Page 80 - Chemical equilibria Volume 4

56 Chemical Equilibria
This particular value of the reaction quotient at equilibrium is known as
the equilibrium constant. Thus, it can be expressed as a function of the
affinity in the reference state or, by virtue of relation [1.25], of the Gibbs
energy associated with the reaction in the reference state:
A 0 Δ G 0
K = Q (equ) = exp r = exp− r [3.2]
r
r
RT RT
The reaction conditions and equilibrium conditions of our system will
thus be easily expressed in terms of the value of the reaction quotient.
Obviously, we shall have:

0
– if A > 0 , or Δ G < , then Q (equ) < K and the reaction occurs
r r r
spontaneously from left to right;
0
– if A r > 0 or Δ G > , then Q r (equ) > K and the reaction is impossible,
r
so the opposite reaction occurs spontaneously;
0
– if A r = 0 or Δ G = then Q r (equ) = K , and the reaction is at
r
thermodynamic equilibrium.
NOTE 3.1.– The equilibrium constant, which depends only on the Gibbs
energies in the reference state, is independent of the state of perfection of the
phases. This means that this constant does not depend on the nature of the
mixture of the reagent species, but only on the nature of the products. For
example, whether we are studying the carbon-oxygen-carbon monoxide
balance using pure carbon or carbon in solution in iron (steel), the
equilibrium constant is the same in both cases, because we can choose
carbon in a pure phase as a reference state.

By comparing the first equality in relation [3.2] and expression [1.28], the
law of mass action becomes:

r ∏
K = a i i ν [3.3]
i
Obviously, the values of the activities are those which reign at
thermodynamic equilibrium, and no longer values in any given state, as was
the case with relation [1.28].