Page 107 - Chemical equilibria Volume 4

Molecular Chemical Equilibria 83

Figure 3.10. Diagrams of the stability of iron oxides

The temperature of 825 K is called the transition temperature.

NOTE 3.8.– there is a fourth possible reaction between the same three
components, which is the dismutation of monoxide, as follows:

4FeO = Fe + Fe 3O 4 [3R.21]
For this reaction, the standard Gibbs energy is easily calculated on the
basis of the relation:

1
0
Δ 0 (Δ g = g − Δ g 0 ) [3.57]
21
2 19 18
Thus, depending on the temperature, we distinguish:

0
0
– if T = T t, we have: A =− Δ g = , and reaction [3R.21] is at
21 21
equilibrium;
0
0
– if T > T t, we have: A 21 =− Δ g < , the oxide FeO cannot exist, so and
21
reaction [3R.21] is shifted entirely to the right;
0
– if T <T t, we have: A 21 =− Δ g > , then the metal iron cannot exist, so
0
21
the reaction [3R.21] is shifted entirely to the left.
NOTE 3.9.– we note that the above reasoning hardly applies at all to iron
sesquioxide Fe 2O 3. Indeed, that compound is not stable in the presence of
iron or of one of the other oxides, and does not reach equilibrium.