Ellingham Diagram and Gibbs Free Energy

Side Note: Sorry about the room echo! Our new studio is still very very empty and the reverberation is very obvious :(

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.

ellingham diagram graph

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:

ellingham diagram equation

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

ellingham diagram 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.

ellingham diagram reaction 1 gradient negative

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

ellingham diagram reaction 1 graph

Reaction 2

ellingham diagram 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.

ellingham diagram reaction 2 gradient positive

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

ellingham diagram reaction 2 graph

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

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