Thermodynamical Principle of Metallurgy – Ellingham Diagram, Applications, Limitations, Practice Problems and FAQ

Have you ever taken part in a running race?

Everyone is aware that the winner will be the one who crosses the finish line first.

But what relevance does a running race have to this concept?

The race for reduction among the metal oxides is comparable to a competition between athletes. The metal with the lowest affinity for oxygen will be reduced more readily than the metal with the highest affinity.

But how do we know which metal has a higher affinity or lower affinity towards oxygen?

The answer to this intriguing question is the ‘Ellingham diagram’. Ellingham diagram is a plot between the change in free energy () and temperature () of oxides. When the plot of a particular metal oxide in the Ellingham diagram is below the other, the former is capable of reducing the later metal oxide into its corresponding metal. This phenomenon is used in the extraction of various metals.

Without any further ado, let’s get to know more about the Ellingham diagram and its applications!

TABLE OF CONTENTS

• Ellingham Diagram – Foundation
• Ellingham Diagram – Important Features
• Ellingham Diagram – Applications
• Ellingham Diagram – Limitations
• Practice Problems
• Frequently Asked Questions – FAQ

Ellingham Diagram – Foundation

For a spontaneous reaction, the change infree energy() should be negative. We know that,

Where, is the change in free energy

is the change in enthalpy during the reaction

is the absolute temperature

is the change in entropy during the reaction

Consider the following reaction.

Oxygen gas is used up in this reaction. We know that gases are the least ordered among the three states of matter. Therefore, gases have high entropy as compared to liquids and solids. In the reaction mentioned above, as oxygen gas is used up to form the metal oxide, the randomness or the entropy decreases. Therefore, is negative. Also the magnitude of decreases, but the rate of decrease of is less than that of , and at some point, both become equal and become zero.

As temperature () increases, becomes more negative. In the equation , is subtracted. Therefore, becomes less negative. Thus, the change in free energy increases with the increase in temperature.

Ellingham Diagram – Important Features

The change in free energy when one gram molecule of dioxygen is plotted against temperature for various metal reactions yielding their corresponding metal oxides. This plot is called the Ellingham diagram for oxides. It is a graph that provides the basis for considering the choice of reducing agents in reducing oxides. Similar plots can be drawn for sulphide and halides.

The important features of the Ellingham diagram for oxide are as follows.

1. Generally, it consists of the plots for the formation of oxides.

1. The slope of the graph for the formation of oxides ( is observed to be positive. further increases with increases in temperature.

1. Generally, free energy follows a straight line, except when the element melts or vaporises.

Example: line changes its slope at because at this temperature boils. Similarly, line changes its slope at because at this temperature boils.

1. There is a point in the curve of the graph above which metal oxides ( decompose on their own and below that point, . This implies that the metal oxide is stable below that point. This point is the point of intersection of two lines. Here, one metal is used to reduce the oxide of another metal at that particular temperature. So, any metal can reduce the oxide of another metal which lies above it in the Ellingham diagram. This is because the becomes more negative by an amount equal to the difference between the two graphs at a particular temperature.

At the point of intersection of the and curves (marked as “A” in the diagram given below), the becomes zero for the two reactions. The temperature at point A is. So, below , the curve of lies below the curve of . Therefore, below this temperature, cannot reduce , but above this temperature, the curve of shifts below the graph of . Therefore, above , can reduce .

Also, reduces the oxides of as it lies below all these metals. The reducing agent forms an oxide when a metal oxide is reduced.

Carbon reacts with dioxygen in two possible ways.

In the first reaction, the volume of carbon dioxide formed is equal to the volume of dioxygen reacting. Thus, the change in entropy () is very small and free energy () hardly changes with temperature (). Therefore, the plot of versus is almost parallel to the x-axis.

In the second reaction, the volume of carbon monoxide is twice the volume of dioxygen reacting. Thus, the change in entropy () is positive and free energy change () becomes more negative with the increase in temperature (). Therefore, the plot of versus slopes downwards.

The plot for both the reactions cross at about . Below this temperature, the formation of carbon dioxide is energetically favoured, and above this temperature, the formation of carbon monoxide is preferred.

