Page 116 - Chemical equilibria Volume 4

92 Chemical Equilibria
f
()
For our reaction, we shall define its reaction quotient Q (we use Q
or
Q () obtained on the basis of the fugacities or partial pressures for mixtures
P
of gases) by:
a 3 β
Q = 3 [3.76]
1 β
aa 2 2 β
1
At equilibrium, this reaction coefficient is equal to the equilibrium
constant K r:
a 3 β
Q (equ) = 3 = K [3.77]
1 β
aa 2 2 β
1
If the polycomponent phase is a perfect solution, then at equilibrium we
have:

x 3 β
Q = 3 = K (I) [3.78]
(I)
1 β
xx 2 2 β
1
and if the components are perfect gases, using relation [3.10], then at
equilibrium we shall have:

P
x 3 β K ()
Q = 3 = K (I) = [3.79]
(I)
1 β
xx 2 β ∑ i β
1 2 ⎛ P ⎞
⎜ 0 ⎟
⎝ P ⎠
The term ∑ β is defined by the relation:
i
∑ β = β − (β + β 2 ) [3.80]
1
i
3
In the triangular diagram, we represent the “iso-parametric” or “iso-Q”
curves, which are not curves per se, formed of equilibrium points, but some
of them, at a given pressure, when relation [3.78] is respected, are the site of
points of composition at equilibrium at a certain temperature.

When we start with a certain composition of the reagents at the initial
time, with that composition being represented by a point M on the segment