Page 51 - Chemical equilibria Volume 4

Properties of States of Physico-Chemical Equilibrium 27
2.1.3. Influence of a pressure disturbance
Now, the temperature and the extent of the reaction are kept constant, and
we provoke a disturbance of the equilibrium by a slight variation in pressure,
which causes a variation of the affinity which is, by virtue of relation [1.39]:
d A =− Δ VP [2.6]
d
r
If we choose to increase pressure (dP > 0), it causes a decrease in
volume. Because the variation of the affinity has the opposite sign to that of
the volume, according to relation [2.6], we obtain:
'0
0
– if Δ V < , d A > 0 and therefore ℜ> ;
r
0
'0
– if Δ V > , d A < 0 and therefore ℜ< .
r
Thus, an increase in pressure will favor the displacement of the
equilibrium toward a new state of equilibrium, in the direction which is
accompanied by a negative associated volume of the reaction ( Δ V < ). If a
0
r
gas is present amongst the reagents and/or reaction products, the number of
gaseous moles decrease. In the case of condensed phases alone, the pressure
will have practically no influence.

2.1.4. Influence of the addition of a component

We now keep the temperature and pressure constant, as well as the
quantities of all the different components of a system except for one, because
the system is disturbed by adding dn i moles of component A i. The affinity of
the disturbed transformation will, in light of relation [1.23], be:
∑
d A =− ν k ∂μ k dn i [2.7]
k n ∂ i
By separating the contribution of the added component A i, this
expression can also be written as:

∑
d A =− ν k ∂μ k dn −ν i ∂μ i dn i [2.8]
i
ki ≠ n ∂ i n ∂ i