Page 112 - Chemical equilibria Volume 4

88 Chemical Equilibria
If the solutions are perfect, the activities are replaced by
the molar fractions of the components in their respective solutions, and we
have:
⎛ P x ⎞
Δ g = Rln ⎜ O 0 2 × M ⎟ [3.65]
0
T
r
⎝ P x MO ⎠
P x
The oxygen pressure scale then gives us the product O 2 × M .
P 0 x
MO

3.4. Binary diagrams for chemical equilibrium

For the equilibria which involve a single phase with two components
(whether pure-phase or not), gaseous or liquid solution or solid, we use
diagrams with the same coordinates as the binary phase diagrams, meaning
that they give the temperature as a function of the composition of the two-
component phase (or vice versa). These are binary diagrams for chemical
equilibrium.

As an example, we shall examine the case of the Boudouard
equilibrium for reaction [3R.25] between monoxide, carbon dioxide and
carbon.

CO 2 + C = 2CO [3R.25]

At a given pressure, we can represent the composition of the gaseous
phase – e.g. the molar fraction of monoxide – as a function of the
temperature as a result of the three relations [3.66], [3.67] and [3.68]. The
second of these relations assumes the carbon to be in a pure phase:

P CO 2 + P = P [3.66]
CO

()
2
P CO / P CO 2 = K 25 P [3.67]
P
d ln K () = Δ h 0 [3.68]
25
25
dT RT 2