Electrolytic dissociation, development theory

This system of nomenclature has withstood the impact of later experimental discoveries and theoretical developments that have since the time of Guyton de Morveau and Lavoisier greatiy altered the character of chemical thought, eg, atomic theory (Dalton, 1802), the hydrogen theory of acids (Davy, 1809), the duahstic theory (Berzehus, 1811), polybasic acids (Liebig, 1834), Periodic Table (Mendeleev and Meyer, 1869), electrolytic dissociation theory (Arrhenius, 1887), and electronic theory and modem knowledge of molecular stmcture. 

Between 1865 and 1887, Dmitri 1. Mendeleev developed the chemical theory of solutions. According to this theory, the dissolution process is the chemical interaction between the solutes and the solvent. Upon dissolution of salts, dissolved hydrates are formed in the aqueous solution which are analogous to the solid crystal hydrates. In 1889, Mendeleev criticized Arrhenius s theory of electrolytic dissociation. Arrhenius, in turn, did not accept the idea that hydrates exist in solutions. 

According to modem views, the basic points of the theory of electrolytic dissociation are correct and were of exceptional importance for the development of solution theory. However, there are a number of defects. The quantitative relations of the theory are applicable only to dilute solutions of weak electrolytes (up to 10 to 10 M). Deviations are observed at higher concentrations the values of a calculated with Eqs. (7.5) and (7.6) do not coincide the dissociation constant calculated with Eq. (7.9) varies with concentration and so on. For strong electrolytes the quantitative relations of the theory are altogether inapplicable, even in extremely dilute solutions. 

The theory of complete electrolytic dissociation at infinite dilution was developed by Debye and Hiickel (1923) and has been further extended by Onsager and later by Fuoss and Krauss. 

Arrhenius acid-base theory - Arrhenius developed the theory of the electrolytic dissociation (1883-1887). According to him, an acid is a substance which delivers hydrogen ions to the solution. A base is a substance which delivers hydroxide ions to the solution. Accordingly, the neutralization reaction of an acid with a base is the formation of water and a salt. It is a so-called symmetrical definition because both, acids and bases must fulfill a constitutional criterion (presence of hydrogen or hydroxide) and a functional criterion (to deliver hydrogen ions or hydroxide ions). The theory could explain all of the known acids at that time and most of the bases, however, it could not explain the alkaline properties of substances like ammonia and it did not include the role of the solvent. -> Sorensen (1909) introduced the -> pH concept. 

Feb. 19,1859, Wijk, Sweden - Oct. 2,1927, Stockholm, Sweden). Arrhenius developed the theory of dissociation of electrolytes in solutions that was first formulated in his Ph.D. thesis in 1884 Recherches sur la conductibilit galvanique des dectrolytes (Investigations on the galvanic conductivity of electrolytes). The novelty of this theory was based on the assumption that some molecules can be split into ions in aqueous solutions. The - conductivity of the electrolyte solutions was explained by their ionic composition. In an extension of his ionic theory of electrolytes, Arrhenius proposed definitions for acids and bases as compounds that generate hydrogen ions and hydroxyl ions upon dissociation, respectively (- acid-base theories). For the theory of electrolytes Arrhenius was awarded the Nobel Prize for Chemistry in 1903 [i, ii]. He has popularized the theory of electrolyte dissociation with his textbook on electrochemistry [iv]. Arrhenius worked in the laboratories of -> Boltzmann, L.E., -> Kohlrausch, F.W.G.,- Ostwald, F.W. [v]. See also -> Arrhenius equation. 

The integration of these three basic developments established the foundations of modem solution theory and the first Nobel prizes in chemistry were awarded to van t Hoff (in 1901) and Arrhenius (in 1903) for their work on osmotic pressure and electrolytic dissociation, respectively. 

Many studies of electrolyte conductivity have been carried out [7]. This work certainly helped to confirm modern ideas about electrolyte solutions. One aspect of the behavior of strong electrolytes which was initially not well understood is the fact that their molar conductance decreases with increase in concentration. Although this is now attributed to ion-ion interactions, early work by Arrhenius [8] ascribed the decrease in all electrolytes to partial dissociation. However, it is clear from the vast body of experimental data that one can distinguish two types of behavior for these systems, namely, that for strong electrolytes and that for weak electrolytes, as has been illustrated here. The theory of the concentration dependence of the molar conductance of strong electrolytes was developed earlier this century and is discussed in detail in the following section. 

