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Ellingham Diagram

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Ellingham Diagram - Lesson Summary

Thermodynamics useful to understand the variation in temperature required for the thermal reduction of oxides and to predict which element will suit as the reducing agent for a given metal oxide. The Gibbs energy is the most important thermodynamic term in metal extraction.

For a spontaneous reaction the change in the Gibbs energy, ∆G, must be negative. ∆G for any process at a given temperature is given by the equation

∆G = ∆H- T∆S

Where, ∆H = enthalpy change

T = absolute temperature

∆S = change in entropy during the reaction.

The change in the Gibbs energy when 1gram molecule of oxygen, sulphur (or) halogen is used to form oxides, sulphides (or) halides of metals plotted against temperature. This graphical representation is called an Ellingham diagram.

These plots are useful to determine the relative ease of reducing a given metal oxide to the metal and also to predict the feasibility of the thermal reduction of an ore.

An Ellingham diagram normally consists of plots of change in the Gibbs energy with temperature for the formation of oxides. An Ellingham diagram for oxides has several important features.

(i) The graphs for most metal to metal oxide reactions show a positive slope.

Ex: 2M + O2 →2MO.

In this reaction, the entropy (or) randomness decreases from left to right due to the consumption of gases. Hence, ∆S becomes negative. If the temperature is raised, then T ∆S becomes more negative. So, ∆G becomes less negative.

(ii) The Gibbs energy changes follow a straight line, unless the materials melt (or) vaporise. The temperature at which such a change occurs is indicated by an increase in the slope on the positive side.

(iii) When the temperature is raised, a point will be reached where the graph crosses the line "∆G is zero." Below this temperature, the free energy of formation of the oxide is negative, so the oxide is stable. Above this temperature, the free energy of formation of the oxide is positive the oxide becomes unstable and will decompose into the metal and dioxygen.

Any metal will reduce an oxide of another metal that lies above it in an Ellingham diagram. Ex: Al reduces FeO, CrO and NiO in termite reaction but Al will not reduce MgO at a temperature below 1500 0C.


Limitations:

The reactants and products are in equilibrium, which is not often true.

It does not explained about the rate of the reaction.

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