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.
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° = RTlnKeq
Keq 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.
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
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
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
Limitations of Ellingham Diagram
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
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.
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
Applications of Ellingham Diagram
Few applications of Ellingham diagram are listed below
Limitations of Ellingham Diagram
It has few limitations as well which are listed below