Incompletely dissociated salts

If the salt completely dissociates, the number of ions present in solution is equal to the number of ionic species in the salt. However, if the salt is incompletely dissociated, the van t Hoff factor is determined as follows ... 

Where both the acid and the base are strong electrolytes, the neutralization point will be at pH = 7 and the end point break will be distinct unless the solutions are very dilute (< 10" 3 mol dm"3). The composition of the titrand at any point in the titration may W computed from the total amount of acid and base present. However, when one of the reactants is a weak acid or base the picture is less clear. The incomplete dissociation of the acid or base and the hydrolysis of the salt produced in the reaction must be taken into account when.calculations of end points and solution composition are made. These points have been considered in chapter 3 and are used in the indicator selection procedure outlined in the preceding section of this chapter. 

The nucleophilic reactivity of an anion depends not only on the extent of its specific solvation, but also on the degree of association with the corresponding cation. An ion-pair associated anion (or cation) is much less reactive than a free, non-associated ion. As early as 1912, Acree postulated that the reactivity of an anionic nucleophile should be depressed when its salt is incompletely dissociated [332]. Due to incomplete dissociation of the ionophore, the reaction rate constant will fall as its concentration increases. The simple model given in Eq. (5-124) is consistent with the observation that in aU cases ion association deactivates the nucleophile [289],... 

It should be realized that the method described actually gives the ionic concentration in the saturated solution, and it is only when dissociation is virtually complete that the result is identical with the solubility. This fact is brought out by the data for thallous chloride the solubility at 18 calculated from Kohlrausch s conductance measurements is 1.28 X 10 2 equiv. per liter, but the value obtained by direct solubility measurement is 1.32 X 10 equiv. per liter. The discrepancy, which is not very large in this instance, is probably to be ascribed to incomplete dissociation of the salt in the saturated solution the degree of dissociation appears to be 128/132, i.e., 0.97. 

In addition to the reason for incomplete dissociation just considered, there are some cases, e.g., weak acids and many salts of the transition and other metals, in which the electrolyte is not wholly ionized. These substances exist to some extent in the form of un-ionized molecules a weak acid, such as acetic acid, provides an excellent illustration of this type of behavior. The solution contains un-ionized, covalent molecules, quite apart from the possibility of ion-pairs. With sodium chlorides, and similar electrolytes, on the other hand, there are probably no actual covalent molecules of sodium chloride in solution, although there may be ion-pairs in which the ions are held together by forces of electrostatic attraction. 

The experimental data show that the deviations from the Onsager equation which may be attributed to incomplete dissociation occur more readily the smaller the ions, the higher their valence and the lower the dielectric constant of the medium. This generalization, as far as ionic size is concerned, appears at first sight not to hold for the salts of the alkali metals, for the deviations from the Onsager equation become more marked as the atomic weight of the metal increases owing to the effect... 

Fig. 59 this is particularly true if both the saturating salt and the added electrolyte are of high valence types. The deviations are often due to incomplete dissociation, and also to the approximations made in the derivation of the Debye-Hiickel equations as already seen, both these factors become of importance with ions of high valence. 

Infrared absorption spectra have been measured for NaF and KF isolated in solid argon. The fundamental modes of these species and the B3u, B2u, and Blu modes of (NaF)2 and (KF)2 are assigned assuming a planar rhomboid structure of D2h symmetry for the dimers.171 Surface ionization of lithium and its halides has been studied by a double-filament technique. At relatively high temperatures the temperature dependence of the lithium surface ionization current from all molecules studied was identical with that from lithium. Incomplete dissociation of LiCl can account for Li ionization threshold temperatures well above that for surface ionization of Li atoms. Dissociation energies of 4.8 0.1 eV for LiCl(g) and 4.3 0.1 eV for LiBr(g) were obtained.172 The equilibrium ionic forms in salt vapours have been studied. The concentration of M+ and X ions in the saturated vapours of MX (M = Na or K, X = halogen) at 850 °C is 107—1010 ions cm-3 and the concentration of M2X+ and M2X ions is 109—1011 ions cm-3, so that the vapours exhibit measurable electric conductivity. The equilibrium constants for the gaseous reactions are as follows ... 

