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This section is about using Gibb’s free energy to do calculations involving equilibria, so make sure you’re comfortable with this first!

Gibb’s free energy of A, *G*_{A}, is:

For the whole reaction, the total change in *G* is:

In line 3, we assume n = 1. In line 4, we combine the partial pressures using the rules of logs.

At equilibrium *ΔG* is zero, which gives:

This is a very important equation. It has another form:

Combining this with another equation for Gibb’s free energy:

we get:

and this can be plotted on a graph.

Back to **Contents: Chemistry: Equilibria**

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