The theory of electrolytic dissociation

The electrolytic dissociation (ionization) of compounds may therefore be represented by the reaction equations  

Ions carry positive or negative charges. Since the solution is electrically neutral, the total number of positive charges must be equal to the total number of negative charges in a solution. The number of charges carried by an ion is equal to the valency of the atom or radical. 

The phenomenon of electrolysis also receives a simple explanation on the basis of the theory of electrolytic dissociation. The conductance of electrolyte solutions is due to the fact that ions (charged particles) are present in the solution, which, when switching on the current, will start to migrate towards the electrode with opposite charge, owing to electrostatic forces. In the case of hydrochloric acid we have hydrogen and chloride ions in the solution  

Pairs of hydrogen atoms will then form hydrogen molecules, which are discharged in the form of hydrogen gas  

On the anode the chloride ions release electrons, forming chlorine atoms  

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The first substantial constitutive concept of acid and bases came only in 1887 when Arrhenius applied the theory of electrolytic dissociation to acids and bases. An acid was defined as a substance that dissociated to hydrogen ions and anions in water (Day Selbin, 1969). For the first time, a base was defined in terms other than that of an antiacid and was regarded as a substance that dissociated in water into hydroxyl ions and cations. The reaction between an acid and a base was simply the combination of hydrogen and hydroxyl ions to form water. 

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... 

Soon after inception of the theory of electrolytic dissociation, it was shown that two types of componnds exist that can dissociate upon dissolution in water (or other solvents) ... 

The theory of electrolytic dissociation also provided the possibility for a transparent definition of the concept of acids and bases. According to the concepts of Arrhenius, an acid is a substance which upon dissociation forms hydrogen ions, and a base is a substance that forms hydroxyl ions. Later, these concepts were extended. 

The theory of electrolytic dissociation was not immediately recognized universally, despite the fact that it could qualitatively and quantitatively explain certain fundamental properties of electrolyte solutions. For many scientists the reasons for spontaneous dissociation of stable compounds were obscure. Thus, an energy of about 770kJ/mol is required to break up the bonds in the lattice of NaCl, and about 430kJ/mol is required to split H l bonds during the formation of hydrochloric acid solution. Yet the energy of thermal motions in these compounds is not above lOkJ/mol. It was the weak point of Arrhenius s theory that this mismatch could not be explained. 

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 elucidation of the electrical behavior of electrolytes owes much to Arrhenius, who was the originator of the theory of electrolytic dissociation, generally, known as the ionic theory. 

Louis Kahlenberg s Opposition to the Theory of Electrolytic Dissociation. Proc. Symposium on Selected Topics in the History of Electrochemistry, Geo. Dubpernell and J.H. Westbrook, eds. (Proc. vol. 78-6, 1978, The Electrochem. Society, Princeton, N.J.)>pp. 299-312. 

Such a chemical approach which links ionic conductivity with thermodynamic characteristics of the dissociating species was initially proposed by Ravaine and Souquet (1977). Since it simply extends to glasses the theory of electrolytic dissociation proposed a century ago by Arrhenius for liquid ionic solutions, this approach is currently called the weak electrolyte theory. The weak electrolyte approach allows, for a glass in which the ionic conductivity is mainly dominated by an MY salt, a simple relationship between the cationic conductivity a+, the electrical mobility u+ of the charge carrier, the dissociation constant and the thermodynamic activity of the salt with a partial molar free energy AG y with respect to an arbitrary reference state ... 

State the fundamental propositions of the theory of electrolytic dissociation. 

At the end of the 19th century, the theory of electrolytic dissociation became an important part of physical chemistry. Wilhelm Ostwald, Svante Arrhenius, and Walther Nemst were among the most vigorous supporters of that theory, which also had some severe critics. The ensuing debate has been discussed in a paper, which analyses the arguments on both sides and shows how the proponents of the theory attempted to resolve its difficulties.90... 

R. Maiocchi, Difficult beginnings the theory of electrolytic dissociation in the 19th century , Nuncius, 1993, 8, 121-167 [in Italian],... 

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. 

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. 

Evidence for the Ionic Theory.—There is hardly any branch of electrochemistry, especially in its quantitative aspects, which does not provide arguments in favor of the theory of electrolytic dissociation without the ionic concept the remarkable systematization of the experimental results which has been achieved during the past fifty years would certainly not have been possible. It is of interest, however, to review briefly some of the lines of evidence w hich support the ionic theory. 

The explanation of this striking regularity follows at once from the theory of electrolytic dissociation. According to this, theory the neutrahsation of a strong base with a strong acid is due simply to the combination of H and OH ions to form undissociated water according to equations such as... 

The theory of electrolytic dissociation, AVhereas the osmotic pressure and the other colligative properties of aqueous solutions of substances, such as cane sugar, obey van t Hoff s laws, marked deviations are met with in aqueous solutions of acids, bases, and salts, even at great dilutions. The osmotic pressure and lowering of the freezing point for these solutions are still found to be approximately proportional to the molecular concentration, but are considerably greater than the theoretical values. To allow for this van t Hoff introduced a new term into his osmotic pressure equation, writing for such solutions... 

The agreement at low temperatures is remarkably good. The heat of dissociation diminishes as the temperature rises. From this it follows that the free ions H and OH must have a smaller specific heat than the unionised molecules. The calculation of the ionisation of water is one of the most convincing proofs of the correctness of the theory of electrolytic dissociation, as well as of the validity of van t Hoff s osmotic pressure laws on which the deduction of these valuable equations is based. 

There is nothing m the simple form of the electrolytic dissociation theory which necessitates the conclusion that the 10ns and the undis-sociated molecules of an electrolyte should obey van t Hoff s law If they do not obey this law the law of mass action cannot be obeyed Hence the fact that solutions of strong electrolytes deviate fiom the requirements of the law of mass action is no argument against the theory of electrolytic dissociation 1... 

Because of its historic importance and the fact that all of the more recent theories of electrolytes are an outgrowth of Arrhetlius, original statement of the theory of electrolytic dissociation in the form given in equation (IS), the rest of this chapter will be devoted to a survey of some of the evidence for and against the theory. 

The Arrhenius definition, known as the theory of electrolytic dissociation (TED) appropriate for the description of aqueous solutions, was historically the first definition of this kind. It states that an acid is a substance prone to dissociation with the formation of protons, H+ ... 

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. 

Figure 3.37 Svante Arrhenius (1859-1927), who proposed the theory of electrolytic dissociations (1887), investigated the viscosity of solutions, the effect of temperature on reaction rates (1889), etc. (Published with permission from the Deutsches Museum, Munich.)...

Fig. 1.2 Svante Arrhenius (1859-1927). The Swedish physical chemist. The founder of the theory of electrolytic dissociation. The Nobel Prize Laureate in chemistry...

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