Page 79 - Chemical equilibria Volume 4

3

Molecular Chemical Equilibria

In this chapter, we shall apply the general principles of transformations at
equilibrium, which we examined in Chapter 1, to the case of chemical
reactions. Hence, we define the equilibrium constant, and its different
variants, the value of which we shall see how to determine
in Chapter 4.
Numerous graphical representations are used to represent equilibrium
states. The main ones which are used for molecular reactions have been
included in this chapter.

3.1. Law of mass action – equilibrium constants

In this section, we introduce the equilibrium constant, its different forms
and its properties.

3.1.1. Expression of the law of mass action

The law of mass action is obtained from expression [1.29], and by writing
that the affinity is null at thermodynamic equilibrium.

Immediately we can deduce that at equilibrium, the reaction quotient
takes the particular value Q (equ ) , and therefore the equilibrium condition is:
r
T
RlnQ r (equ) = r A 0 [3.1]

Chemical Equilibria, First Edition. Michel Soustelle.
© ISTE Ltd 2015. Published by ISTE Ltd and John Wiley & Sons, Inc.