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Ellingham Diagram

Last Updated : 15 Dec, 2022
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The Gibbs equation enables us to predict the spontaneity of a process based on enthalpy and entropy measurements. The Ellingham diagram was developed by H.G.T. Ellingham to predict the spontaneity of metal oxide reduction. One of the most straightforward graphical representations of Thermodynamic statements that exist in metal synthesis is the Ellingham diagram. In this article, you will study the reduction of several metals by selecting a suitable reducing agent using the Ellingham diagram. The characteristics, benefits, and drawbacks of the Ellingham diagram will also be familiar to you.

Thermodynamic Principles of Metallurgy

When it comes to metallurgy, Gibbs’s Free Energy is the most important thermodynamic notion to understand. Gibbs Free Energy determines whether a process will occur spontaneously or not in thermodynamics. The letter ΔG. If the value of ΔG is negative, the reaction will happen on its own. To arrive at ΔG, we’ll look at two equations.

ΔG = ΔH – TΔS

The change in enthalpy is denoted by ΔH. An endothermic reaction will be represented by a positive value, while a negative value will represent an exothermic reaction. As a result, when the reaction is exothermic, ΔG is negative. Entropy, or the unpredictability of molecules, is denoted by the letter ΔS. When the state of the matter changes, this changes dramatically. Another equation that connects Gibbs Free Energy and the equilibrium constant is

ΔG° = RTlnKeq

The equilibrium constant is Keq. The active mass of products is divided by the active mass of reactants to arrive at this figure. The universal gas component is R. The equilibrium value must now be kept positive in order to get a negative value of ΔG (which is desirable).

Also Read: Laws of Thermodynamics

Ellingham Diagram

An Ellingham diagram depicts the relationship between temperature and a compound’s stability. It’s a graphical illustration of the Gibbs Energy Flow. The Ellingham diagram is used in metallurgy to plot the reduction process equations. This help us in determining the best reducing agent to use when reducing oxides to produce pure metals. 

Ellingham Diagram represents the following important characteristics:

  1. ΔG is plotted in relation to temperature in this graph. The entropy is represented by the slope of the curve, whereas the enthalpy is represented by the intercept.
  2. As you may be aware, the ΔH (enthalpy) is unaffected by temperature.
  3. The temperature has no effect on ΔS, which is the entropy. However, there is a stipulation that no phase shift should occur.
  4. The temperature will be plotted on the Y-axis, while the ΔG will be plotted on the X-axis.
  5. Metals with curves near the bottom of the diagram are less common than metals found higher up.
Ellingham Diagram

 

The reaction of metal with air can be added up as follows:

M(s) + O2(g) → MO(s)

When it comes to reducing metal oxides, the ΔH is usually always negative (exothermic). ΔS is also negative because we are going from a gaseous to a solid state in the reaction (as seen above). As a result, as the temperature rises, the value of TΔS rises as well, and the reaction slope rises.

Read More: 

Observations from the Ellingham Diagram

  1. The slope is positive for the majority of metal oxide production. It is possible to explain it as follows. The creation of metal oxides consumes oxygen gas, resulting in a decrease in unpredictability. As a result, ΔS becomes negative, and the term TΔS in the straight-line equation becomes positive.
  2. Carbon monoxide formation is shown by a straight line with a negative slope. In this scenario, ΔS is positive because the consumption of one mole of oxygen gas results in two moles of CO gas. It implies that CO becomes more stable at higher temperatures.
  3. As the temperature rises, the ΔG value for the creation of metal oxide becomes less negative until it reaches zero at a certain point. Below this temperature, ΔG is negative and the oxide is stable; above this temperature, ΔG is positive and the oxide is unstable. This overall pattern shows that as temperatures rise, metal oxides become less stable and decompose more easily.
  4. Some metal oxides, such as MgO and HgO, have a sharp change in slope at a specific temperature. This is because of a phase shift (melting or evaporation).

Limitations of Ellingham Diagram

  1. Ellingham diagrams are built solely on thermodynamic considerations. It provides information on a reaction’s thermodynamic feasibility. It provides no information regarding the rate of the reaction. Furthermore, it provides no indication of the probability of other reactions occurring.
  2. It also lacks comprehensive information on the oxides and their formations. Let’s say there’s the possibility of more than one oxide. This scenario is not represented in the diagram.
  3. The interpretation of ΔG is predicated on the assumption that the reactants and products are in equilibrium, which is not necessarily true.

Significance of Ellingham Diagram

  1. On the graph, the Ellingham curve is lower than that of most other metals, such as iron. This effectively suggests that all of the metals above it in the graph can be utilized as a reducing agent for their oxides. 
  2. A blast furnace is used to separate iron from its oxide. In the furnace, the ore is mixed with coke and limestone. The reduction of iron oxides takes place at a variety of temperatures. The temperature in the bottom part of the furnace is substantially higher than in the top. Thermodynamics was used to explain the reactions, which led to the development of this technique.
  3. At the top of the diagram is the Ellingham diagram for the formation of Ag2O and HgO, with breakdown temperatures of 600 and 700 K, respectively. 
  4. The Ellingham diagram forecasts the thermodynamic feasibility of reducing oxides of one metal by oxides of another. Any metal can diminish the oxides of other metals in the figure above it. In the Ellingham figure, for example, the formation of chromium oxide is above that of aluminum, indicating that Al2O3 is more stable than Cr2O3
  5. Because the carbon line crosses the lines of many metal oxides, it can decrease all of them at sufficiently high temperatures. Let’s look at the thermodynamically favorable circumstances for iron oxide reduction by carbon. Around 1000 K, the Ellingham diagram for the production of FeO and CO intersects. 

FAQs on Ellingham Diagram

Question 1: According to Ellingham’s diagram which of the following given elements can be utilized to decrease Alumina?

Zinc, Magnesium, Iron or Copper.

Answer:

Magnesium can be used toreduce Alumina because according to Ellingham’s diagram the metal should be more reactive than Aluminium.

Question 2: Give the Significance of the Ellingham Diagram.

Answer:

  • The positive slope of metal oxides shows that their stabilities decrease with increase in temperature. The decrease in their stabilities is due to an increase in ΔG° value.
  • The sudden change in the graph shows a phase change, that is, change from solid to liquid or from liquid to vapour.
  • The negative slope of CO shows that it becomes more stable with increase in temperature (this is the opposite of that taking place in metal oxides).

Question 3: What is the Ellingham diagram?

Answer:

An Ellingham diagram depicts the relationship between temperature and a compound’s stability. It’s a graphical illustration of the Gibbs Energy Flow. The Ellingham diagram is used in metallurgy to plot the reduction process equations. This help us in determining the best reducing agent to use when reducing oxides to produce pure metals. 

Question 4: Write two Limitations of the Ellingham diagram.

Answer:

Following are the Limitations of Ellingham diagram:

  • Ellingham diagrams are built solely on thermodynamic considerations. It provides information on a reaction’s thermodynamic feasibility. It provides no information regarding the rate of the reaction. Furthermore, it provides no indication of the probability of other reactions occurring.
  • It also lacks comprehensive information on the oxides and their formations. Let’s say there’s the possibility of more than one oxide. This scenario is not represented in the diagram.

Question 5: How to draw an Ellingham diagram?

Answer:

The Gibbs energy change with temperature rise for oxide production is represented on the Ellingham diagram.



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