The oxide of the metal decomposes as

………

Carbon as reducing agent:

The oxidation of is given as

………

Adding and ,

Here, (coke) act as a reducing agent.

On increasing the temperature, of formation of metal oxides become less negative; of reaction becomes more negative. So can be used to reduce any metal from its corresponding metal oxide.

Carbon monoxide as a reducing agent:

The oxidation of is given as:

………

Adding and ,

………

Here, will act as a reducing agent.

Ellingham Diagram – Applications

1. It is used to assess how easily metal oxides can be reduced.

1. In metallurgy, it is used in predicting the temperature at which metal, oxide, and oxygen are in equilibrium. Additionally, it helps in predicting how metals will react with nonmetals, sulfur, and nitrogen.

1. It helps in predicting the circumstances required for reducing a metal ore into its metal.

1. It is used to identify the appropriate reducing agent that will reduce metal oxides.

1. It is used to determine whether the thermal reduction of an ore is feasible.

1. The plot of aluminium lies below the metals like iron, tin and chromium. Therefore, aluminium can be used to reduce the oxides of the metals whose plot lies above the plot of aluminium. As aluminium oxide is more stable, it is used in the extraction of these metals.

1. Ellingham diagram is useful in theextraction of iron from iron oxide in a blast furnace. The blast furnace operates at two different temperatures wherein, the top of the blast furnace is at a lower temperature than the bottom. The following reactions take place during the extraction of iron from iron oxide.

At ,

At ,

Ellingham Diagram – Limitations

1. It does not give any insights into the reduction reaction's kinetics. In other words, it overlooks reaction kinetics.

1. The reactions predicted by the Ellingham diagram may occur slowly as it is only a thermodynamic analysis.

1. It considers the products and the reactants to be in equilibrium always, which is not the case.

Practice Problems

1. Which of the following metal pairs can be extracted from their ores using the carbon reduction method?

1. Cu, Zn
2. Cu, Fe
3. Zn, Sn
4. Zn, Fe

Answer:C

Solution: Zinc and tin are extracted commercially from their respective ores using the carbon reduction method.

The reaction used in the carbon reduction of zinc oxide is as follows.

The reaction used in the carbon reduction of tin oxide is as follows.

So, option C is the correct answer.

1. With respect to an ore, the Ellingham diagram helps to predict the feasibility of its

1. Electrolysis
2. Zone refining
3. Thermal reduction
4. Vapour phase refining

Answer: C

Solution: Ellingham diagram is the graph between and of various oxides. These help in predicting the feasibility of thermal reduction of ores.

So, option C is the correct answer

1. The correct statement regarding the given Ellingham diagram is:

1. At , can be used to extract from
2. At , can be used to extract from
3. At , coke can be used to extract from
4. Coke cannot be used to extract from

Answer: B

Solution: According to the given diagram, can reduce . This is because, in Ellingham diagram, the metal whose plot lies below can reduce the metal oxide whose plot lies above. As the plot of aluminium lies below the plot of zinc, aluminium can reduce zinc oxide to zinc.

At , can be used for the extraction of from

So, option B is the correct answer.

1. According to the following diagram, A reduces BO2 when the temperature is:

Answer: A

Solution: In the Ellingham diagram, the line of the element that lies below can reduce the oxide of the element which lies above it. Therefore, for A to reduce , the temperature at which the line for element A is below that of according to the graph is when T .

So, option A is the correct answer.

Frequently Asked Questions – FAQ

Q1. Many lines on the Ellingham diagram are similar, why?

Ans. The change in standard enthalpy of many reactions is comparable as one mole of gas is removed in most of the reactions. This is why many lines on the Ellingham diagram are similar.

Q2. What is the reason behind the change in the slope of lines in the Ellingham diagram?

Ans. This is because there is a large change in the entropy associated with the change in state which causes a change in the slope of formation of metal oxides from their corresponding metal.

Q3. What does the difference between two values signify?

Ans. It determines whether the reduction of an oxide of the upper line is feasible by an element present in the lower line. Larger the difference between the upper and lower lines, the easier the reduction.

Q4. What does a positive value of slope signify?

Ans. At high temperatures, of formation of metal oxide become positive, indicating that metal oxide has become unstable and can decompose into the corresponding metal and oxygen.