Ellingham Diagram and Gibbs Free Energy

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In this video we want to understand the relationship between the Ellingham diagram and Gibbs free energy.

The Ellingham diagram is a graph that plots the free energy change of a reaction as a function of temperature.

In A Level Chemistry syllabus we know that Gibbs free energy (ΔG) is related to enthalpy change (ΔH) and entropy change (ΔS) by the following expression:

We can rewrite this equation as a straight line equation (y = mx + c) with:

ΔG as the y coordinate
Temperature, T as the x coordinate
-ΔS as the gradient
ΔH as the y-intercept

With information about enthalpy change and entropy change, we can plot the Ellingham diagram.

Let's have 2 examples to illustate this.

Reaction 1

We can deduce the entropy change by comparing moles of gaseous reactants and products.

There is an increase in moles of gaseous species and increase in degree of disorder.

Hence ΔS is positive, and gradient -ΔS will be negative.

Therefore the Ellingham diagram for reaction 1 would be as follows:

Reaction 2

There is a decrease in moles of gaseous species and decrease in degree of disorder.

ΔS is negative and gradient -ΔS will be positive.

Therefore the Ellingham diagram for reaction 2 would be as follows:

Topic: Energetics, Physical Chemistry, A Level Chemistry, Singapore

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