Page 27 - Chemical equilibria Volume 4

Physico-Chemical Transformations and Equilibria 3
Thus, we speak of the enthalpy, Gibbs energy, entropy, variations in
volume, etc. associated with the transformation.
1.1.3. Standard values associated with a transformation

We shall see that in the particular case where we choose pure components
in their phases, at a pressure of 1 bar, the standard value A associated with
the reaction [1R.1] is defined by the relation:

Δ A = ∑ ν i i o [1.2]
0
a
r
i
o
a is the standard value of the parameter A of the i, meaning that the
i
value of A taken in the pure state at a pressure of 1 bar at the temperature of
298 K (25ºC), the substance being in its normal state of condensation at that
temperature and pressure. We thus define a standard value at temperature T
as the above standard value chosen at the temperature T instead of 298 K.

1.1.4. Extent and rate of a transformation

A transformation is also characterized by its extent ξ at a time t and its
rate ℜ at that instant. The rate is the derivative in relation to time of the
extent. If n k is the number of moles of the component A k, we have:
dξ = 1 dn k [1.3a]
dt ν k dt

and:

dξ
ℜ= [1.3b]
dt

In the case where multiple transformations take place in the system,
equations [1.3a] and [1.3b] are replaced by:

dn
k ρ = ν ℜ [1.4a]
dt k ρ ρ