Department of Chemistry

Teacher: Juli Nanda Goswami

Class: Semester-IV

Paper: C9T

Unit: General principles of Metallurgy

Module 2: i)Chief mode of occurrence of metals based on standard electrode potential.

2.Ellingham diagram for reduction of metal oxides using carbon and CO as reducing agent

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BELDA COLLEGE

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i)Electrochemical Series

ii) Reduction Potential & Extraction Methods

iii) Ellingham Diagram

2. Extractive techniques
3. Metals and their ores
4. Metals and its extracting techniques
5. Pyro-metallurgy

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CONTENTS

���i)Electrochemical Series���

• The standard reduction potentials of a large number of electrodes have been measured using standard hydrogen electrode as the reference electrode.
• The arrangement of elements in order of increasing reduction potential values is called electrochemical series. It is also called activity series, of some typical electrodes.

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�Reduction Potential & Extraction Methods

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Electrolysis of fused salts, usually chlorides

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Electrolysis of MgCl₂ High temp. reduction with Carbon

Electrolysis of Al₂O₃dissolved in Na₃[AlF₆]

Chemical reduction of oxides with Carbon

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Found as native metal or compounds easily decomposed by heat.

Ellingham Diagram

• Gibbs equation helps us to predict the spontaneity of a reaction on the basis of enthalpy and entropy values directly.

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• H.G.T Ellingham proposed the Ellingham diagram to predict the spontaneity of reduction of various metal oxides. Ellingham diagram was basically a curve which related the Gibbs energy value with the temperature.

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• Gibbs energy is given as: ΔG = ΔH – TΔS Where ΔH is the change in enthalpy and ΔS is the change in entropy. Thus, when the reaction is exothermic, enthalpy of the system is negative, thus making Gibbs free energy negative.

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• Hence, we can say that the reaction will proceed in the forward direction due to a positive value of the equilibrium constant.

Ellingham Diagram

• A general reaction expressing oxidation is given by:

2M(s) + O2(g) → 2MO(s)

ΔG = ΔH – TΔS

As is evident from the reaction, the gaseous amount of reactant is decreasing from left to right as the product formed is solid metal oxide on the right side. Hence, we can say that molecular randomness is also decreasing. Thus, ΔS is negative and ΔG shifts towards higher side despite rising T. Hence, for most of the reactions shown above for the formation of M_xO (s), the curve is positive. The metal oxide (M_xO) is stable at the point in a curve below which ΔG is negative. Above this point, the metal oxide is unstable and decomposes on its own.

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Ellingham diagram for Reduction of Oxides

Oxidation of Carbon: An Exception

• The majority of the lines slope upwards, because both the metal and the oxide are present as condensed phases (solid or liquid). The reactions are therefore reacting a gas with a condensed phase to make another condensed phase, which reduces the entropy.
• A notable exception to this is the oxidation of solid carbon. The line for the reaction C+O₂ → CO₂ is a solid reacting with a mole of gas to produce a mole of gas, and so there is little change in entropy and the line is nearly horizontal.
• For the reaction 2C+O₂ → 2CO we have a solid reacting with a gas to produce two moles of gas, and so there is a substantial increase in entropy and the line slopes rather sharply downward.

Ease of Reduction

• The position of the line for a given reaction on the Ellingham diagram shows the stability of the oxide as a function of temperature.
• Reactions closer to the top of the diagram are the most “noble” metals (for example, silver), and their oxides are unstable and easily reduced.
• As we move down toward the bottom of the diagram, the metals become progressively more reactive and their oxides become harder to reduce.

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• A given metal can reduce the oxides of all other metals whose lines lie above theirs on the diagram.
• For example, the 2Mg + O₂ → 2MgO line lies below the Ti + O₂ → TiO₂ line, and so magnesium can reduce titanium oxide to metallic titanium.
• Since the 2C + O₂ → 2CO line is downward-sloping, it cuts across the lines for many of the other metals. This makes carbon unusually useful as a reducing agent, because as soon as the carbon oxidation line goes below a metal oxidation line, the carbon can then reduce the metal oxide to metal.
• So, for example, solid carbon can reduce chromium oxide once the temperature exceeds approximately 1225°C, and can even reduce highly-stable compounds like silicon dioxide and titanium dioxide at temperatures above about 1620°C and 1650°C, respectively.

CrO₂+2C → Cr+ 2CO

2C + SiO₂ → Si+ 2CO

Applications of the Ellingham diagram

• Ellingham diagram helps us to select a suitable reducing agent and appropriate temperature range for reduction.
• The reduction of a metal oxide to its metal can be considered as a competition between the element used for reduction and the metal to combine with oxygen. If the metal oxide is more stable, then oxygen remains with the metal and if the oxide of element used for reduction is more stable, then the oxygen from the metal oxide combines with elements used for the reduction.
• From the Ellingham diagram, we can infer the relative stability of different metal oxides at a given temperature.

1. Ellingham diagram for the formation of Ag2O and HgO is at upper part of the diagram and their decomposition temperatures are 600 and 700 K respectively. It indicates that these oxides are unstable at moderate temperatures and will decompose on heating even in the absence of a reducing agent.

2. Ellingham diagram is used to predict thermodynamic feasibility of reduction of oxides of one metal by another metal. Any metal can reduce the oxides of other metals that are located above it in the diagram.

For example, in the Ellingham diagram, for the formation of chromium oxide lies above that of the aluminium, meaning that Al₂O₃ is more stable than Cr₂O₃. Hence aluminium can be used as a reducing agent for the reduction of chromic oxide.

However, it cannot be used to reduce the oxides of magnesium and calcium which occupy lower position than aluminium oxide.

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Applications of the Ellingham diagram �

• The carbon line cuts across the lines of many metal oxides and hence it can reduce all those metal oxides at sufficiently high temperature.
• Let us analyse the thermodynamically favourable conditions for the reduction of iron oxide by carbon. Ellingham diagram for the formation of FeO and CO intersects around 1000 K. Below this temperature the carbon line lies above the iron line which indicates that FeO is more stable than CO and hence at this temperature range, the reduction is not thermodynamically feasible.
• However, above 1000 K carbon line lies below the iron line and hence, we can use coke as reducing agent above this temperature. The following free energy calculation also confirm that the reduction is thermodynamically favoured.

Limitations of Ellingham diagram

1. Ellingham diagram is constructed based only on thermodynamic considerations. It gives information about the thermodynamic feasibility of a reaction. It does not tell anything about the rate of the reaction. More over, it does not give any idea about the possibility of other reactions that might be taking place.

2. The interpretation of ΔG is based on the assumption that the reactants are in equilibrium with the products which is not always true