Strong completely dissociated electrolytes

We have already discussed qualitatively the relaxation and electrophoretic effects which retard the motion of ions, surrounded by their ion atmospheres, through a solution. Such effects will show themselves at the experimental level in conductivity measurements. For the ion conductivity Xj of an ion species i in a very dilute solution of a strong electrolyte, Onsager derived the expression 

When appropriate numerical values for universal constants are inserted. Equation (4.18) becomes 

For a completely dissociated electrolyte the addition of two such terms, one for cationic and the other for anionic species, gives the molar conductivity by the Kohlrausch principle 

Equation (4.22) is of the same form as the empirical square root law (Equation 4.5)) found by Kohlrausch. 

Variation of molar conductivity, A, with JC for 1 1 electrolytes. Up to -JC 0-10 the graphs are linear and have approximately the Onsager slope. 

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Although Debye and Hiickel worked out their theory to solve the problem of strong, completely dissociated electrolytes, the results may be applied to weak and transition electrolytes as well, if the actual ionic concentration is substituted in the equation for ionic strength. With strong electrolytes, which are completely dissociated, it is possible to substitute in the term directly the analytical concentration of the substance, but with weak electrolytes their dissociation degree a has to be considered. For example with uni-... 

Schematic plot of A versus dilution for a strong, completely dissociated electrolyte.

Generally, strong acids in hydrogen peroxide remain strong. For example, plots of equivalence conductance versus the half-power of concentration yield straight lines which are characteristic of completely dissociated electrolytes. 

The evidence just given, which is typical of that obtained from all recent measurements, shows that the Onsager equation is valid for very dilute aqueous solutions of strong electrolytes. This fact is important as it lends additional and strong support to the correctness and utility of the interionic attraction theory. As has already been emphasized Onsager s equation is a limiting equation and deviations from it, even for completely dissociated electrolytes, are to be expected as the concentration is increased. 

Completely dissociated electrolytes under ambient conditions has long been a major topic in solution chemistry, so the seeond option is traditionally used. That is, for the mole fraction of an aqueous solution of a strong eleetrolyte such as NaCl or KCl, Equation (7.1) is modified to... 

Let us find out where the relationship (1.17) is coming from taking into account that in a completely dissociated electrolyte b+ = v+ b, and b = v. bi, where fc is the concentration of a strong (completely dissociated to ions) electrolyte ... 

Alkali-metal halides are textbook examples of strong electrolytes. However, conductance and potentiometric measurements reveal that there are some salts which behave differently and do form ion pairs by the strong attraction of the unlike ions. For these systems, a chemical model of ion pairing as proposed in Refs. 17 to 20 can be applied to consider the equilibrium between the completely dissociated electrolyte and the ion pair... 

Complete and Incomplete Ionic Dissociation. In the foregoing chapter mention has been made of electrolytes that are completely dissociated in solution, and of weak electrolytes where free ions are accompanied by a certain proportion of neutral molecules. In the nineteenth century it was thought that aqueous solutions of even the strongest electrolytes contained a small proportion of neutral molecules. Opinion as to the relation between strong and weak electrolytes has passed through certain vicissitudes and we shall describe later how this problem has been resolved. 

Incomplete Dissociation into Free Ions. As is well known, there are many substances which behave as a strong electrolyte when dissolved in one solvent, but as a weak electrolyte when dissolved in another solvent. In any solvent the Debye-IIiickel-Onsager theory predicts how the ions of a solute should behave in an applied electric field, if the solute is completely dissociated into free ions. When we wish to survey the electrical conductivity of those solutes which (in certain solvents) behave as weak electrolytes, we have to ask, in each case, the question posed in Sec. 20 in this solution is it true that, at any moment, every ion responds to the applied electric field in the way predicted by the Debye-Hiickel theory, or does a certain fraction of the solute fail to respond to the field in this way In cases where it is true that, at any moment, a certain fraction of the solute fails to contribute to the conductivity, we have to ask the further question is this failure due to the presence of short-range forces of attraction, or can it be due merely to the presence of strong electrostatic forces ... 

It is important to realise that whilst complete dissociation occurs with strong electrolytes in aqueous solution, this does not mean that the effective concentrations of the ions are identical with their molar concentrations in any solution of the electrolyte if this were the case the variation of the osmotic properties of the solution with dilution could not be accounted for. The variation of colligative, e.g. osmotic, properties with dilution is ascribed to changes in the activity of the ions these are dependent upon the electrical forces between the ions. Expressions for the variations of the activity or of related quantities, applicable to dilute solutions, have also been deduced by the Debye-Hiickel theory. Further consideration of the concept of activity follows in Section 2.5. 

The following facts must be borne in mind. All strong electrolytes are completely dissociated hence only the ions actually taking part or resulting from the reaction need appear in the equation. Substances which are only slightly ionised, such as water, or which are sparingly soluble and thus yield only a small concentration of ions, e.g. silver chloride and barium sulphate, are, in general, written as molecular formulae because they are present mainly in the undissociated state. 

The above examples assume that the strong base KOH is completely dissociated in solution and that the concentration of OH ions was thus equal to that of the KOH. This assumption is valid for dilute solutions of strong bases or acids but not for weak bases or acids. Since weak electrolytes dissociate only slightly in solution, we must use the dissociation constant to calculate the concentration of [H" ] (or [OH ]) produced by a given molarity of a weak acid (or base) before calculating total [H" ] (or total [OH ]) and subsequendy pH. 

Debye-Huckel theory assumes complete dissociation of electrolytes into solvated ions, and attributes ionic atmosphere formation to long-range physical forces of electrostatic attraction. The theory is adequate for describing the behaviour of strong 1 1 electrolytes in dilute aqueous solution but breaks down at higher concentrations. This is due to a chemical effect, namely that short-range electrostatic attraction occurs... 

