PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 373

11.5 Dissociation in solutions of weak electrolytes

This section is focused on the dissociation of weak acids, bases and their salts. This is nothing
but application of the theory of chemical equilibrium to electrolyte solutions.

11.5.1 Some general notes

When solving dissociation equilibria we observe the following rules:

1. The activity of water is assumed to be unity. For the activity of an undissociated
   electrolyte and for the activities of ions, the standard state of unit concentration is
   chosen, see11.4.1.


2. The activity coefficient of an undissociated electrolyte is in most calculations assumed
   to be unity. It is considered only in very accurate calculations, but in that case we have
   to know its concentration dependence.


3. The activity coefficients of the ions:
   
   
   • in less accurate calculations they are considered to be unity,
   
   
   • if the ionic strength of the solution isI <0.01, they are calculated from the Debye-
     H ̈uckel relation,
   
   
   • if the ionic strength is higher, we have to know their dependence on this ionic
     strength or on the concentration.
   
   
   
   


4. If the concentration of hydrogen ions is comparable in order with their concentration in
   pure water, we also have to consider equation (11.47) for the dissociation of water when
   solving a chemical equilibrium.

11.5.2 Ionic product of water

Water dissociates according to the reaction

H 2 O = H++ OH−. (11.47)

The equilibrium constant of this reaction is called theionic product of waterKw. We have

Kw=

aH+aOH−

aH 2 O

=

cH+cOH−γ^2 ±

(cst)^2

, (11.48)