Page 100 - Chemical equilibria Volume 4

76 Chemical Equilibria
molecules, the standard entropy will be positive, and therefore the slope of
the line will be negative (Figure 3.6(b)).
0
If we look above the line, we have RlnT Q (I) > Δ g By virtue of the
.
r
same reasoning as above, the reaction from left to right is favored. We then
say that the part above the line is the domain of predominance of carbon
dioxide. On the other hand, below the curve is the domain of predominance
of carbon monoxide.
We speak of the domain of predominance because at every point of the
diagram, we have a mixture of the two gases, but each of them is present in a
greater quantity in its domain of predominance.

On the other hand, for carbon, above the curve the system contains
carbon, whilst below the curve the carbon will have completely disappeared,
so we say that above the curve is the domain of existence of carbon.

More generally, for the components belonging to a phase with multiple
components (mixtures of gases or solutions), the Ellingham curve delimits
domains of predominance, whereas for components in a pure phase, the
Ellingham curve delimits domains of existence.

3.3.2.3. Ellingham diagram for metal–oxygen–oxide systems
At the heart of it, the Ellingham diagrams were constructed and used for
the stability of metal oxides. In order to facilitate comparisons and
combinations of reactions, we tend to write the balance equations of those
reactions involving only one oxygen molecule, as in the following examples:

C + O 2 = CO 2 [3R.10]
2C + O 2 = 2CO [3R.11]

[3R.12]
4/3Al + O 2 = 2/3 Al 2O 3
In these conditions, if P is the oxygen pressure at equilibrium, the line
0
represents the quantity Δ g = Rlog P as a function of the temperature.
T
1
The slope of the line is positive or negative depending on the sign of the
standard entropy of the reaction, and in practice, depending on the creation
or consumption of gas molecules in the reaction. Thus, in reaction [3R.10],