Page 106 - Chemical equilibria Volume 4

82 Chemical Equilibria
Temperature T i is known as the temperature of inversion of
reaction [3R.17].
3.3.2.7. Ellingham diagram for a metal with multiple oxides – diagram
for iron
Certain metals are capable of forming several different oxides. A
particularly interesting case is that of iron oxides, with three compounds
elementary iron, iron monoxide FeO, and tri-iron tetroxide Fe 3O 4. Between
those components, we can write three reactions represented by an Ellingham
line:

2Fe + O 2 = 2FeO [3R.18]

6FeO + O 2 = 2Fe 3O 4 [3R.19]
3/2Fe + O 2 = 1/2Fe 3O 4 [3R.20]

As these three reactions are not independent, their standard Gibbs
energies are linked to one another by the relation:

1 3
Δ 0 Δ g = 0 Δ g + g 0 [3.56]
20
4 19 4 18
Figure 3.10(a) gives the representation of the three Ellingham lines,
which intersect at temperature T t = 825 K.

The straight line pertaining to equilibrium [3R.20] is situated between
those representing the equilibria [3R.18] and [3R.19].

Let us first place ourselves at a temperature lower than 825 K. By
consideration of the stable components in relation to each curve, we can
easily show that there is only compatibility between the curves if we
conserve that relating to the equilibrium [3R.20] (Figure 3.10(b)), and
therefore two zones of stability: one for metal iron (bottom) and the other for
tetroxide (bottom).

On the other hand, below the temperature of 825 K, it is easy to
demonstrate that we can only conserve (Figure 3.10(b)) the curves relative to
the equilibria [3R.20] and [3R.16], and therefore ultimately the three zones
of stability illustrated in Figure 3.10(c).