In the experimental determination of activity coefficients of strong electrolytes, by the methods described below, the molalities, etc., of the ions are taken as the stoichiometric values, that is, the total possible molality, etc., disregarding incomplete dissociation, For example, in the last problem, the molalities of the sodium and sulfate ions in the 0.5 molal solution of sodium sulfate were taken as exactly 1.0 and 0.5, respectively, without allowing for the possibility that the salt may be only partially dissociated at the specified concentration. The activity coefficients obtained in this manner are called stoichiometric activity coefficients they allow for all variations from the postulated ideal behavior, including that due to incomplete dissociation. If the treatment is based on the actiuil ionic molalities, etc., in the given solution, as in the Debye-Httckel theory (Chapter XVII), there is obtained the true (or actual) activity coefficient. TTie ratio... 

In the last article, Olteanu and Pavel (1999) partially eliminated the main drawback of the theoretical models of Danek (1989) and Olteanu and Pavel (1995, 1996, 1997), i.e. the varying values of the dissociation degree of the same pure salt depending on the nature of the second component. In the new approach Olteanu and Pavel (1999) proposed a model, in which the equilibrium constants of the dissociation processes were set equal to the probabilities of the two dependent processes in order to describe more successfully the incomplete dissociation and its effect on the electrical conductivity of the mixture. One of the versions of this model leads to almost the same value for the dissociation degree of a component regardless of the nature of the second value. 

A similar idea on the incomplete dissociation of molten salts was presented by Klemm and Schafer (1996). Their model was stimulated by a qualitative explanation of the Chemla effect made by Klemm (1984), which is the crossing over of the mobility isotherms at a certain temperature. This means that at high concentrations of a larger sized cation, its mobility is greater than that of the smaller one. This effect was first observed in the LiBr-KBr system by Mehta et al. (1969), was also observed later in many other monovalent systems, and named after one of its discoverers. 

An even simpler case occurs when g = v and/= v, i.e., the aggregate is identical with the salt. Then, these relationships may be used to correct the mean activity coefficient for the incomplete dissociation. 

This last condition is fulfilled when the ionic concentrations are very low, as they are in fact in dilute solutions of weak electrolytes. The dissociation constants of substances such as weak organic acids can be determined by a combination of the formulae of Ostwald and Arrhenius, but the procedure is quite inadmissible for salts. Here the degree of dissociation is large. In fact the value of a is often indistinguishable from unity, and the mutual influences of the ions are considerable. They are calculable in principle by methods due to Debye and Hiickel, and operate differently on different properties. The procedure outlined on p. 276 allows the calculation of the activity coefiicients. In general the thermodynamic properties of the salt in solution correspond to those of a system with apparently incomplete dissociation, not because the concentrations of the ions are reduced by molecule formation but because the activity coefficients are lowered by mutual ionic influences. 

Because solutes disrupt the normal pattern of water structnre, solntions have freezing temperatures lower than that of pure water (Feeney, 1974) and solutions have boiling temperatures higher than that of pure water. Differences in freezing and boiling points are determined almost solely by the number of solute particles present and not by their properties. Totally dissociated compounds, such as table salt (NaCl), will have twice as much freezing point depression and boiling point elevation as will nondissociated compounds such as sucrose. Incompletely dissociated compounds will have intermediate effects. 

The so-called weak electrolytes did not follow Kohlrausch s law. This could be partially explained by incomplete dissociation. The dissociation equilibrium of a salt CA (C cation, A anion) in a diluted electrolyte, where activities a can be approximately substimted by concentrations c, is described by the equation... 

Electrolyte models. Assuming dissociation of the donor salt in the glass matrix, there is either a complete dissociation (strong electrolyte, Anderson-Stuart modeF°) or incomplete dissociation (weak electrolyte, Ravaine-Souqueti model) and the cations usually move in the matrix. 

Although there is ample evidence of its existence, the NaSO ion is generally ignored when calculating activity coefficients in solutions containing sodium and sulfate ions. Sodium sulfate is treated as a completely dissociating electrolyte. As early as 1930. Righellato and Davies (S34) stated that, even in dilute solutions, most uni-bivalent salts are incompletely dissociated. Based on conductance measurements at 18 C, they presented dissociation constants for a number of intermediate ions. For the salt MzX the dissociations were defined ... 

Righellato and Davies (S34) postulated in 1930 that most uni-bivalent salts must be considered as incompletely dissociated even in dilute solutions. It may be expected then that ion association will occur for many bi-univalent salts. This would seem to be particularly true of the doubly charged transition metals. Zinc chloride is one such salt which has been shown to form many complexes. The "complexing tests" described earlier in this chapter are applied to manganous chloride, cobalt chloride, nickel chloride, and cupric chloride. 

Bell, R.P. J.H.B. George, "The incomplete dissociation of some thalloua and calcium salts at different temperatures". Trans. Faraday Soc., v49, pp619-627 (1953)... 

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