When a small amount of a strong molten electrolyte is dissolved in another strong molten electrolyte, the laws of ideal dilute solutions are obeyed until relatively high concentrations are attained, assuming occurrence of a virtually complete dissociation. 

The theory of Debye and Hiickel started from the assumption that strong electrolytes are completely dissociated into ions, which results, however, in electrical interactions between the ions in such a manner that a given ion is surrounded by a spherically symmetrical distribution of other ions mainly of opposite charges, the ionic atmosphere. The nearer to the central ions the higher will be the potential U and the charge density the limit of approach to the central ion is its radius r = a. 

This time a solid sample of a weak base is being added to a solution of its conjugate acid. We let represent the concentration of acetate ion from the added sodium acetate. Notice that sodium acetate is a strong electrolyte, completely dissociated in aqueous solution. 

A wide variety of data for mean ionic activity coefficients, osmotic coefficients, vapor pressure depression, and vapor-liquid equilibrium of binary and ternary electrolyte systems have been correlated successfully by the local composition model. Some results are shown in Table 1 to Table 10 and Figure 3 to Figure 7. In each case, the chemical equilibrium between the species has been ignored. That is, complete dissociation of strong electrolytes has been assumed. This assumption is not required by the local composition model but has been made here in order to simplify the systems treated. 

First, these electrolytes are strong (i.e. they completely dissociate to form ions) so there is a sufficient supply of ions available to carry the charge, thus explaining why the conductivity A is high. Secondly, the transport numbers t of the anions and cation in these two salts happen to be just about the same, thereby minimizing the junction potentials (see Chapter 3). 

Moderately strong electrolytes, such as aqueous HNO3, generally have been treated thermodynamically as completely dissociated substances. Thus, for HN03(aq), the value for AfG of — 111.25 kJ mol hsted in [Ref. 11] refers to the reaction... 

From the point of view of chemical modeling, aqueous solutions are treated as electrolytic solutions —i.e., solutions in which solutes are present partially or totally in ionic form. Speciation is the name for the characteristic distribution of ion species in a given aqueous solution in the form of simple ions, ionic couplings, and neutral molecules. Solutes in aqueous solutions are defined as electrolytes and may be subdivided into nonassociated and associated. Nonassociated electrolytes are also defined as strong and mainly occur in the form of simple or simply hydrated ions. An example of a strong electrolyte is the salt NaCl, which, in aqueous solution of low ionic strength, occurs in the form of completely dissociated Na and CN ions. 

A solution of sulfurous acid is dominated by molecules of H2SO3 with relatively scarce and HSO3 ions. Make sure that you grasp the difference between this case and the previous example of the strong electrolyte Na2C03, which completely dissociates into ions. 

The diquaternary salts of 4,4 -bipyridine are strong electrolytes and are completely dissociated in water.Thermodynamic and viscosity measurements for solutions of paraquat dichloride in water have been obtained,and enthalpy data have been determined. ° Distribution of the positive charges in the paraquat dication has been calculated from a quantum mechanical procedure." There has been much interest in the spectra of diquaternary salts of 4,4 -bipyridine ... 

At high field strengths a conductance Increase Is observed both In solution of strong and weak electrolytes. The phenomena were discovered by M. Wien (6- ) and are known as the first and the second Wien effect, respectively. The first Wien effect Is completely explained as an Increase In Ionic mobility which Is a consequency of the Inability of the fast moving Ions to build up an Ionic atmosphere (8). This mobility Increase may also be observed In solution of weak electrolytes but since the second Wien effect Is a much more pronounced effect we must Invoke another explanation, l.e. an Increase In free charge-carriers. The second Wien effect Is therefore a shift in Ionic equilibrium towards free ions upon the application of an electric field and is therefore also known as the Field Dissociation Effect (FDE). Only the smallness of the field dissociation effect safeguards the use of conductance techniques for the study of Ionization equilibria. 

According to modem theory, many strong electrolytes are completely dissociated in dilute solutions. The freezing-point lowering, however, does not indicate complete dissociation. For NaCl, the depression is not quite twice the amount calculated on the basis of the number of moles of NaCl added. In the solution, the ions attract one another to some extent therefore they do not behave as completely independent particles, as they would if they were nonelectrolytes. From the colligative properties, therefore, we can compute only the "apparent degree of dissociation" of a strong electrolyte in solution. 

The form in which chemical analyses of sea water are given records the history of our thought concerning the nature of salt solutions. Early analytical data were reported in terms of individual salts NaCl, CaSO/i, and so forth. After development of the concept of complete dissociation of strong electrolytes, chemical analyses of sea water were given in terms of individual ions Na+, Ca++, Cl-, and so forth, or in terms of known undissociated and partly dissociated species, e.g., HC03 , In recent years there has been an attempt to determine the thermodynamically stable dissolved species in sea water and to evaluate the relative distribution of these species at specified conditions. Table 1 lists the principal dissolved species in sea water deduced from a model of sea water that assumes the dissolved constituents are in homogeneous equilibrium, and (or) in equilibrium, or nearly so, with solid phases. 

What is the total molar concentration of ions in a 0.350 M solution of the strong electrolyte Na2S04, assuming complete dissociation ... 

Most often sulfonic acid groups serve as hydrophilic substituents, because they are readily introduced and, as strong electrolytes, are completely dissociated in the acidity range used in the dyeing process. Almost invariably the products manufactured and employed are water-soluble sodium salts of the sulfonic acids. 

Strong acids and bases are strong electrolytes, and weak acids and bases are weak electrolytes, so strong acids and bases completely dissociate in water,... 

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