Page 130 - Chemical equilibria Volume 4

106 Chemical Equilibria
either the standard state, in which the different compounds are pure at the
pressure of 1 bar, in their normal state of condensation at the chosen
temperature (reference I) or a reference state in which the solutions are
infinitely dilute at the pressure of 1 bar (reference II), or indeed, for
solutions, a state in which all the solutes are at the concentration of 1 mol/l
(reference III).
The Gibbs energy in the reference state associated with a reaction is
linked to its enthalpy and its entropy in the same reference state at
temperature T by:

Δ r T 0 Δ G = r T 0 T Δ H − r 0 T S [4.1]

The equilibrium constant of a reaction in the chosen reference state is
defined on the basis of its corresponding associated Gibbs energy by:

Δ G 0
K = exp− r T [4.2]
RT

This equilibrium constant varies with temperature, in accordance with the
van ’t Hoff equation:

∂ ln K = Δ H T 0 [4.3]
r
∂ T RT 2

The enthalpy of the reaction r varies with temperature, according to the
Kirschoff relation [4.4] as a function of the specific heat capacity at constant
pressure:

⎛ Δ H ⎞ ∂ 0 0
⎜ r ⎟ = Δ C P
r
⎝ ∂ T P ⎠ [4.4]
0
The term Δ C is linked to the molar specific heat capacities at constant
r
P
pressure in the reference conditions of all the components of the reaction by
the expression [4.6]:
P ∑
0
Δ C = ν C 0
r k P () k
k [4.5]