Discussion Neutralization is the interaction of an acid and a base, as the result of which a salt and water are formed. From the standpoint of the theory of electrolytic dissociation, in neutralization the hydrogen ions furnished by the acid unite with the hydroxyl ions furnished by the base. If the solutions are sufficiently dilute, both the acid and the base are completely dissociated, and the only change that occurs when they are mixed is the formation of undissociated water from its ions. It follows that equal volumes of equivalent solutions of acids and of bases should produce the same amount of heat when neutralization takes place. The experiment described below is designed to test this conclusion. Equal quantities of normal solutions of several acids and bases are mixed and the heat developed in each case is measured. This is done by determining the rise in temperature that occurs when the solutions are mixed. Since dilute solutions are used, it is assumed in the calculations that the specific heat of the resulting salt solutions is equal to that of water, which is 1. If the volume of the solution is multiplied by the rise in temperature, the product is the number of calories set free. 

The study of catalysis by acids and bases played a very important part in the development of chemical kinetics, since many of the reactions studied in the early days of the subject were of this type. The early investigations of the kinetics of reactions catalyzed by acids and bases were carried out at the same time that,the electrolytic dissociation theory was being developed, and the kinetic studies contributed considerably to the development of that theory. The reactions considered from this, point of view were chiefly the inversion of cane sugar and the hydrolysis of esters. 

He employed a range of adds and he correlated the affinity (reactivity) of an acid with its catalytic power. He was therefore in a good position to appreciate Arrhenius s concept of electrolytic dissociation when the latter sent him a copy of his doctoral thesis in 1884. In 1887 Ostwald moved to Leipzig as professor of physical chemistry. For the remainder of his career he championed the ionic theory of Arrhenius against much opposition. He provided additional evidence for the theory, and he developed the theory of add-base indicators. He resigned from Leipzig in 1905, and in his retirement he worked on the theory of colours, as well as espousing many humanistic, educational and cultural causes. 

Even traces of dissolved electrolytes considerably increase the very small conductance of water. This effect is often used to continuously monitor the purity of distilled or demineralized water. The fundamental importance of conductance measurements became obvious when they were used by Svante Arrhenius (1859-1927) to develop his theory of electrolytic dissociation (12). 

Arrhenius, Svante Ausust (1859-1927) Swedish physical chemist who, in 1884, was the first to propose that acids, bases, and salts in solution dissociated into ions. His theory of electrolytic dissociation was well before its time and was not scientifically confirmed until the theory of atomic structure was more fully developed. He also worked on reaction rates, and was the first to recognize the greenhouse effect on climate. He was awarded the Nobel Prize in chemistry in 1903. 

Arrhenius, Svante August (1859-1927) A Swedish physicist and chemist who did fundamental work on physical chemistry. He worked with van t Hoff in Amsterdam and proposed a theory of activated molecules and established a coimecdon between rate of reaction and absolute temperature. He also developed a theory for electrolytic dissociation based on van t Hoff s results and stated that any acid, base, or salt dissolved in water is partly split up into positively and negatively charged ions, and that they move in opposite directions on electrolysis. He was awarded a Nobel Prize for Chemistry in 1903. 

Although Arrhenius developed his theory of electrolytic dissociation to explain the electrical conductivities of solutions, he was able to apply it more widely. One of his first successes came in explaining certain anomalous values of colligative properties described by the Dutch chemist Jacobus van t Hoff (1852-1911). 

Hence, the theory of electrolyte solutions subsequently developed in two directions (1) studies of weak electrolyte solutions in which a dissociation equilibrium exists and where because of the low degree of dissociation the concentration of ions and the electrostatic interaction between the ions are minor and (2) studies of strong electrolyte solutions, in which electrostatic interaction between the ions is observed. 

Raji Heyrovska [18] has developed a model based on incomplete dissociation, Bjermm s theory of ion-pair formation, and hydration numbers that she has found fits the data for NaCl solutions from infinite dilution to saturation, as well as several other strong electrolytes. She describes the use of activity coefficients and extensions of the Debye-Hiickel theory as best-fitting parameters rather than as explaining the significance of the observed results. ... 

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

In all other solutions the so called degree of dissociation, as determined from the measurement of some colligative property, merely indicates the magnitude of interionic forces, it cannot, however, be taken as a measure of the quantity of dissociated and undissociated molecules of the solute. A complete theory of strong electrolytes, at least of their diluted solutions, has been developed by Debye and Hiickel, this theory is the basis of modern electrochemistry. 

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