Electrolytic dissociation degree

Thus, quantitative criteria that could be tested experimentally had now been formulated for the first time in the theory of electrolytic dissociation, in contrast to earlier theories. The good agreement between degrees of dissociation calculated from independent measurements of two different properties with Eqs. (7.5) and... 

At the beginning of the twentieth century the idea was put forward that in solutions of strong electrolytes the degree of dissociation is not simply high but dissociation of the solute is complete (i.e., equilibrium between ions and undissociated molecules does not exist). This point is particularly evident for ionophors, which in the solid state do not possess individual molecules and for which it is unlikely that undissociated molecules should appear in a solution. 

The acidic character of acids depends on the availability ofhydrogen ions in their solution. An acid X3 is said to be stronger than another acid X2 if, in equimolar solutions, X3 provides more hydrogen ions than does X2. This will be possible provided that the degree of dissociation of X3 is greater than that of X2. Based on the Arrhenius theory of electrolytic dissociation, solutions may be classified in the manner shown in Figure 6.1. If the ionization of an acid is almost complete in water, the acid is said to be a strong acid, but if the... 

A comparison of the equivalent conductance at some finite concentration (Ac) with that at infinite dilution (AJ gives a measure of the fraction of electrolyte dissociation at the higher concentration. One introduces a, the degree of dissociation or ionization, and writes... 

Fig. 1.2 Dependence of the dissociation degree a of a week electrolyte on molar concentration c for different values of the apparent dissociation constant K (indicated at each curve)... 

The unavailability of data on dissociation degree and mobility has thus made ion conductivity an alternative metric that has been universally adopted by the battery research and development community to evaluate the transport ability of electrolytes. However, it should always be remembered that such a metric of convenience is based on an unstated assumption that is, the increase in the overall conductivity should originate, at least partially, from the improvement in the cation conductivity. Qualitatively, this assumption holds true, since a correlation does usually exist between ion conductivity and power performance in batteries, although quantitatively the distribution of this increase between anions and cations is unknown. 

The efforts to improve ion conductivity have revolved around eq 1, that is, aiming at increasing either the salt dissociation degree riij or the ionic mobility (mj). Since these two factors are decided simultaneously by the physicochemical natures of the salt and solvents, different approaches involving either of these electrolyte components have been adopted. 

For -weak electrolytes the degree of dissociation found by the freezing point method agrees with that found from conductivity within the limits of experimental error and Ostwald s dilution law is obeyed. 

The great increase in complexity in solution thermodynamics which occurs when a salt is dissolved to substantial concentration in a mixture of two liquid components becomes fully apparent in the realization that the liquid phase so created is a concentrated solution of an electrolyte whose degree of dissociation is a function of the relative proportions of the other two components present, and... 

Electrolytes, depending upon their strength, dissociate to a greater or less extenl in polar solvents. The extent to which a weak electrolyte dissociates may be determined by electrical conductance, electromotive force, and freezing point depression methods. The electrical conductance method is the most used because of its accuracy and simplicity. Arrhenius proposed that the degree of dissociation, a. of a weak electrolyte at any concentration in solution could be found from the rutio of the equivalent conductance. A. of the electrolyte at the concentration in question to (he equivalent conductance at infinite dilution A0 of the electrolyte. Thus... 

The thermodynamic behaviour of weak electrolytes is based on the conditions for equilibrium between dissociated ions and undissociated portion in the solution. Prove that, for most weak electrolytes, the degree of dissociation increases as the electrolyte concentration decreases. 

Combination between solute and solvent is undoubtedly responsible for the effect of solvents in the majority of cases. This solvation may give an intermediate complex of high stability or low stability. If the complex is stable the reaction will go slowly if unstable it will go rapidly. In other cases the solvent effect can sometimes be explained as due to a varying degree of electrolytic dissociation. Specific reactions will be given presently, illustrating these different factors. 

According to the Arrhenius conception electrolytes dissociate in solutions to a certain degree that depends solely on the concentration at a given temperature. The dissociation is partial at finite concentration but it increases with increasing dilution and it becomes complete at infinite dilution. At a given temperature... 

