Dissociation of weak electrolyte

DETERMINATION OF EQUILIBRIUM CONSTANTS FOR DISSOCIATION OF WEAK ELECTROLYTES... 

There are some cases where a reaction, that is, the formation or dissolution of a chemical bond, is involved along with ion exchange phenomena (Helfferich, 1983). Examples of this are acid-base neutralization, dissociation of weak electrolytes in solution or weak ionogenic groups in ion exchangers, complex formation, or combinations of these (Table 5.2). With some of these, very low apparent D in ion exchangers have been noted. 

From Eqn. (14) it follows that with an exothermic reaction - and this is the case for most reactions in reactive absorption processes - decreases with increasing temperature. The electrolyte solution chemistry involves a variety of chemical reactions in the liquid phase, for example, complete dissociation of strong electrolytes, partial dissociation of weak electrolytes, reactions among ionic species, and complex ion formation. These reactions occur very rapidly, and hence, chemical equilibrium conditions are often assumed. Therefore, for electrolyte systems, chemical equilibrium calculations are of special importance. Concentration or activity-based reaction equilibrium constants as functions of temperature can be found in the literature [50]. 

Also the so called degree of dissociation, determined from the colligative properties, does not agree with the result obtained from the measurement of the electrical conductance. Finally the law of chemical equilibrium, applicable to the dissociation of weak electrolytes, cannot be applied to the strong ones. 

Table XV.7. Effect of Dielectric Constant on Dissociation OF Weak Electrolytes at 25 C ...

On this basis, the dissociation of weak electrolytes may be written ... 

The 1957 Fuoss-Onsager equation can be adapted to take account of association of ions to form ion pairs and to account for incomplete dissociation of weak electrolytes. Chemically these are two different types of situation, but physically they are the same, viz. some of the ions are removed from solution by formation of ion pairs, or by formation of undissociated molecular species. The physical manifestation is that not all of the solute will be able to conduct the current, and so the observed conductance will be lower than that predicted by the... 

The dissociation of weak electrolytes or the solubility of slightly soluble substances can be quantitatively described by equilibrium constants. Equilibrium constants for completely dissolved and dissociated electrolytes are effectively infinite. Consider the dissociating species AB ... 

We mentioned at the begmning of the last section on activity that the presence of diverse salts will generally increase the dissociation of weak electrolytes due to a shielding (or decrease in the activity) of the ionic species produced upon dissociation. We can quantitatively predict the extent of the effect on the equilibrium by taking into account the activities of the species in the equilibrium. 

In Chapter 6, we discussed the thermodynamic equilibrium constant based on activities rather than on concentrations. Diverse salts affect the activities and therefore the extent of dissociation of weak electrolytes such as weak acids or bases. 

One of the early uses of limiting conductances was to determine the degree of dissociation of weak electrolytes. Arrhenius suggested that, at any given concentration, the measured equivalent conductance (when compared to the limiting equivalent conductance where all ions are dissociated) should be a measure of the degree of dissociation, a. This can be expressed as... 

As we have seen in several examples in this chapter, HCN acts as an acid in aqueous solutions. We introduced a few fundamental concepts of acids and bases in Chapter 3, but the context of equilibrium allows us to explore them further. Recall that we distinguished between strong acids (or bases), which dissociate completely in solution, and weak acids (or bases), which dissociate only partially. At this point in our study of chemistry, we should realize that this partial dissociation of weak electrolytes was an example of a system reaching equilibrium. So we can use equilibrium constants to characterize the relative strengths of weak acids or bases. One common way to do this is to use the pH scale, which we will define in this section. 

Determination of the degree of dissociation of weak electrolytes is a common application of conductivity measurements. This approach will be briefly outlined here with a sample calculation illustrating the utility of the method. From the data presented in the next section, the limiting molar conductivity of acetic acid can be seen to be 389.9 S cm mol (from addition of the limiting values for the proton and the acetate ion). At finite concentrations, this weak acid will only be partially deprotonated. The ratio of observed to predicted electrolytic conductivity can be used to determine the degree of dissociation, a. At 0.01 M, the observed molar conductivity of acetic acid was found to be 14.30 S cm mol . Thus... 

Electric field methods for the study of fast kinetics encompass a number of diverse experimental techniques. If the applied field is small the techniques are usually considered under the heading of dielectric relaxation techniques. These will be discussed elsewhere in this volume. At high field strengths there is an increased dissociation of weak electrolytes, the so called second Wien effect. Quantitatively this effect is given by the equation of Onsager (1)... 

Simms HS (1928) The effect of salts on weak electrolytes. 1. Dissociation of weak electrolytes in presence of salts. J Phys Chem 33 1121-1141... 

Rapid changes in electric field can also be used to perturb chemical equilibria. As high electric fields favour the production of ions from neutral species, it is possible to increase rapidly the dissociation of weak electrolytes. In the most common application of this technique. 

---