Theory of electrolytic dissociation

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 

For all monobasic acids (HCl), monoacid bases (NaOH), and their salts, i is nearly equal to 2, for dibasic acids and their salts approximately 3. 

The factor i only occurs in solutions which are good conductors of electricity, and in 1887 Arrhenius succeeded in explaining these apparent deviations from the simple laws by his electrolytic dissociation theory. The molecules of an electrolyte are broken up to a greater or less extent into their free ions, even when the solution is not conducting a current of electricity. Thus we have the equation HCl H - - CL 

For monobasic acids and monoacid bases and their salts, we have w = 2, and hence 

Substances of this kind are called binary electrolytes. For dibasic acids and their alkali salts (ternary electrolytes), we have w = 3, and therefore 

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Amplitude of a process, 114. Andrew s diagram, 173 Anisotropic bodies, 193 Aphorism of Clausius, 83, 92 Arrhenius theory of electrolytic dissociation, 301 Aschistic process, 31, 36, 51 Atmosphere, 39 Atomic energy, 26 Availability, 65, 66 Available energy, 66, 77, 80, 98, 101... 

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. 

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. 

Numerous measurements of the conductivity of aqueous solutions performed by the school of Friedrich Kohhansch (1840-1910) and the investigations of Jacobns van t Hoff (1852-1911 Nobel prize, 1901) on the osmotic pressure of solutions led the young Swedish physicist Svante August Arrhenius (1859-1927 Nobel prize, 1903) to establish in 1884 in his thesis the main ideas of his famous theory of electrolytic dissociation of acids, alkalis, and salts in solutions. Despite the sceptitism of some chemists, this theory was generally accepted toward the end of the centnry. 

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... 

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],... 

These phenomena that were previously considered anomalies of the mentioned colligative properties of the solutions, have been dealt with by Arrhenius in his effort to explain such anomalies by his well known theory of electrolytic dissociation. According to this explanation the molecules of a dissolved electrolyte partly split to form smaller particles, i. e. ions, which from the thermodynamic point of view are as effective as the undissociated molecules themselves. As the number of particles of matter is thus greater, the manifestations of colligative properties are increased, compared to what they would be with an undissociated electrolyte. 

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 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 ... 

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. 

This theory of electrolytic dissociation, or the ionic theory, attracted little attention until 1887 when vanT IIoff s classical paper on the theory of solutions was published. The latter author had shown that the ideal gas law equation, with osmotic pressure in place of gas pressure, was applicable to dilute solutions of non-electrolytes, but that electrolytic solutions showed considerable deviations. For example, the osmotic effect, as measured by depression of the freezing point or in other ways, of hydrochloric acid, alkali chlorides and hydroxides was nearly twice as great as the value to be expected from the gas law equation in some cases, e.g., barium hydroxide, and potassium sulfate and oxalate, the discrepancy was even greater. No explanation of these facts was offered by vanT Iloff, but he introduced an empirical factor i into the gas law equation for electrolytic solutions, thus... 

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 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. 

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