According to the Arrhenius theory the decrease of A with increasing concentration of the solution with all electrolytes is merely due to the lowering of the dissociation degree as this theory does not take into account the mutual attraction of ions and the lowering of ion mobility in more concentrated solutions, the velocity of the ions in equation (111-25) should be equal both at finite concentrations and infinite dilution, i. e. ( + + v ) = (v + ). As... 

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-... 

DEGREE OF DISSOCIATION. STRONG AND WEAK ELECTROLYTES When discussing the theory of electrolytic dissociation, it was stated that it is a reversible process and its extent varies with concentration (and also with other physical properties, like temperature). The degree of dissociation (a) is equal to the fraction of the molecules which actually dissociate. 

The Electrolytic Dissociation Theory. —From his studies of the conductances of aqueous solutions of acids and their chemical activity, Arrhenius (1883) concluded that an electrolytic solution contained two kinds of solute molecules these were supposed to be active molecules, responsible for electrical conduction and chemical action, and inactive molecules, respectively. It was believed that when an acid, base or salt was dissolved in water a considerable portion, consisting of the so-called active molecules, was spontaneously split up, or dissociated, into positive and negative ions it was suggested that these ions are free to move independently and are directed towards the appropriate electrodes under the influence of an electric field. The proportion of active, or dissociated, molecules to the total number of molecules, later called the degree of dissociation, was considered to vary with the concentration of the electrolyte, and to be equal to unity in dilute solutions. 

S) Electrolytic dissociation of water and hydrolysis. If one is not previously convinced of the correctness of the electrolytic dissociation theory, hardly any result won by means of it is so convincing as the agreement between the conclusions drawn in completely different ways as to the degree of dissociation of water itself. 

Remember Not all electrolytes dissociate completely in water. We said that a strong electrolyte ionizes to a greater degree than a weak electrolyte. The same idea is true for acids and bases, which, as you know, are... 

The model of electrical conductivity of molten salt mixtures based on incomplete electrolytic dissociation of components was proposed by DanCk (1989). The dissociation degree of a component is affected by the presence of second component. Consequently, the dissociation degree of both components in the system is not constant, but changes with composition, affecting the concentration of the conducting particles in the electrolyte. This effect is caused by interactions of components, given by the nature of the repulsive forces between ions, determining their actual coordination sphere. 

Synthetic polymers are widely applied to modify the surface properties of materials, and their adsorption mechanism is very different from small ions or molecules discussed in previous sections. Moreover, special methods are applied to study polymer adsorption, thus, polymer adsorption became a separate branch of colloid chemistry. Polymers that carry ionizable groups are referred to as polyelectrolytes. Their adsorption behavior is more sensitive to surface charging than adsorption of neutral polymers. Polyelectrolytes are strong or weak electrolytes, and the dissociation degree of weak polyelectrolytes is a function of the pH. The small counterions form a diffuse layer similar to that formed around a micelle of ionic surfactant. 

That fraction of a gram molecular weight which splits into ions is called the degree of electrolytic dissociation a. For example, in a 0.01 N sodium chloride solution a equals 0.9. Then the concentration of undissociated salt is 0.1 X 0.01 = 0.001, and the concentration of chloride and sodium ions is 0.9 X 0.01 = 0.009. 

Not only was it possible to explain, by the new theory, the anomalous osmotic behavior of strong electrolytes, but intensive quantitative studies indicated clearly the existence of a simple relationship between the irrationality factor i and the degree of electrolytic dissociation a. In a solution of a uni-univalent electrolyte of concentration c, the sum of concentrations of all particles evidently exceeds c. This summation is equal to (1 — a)c -f 2ac = c(l -1- a). From the definition of irrationality factor. 

Since i could be determined from freezing point and boiling point measurements, it was possible to calculate a. Other methods, such as determining the electrical conductivity of solutions, likewise permit the calculation of the degree of dissociation a. The equivalent conductance A of a solution is a function of the concentration. It is greatest in infinitely dilute solution where the electrolyte is completely dissociated into ions, and diminishes with increasing concentration due to the decrease in electrolytic dissociation. If we assume provisionally that the conductivity is determined only by the concentration of ions, it follows that... 

In the following table are included data found by A. A. Noyes and McInnes for potassium chloride. Here f indicates the degree of electrolytic dissociation, calculated from conductivity measurements ... 

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