Principles & applications of Ellingham diagram | Inorganic Chemistry PDF Download

Thermodynamics

There is an overlap between the study of physics and chemistry, known as Physical Chemistry. And here is where the concept of thermodynamics exits. Thermodynamics is the branch of science that deals with a relationship between thermal energy i.e. heat and other forms of energy.
Thermodynamics is the study of the energy transfer that occurs during chemical as well as physical changes. It also allows us to predict and measure these changes.

Thermodynamics in Metallurgy

The main thermodynamic concept we must concern ourselves with when it comes to metallurgy is Gibbs Free Energy. In thermodynamics, whether a process will happen spontaneously or not will be determined by Gibbs Free Energy. The symbol ΔG. If this value of ΔG is negative then the reaction will occur spontaneously. We will now look at two equations to arrive at ΔG
ΔG = ΔH – TΔS
ΔH is the change in enthalpy. Here a positive value will depict an endothermic reaction, while a negative value will be an exothermic reaction. So when the reaction is exothermic, it makes ΔG negative. ΔS is the Entropy or the randomness of molecules. This changes very sharply when the state of the matter changes. Another equation which relates the Gibbs Free Energy to the equilibrium constant is
ΔG° = RTlnK_eq
K_eq is the equilibrium constant. It is calculated by dividing the active mass of products by the active mass of reactants. R is the universal gas component. Now to attain a negative value of ΔG (which is desirable) the value of the equilibrium must be kept positive.

Ellingham Diagram

An Ellingham diagram shows the relation between temperature and the stability of a compound. It is basically a graphical representation of Gibbs Energy Flow.
In metallurgy, we make use of the Ellingham diagram to plot the reduction process equations. This helps us to find the most suitable reducing agent when we reduce oxides to give us pure metals. Let us take a look at some important properties of the Ellingham Diagram

• Here ΔG is plotted in relation to the temperature. The slope of the curve is the entropy and the intercept represents the enthalpy.
• As you know the ΔH (enthalpy) is not affected by the temperature
• Even ΔS that is the entropy is unaffected by the temperature. However, there is a condition here, that a phase change should not occur.
• We will plot the temperature on the Y-axis and the ΔG on the X axis
• Metals that have curves at the bottom of the diagram reduce the metals found more towards the top

The reaction of metal with air can be generally represented as
M (s) + O₂ (g) → MO (s)
Now when reducing metal oxides the ΔH is almost always negative (exothermic) reaction. Also since in the reaction (as seen above), we are going from the gaseous state to the solid state ΔS is also negative. Hence as the temperature increases, the value of TΔS will also increase, and the slope of the reaction goes upwards

Exceptions to Ellingham Diagram

There are cases when the entropy is not negative, and the slope will not be upwards. Let us take a look at few such examples

• C(s) + O₂ (g) → CO₂ (g): Entropy of solids is negligible. So here one molecule of gas is resulting in one molecule of gas. Hence there is almost no net entropy. So there will be no slope, it is completely horizontal.
• 2C (s)+ O₂ (g) → 2CO (g): Here one mole of gas is giving you two moles of gas as products. So here the entropy will be positive. And as a result, this curve will go downwards.

Limitations of Ellingham Diagram

• It does not consider the kinetics of the reactions.
• Also, it does not provide complete information about the oxides and their formations. Say for example more than one oxide is possible. The diagram gives us no representation of this scenario

Uses of Ellingham Diagram
1) Alumino Thermic Process: The Ellingham curve on the graph actually lies lower than most of the other metals such as iron. This essentially means Aluminium can be used as a reducing agent for oxides of all the metals that lie above it in the graph. Since aluminium oxide is more stable it is used in the extraction of chromium by a thermite process.
2) Extraction of Iron: Extraction of iron from its oxide is done in a blast furnace. Here the ore mixes with coke and limestone in the furnace. Actually, the reduction of the iron oxides happens at different temperatures. The lower part of the furnace is kept at a much higher temperature than the top. This process was developed after understanding the reactions with the help of thermodynamics. These reactions are as follows
At temperatures of 500-800 K
3Fe₂O₃ + CO → 2 Fe₃O₄ + CO₂
Fe₃O₄ + 4CO → 3Fe + 4 CO₂
Fe₂O₃ + CO → 2FeO + CO₂
At temperatures of 900-1500 K
C + CO2 → 2CO
FeO + CO → Fe + CO₂

Solved Question

Question: In which of the following pair of metals, both are commercially extracted from their respective ores by carbon reduction method?
(a) Zn, Cu
(b) Fe, Cu
(c) Sn, Zn
(d) Fe, Zn
Ans: The correct option is “C”. The oxide ores of Tin and Zinc are reduced with carbon to form metals. And so Tin and Zinc are commercially extracted from their respective ores by carbon reduction.

Ellingham Diagram

Salient features of Ellingham Diagram
Ellingham diagrams were 1^st 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.