metal can be reduced.
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Keyterms: Ellingham Diagram, Metallurgy, Gibbs free energy, Oxides, Temperature, Metals, Sulphur, Oxygen, Halogen, Molecule, Nitrogen, Non-metals
Ellingham Diagram
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An Ellingham diagram is referred to as a graphical representation that shows the dependence of a compound stability on the temperature. The analysis done through the Ellingham diagram helps in determining the equilibrium temperature between the metal, its oxides and oxygen and through the process of extension helps in predicting the reaction between a metal with sulphur, nitrogen and other non-metals. The Ellingham diagram helps in determining the favourable conditions under which an ore will turn into its metal form.
Ellingham Diagram
The video below explains this:
Ellingham Diagram Detailed Video Explanation:
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Features of the Ellingham Diagram
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- It is a plot of ΔfGo in kJ/mol of oxygen versus the temperature for the formation of oxides.
- The graphs of metal to metal oxide reactions for a majority of metals shows a positive slope.
- The line for the change of Gibbs energy is a straight line unless the substance does not change its state to liquid or gas.
- When the substance changes its state, an increase in the slope on the positive side is observed.
- According to the Ellingham diagram on increasing the temperature, after a certain point, ΔG will be zero.
- Beyond this temperature, the free energy for the formation of the oxide is positive. This leads to the oxide being unstable which results in its decomposition into its respective metal and oxygen.
- For temperatures up to this temperature, the free energy for the formation of oxides is negative. This leads to the oxide being stable.
Observations from the Ellingham Diagram
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- Any metal that lies above another metal in the Ellingham diagram has the ability to reduce it.
- For a majority of metals, the slope for the formation of their respective oxide is found to be positive.
- Some metal oxides like HgO, MgO show a drastic change in their slope at a given temperature.
Exceptions to the Ellingham Diagram
Some cases can be observed for which when the entropy is positive, the slope for the substances is downwards. Few examples of such exceptions are:
- C (s) + O2 (g) → CO2 (g): In this reaction, a single molecule is leading to the formation of another single molecule of gas. Hence, the entropy is null and the slope is zero which is represented by a horizontal line.
- 2C (s) + O2 (g) → 2CO (g): In this reaction, a single molecule is leading to the formation of two molecules of gas. Hence, the entropy is positive resulting in a curve that goes downwards.
Applications of the Ellingham Diagram
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- Ellingham Diagram facilitates a more efficient selection of the best reducing agent for a range of ores.
- It provides a reference on how to best purify the metals and for the removal of impurities.
- By referring to the Ellingham curve for Aluminum, it was inferred that it is an efficient reducing agent in the thermite process, aiding in the extraction of Chromium and Manganese through the reduction of their respective oxides.
- The Ellingham Diagram is used to identify the point of equilibrium of the oxidation-reduction process.
Limitations of the Ellingham Diagram
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- The kinetics of the reactions are not taken into consideration in the Ellingham Diagram.
- The diagram fails to provide complete information regarding the oxides and their formations. For instance, if more than 1 oxide of a metal can be formed, the information for the same through the Ellingham diagram cannot be attained.
Things to Remember
- The Ellingham Diagram is a graph of Gibbs energy (ΔG) versus Temperature (T) for the formation of oxide of a metal.
- The diagram represents how the stability of compounds is dependent on the temperature.
- It is widely used in the identification of the most suitable reducing agent for the reduction of metal ores.
- It helps to predict reactions of metals with non-metals such as nitrogen and sulphur.
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Sample Questions
Ques. Which thermodynamic quantity is shown as a function of temperature in the Ellingham Diagram? [3 marks]
Ans. The Gibbs energy change (ΔG) of a reaction is charted against a range of temperatures to show a near accurate representation on the feasibility of thermodynamic reactions. The change in the Gibbs energy is given by the equation:
ΔG = ΔH – TΔS
Where, ΔH = enthalpy change
T = absolute temperature
ΔS = change in entropy during the reaction.
Ques. As observed in the Ellingham diagram, why are slopes of a majority of lines found to be identical? [3 marks]
Ans. As most reactions represented in the Ellingham Diagram involve the formation of a solid product while eliminating gas and it is known that the entropy of gases is higher than that of solids while within different gases, the entropy is observed to be similar. Therefore, most reactions are found to have a similar change in entropy which can be observed through identical slopes.
Ques. Which reducing agent is the most efficient in the extraction of iron from hematite ores? [3 marks]
Ans. According to the Ellingham diagram, carbon mono-oxide is the most efficient reducing agent since its reaction has a less negative free energy change than dioxide formation.
In the blast furnace, hematite is reduced by a mono-oxide mainly because the kinetics for its gaseous form react better with the ore.
Ques. Which pair of metals are commercially extracted from their ores using carbon reduction? [2 marks]
Ans. The Ellingham Diagram can be used to infer that the ores of Tin and Zinc are reduced using carbon extract, a purer form of metals. Thus, their oxide ores are reduced and we are able to commercially extract Tin and Zinc with this process.
Ques. Which calcined ores can be reduced using carbon as the reducing agent? [1 mark]
Ans. Hematite (Fe2O3) and Zincite (ZnO) can be reduced by carbon.
Ques. Given that the Gibbs free energy for CaO is – 604.2 kJ/mol and the free energy for Al2O3 is – 1582 kJ/mol, is the below reaction feasible and why?
3Ca + Al2O3 → 2Al + 3CaO [4 marks]
Ans. 3Ca + Al2O3 → 2Al + 3CaO
ΔG = 3 X ΔG(CaO) – ΔG(Al2O3)
= 3 X (-604.2) – (-1582)
= -1812.6 + 1582
= -230.6 kJ
This shows that ΔG is negative. Therefore, this reaction is feasible.
Ques. What thermodynamic quantity does the intercept at T=0 K for any standard free energy vs T line signify? [3 marks]
Ans. As the Ellingham diagram is plotted between ΔG and T, the intercept will exhibit the properties of ΔG at corresponding temperatures. It is known that ΔG = ΔH – TΔS and thus at T=0, ΔG = ΔH and so at that point the intercept will represent the Standard Enthalpy Change i.e., ΔH.
Ques. In the Ellingham Diagram, why do the metals found at the lower end of the graph reduce metals and their oxides found at the top of the diagram? [3 marks]
Ans. The stability of Metal Oxides (MxO) slowly deteriorates after reaching a point in the curve where ΔG is negative and therefore, the metal oxides that are found at the top have a higher chance to be unstable and decompose on their own making them susceptible to being reduced by metals found in lower part of diagram.
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