Dissociation theories

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. 

Hydrogen was recognized as the essential element in acids by H. Davy after his work on the hydrohalic acids, and theories of acids and bases have played an important role ever since. The electrolytic dissociation theory of S. A. Arrhenius and W. Ostwald in the 1880s, the introduction of the pH scale for hydrogen-ion concentrations by S. P. L. Sprensen in 1909, the theory of acid-base titrations and indicators, and J. N. Brdnsted s fruitful concept of acids and conjugate bases as proton donors and acceptors (1923) are other land marks (see p. 48). The di.scovery of ortho- and para-hydrogen in 1924, closely followed by the discovery of heavy hydrogen (deuterium) and... 

Stoyanov, A.V. and Righetti, P.G., Ampholyte dissociation theory and properties of ampholyte aqueous solutions, Electrophoresis, 18, 1944, 1997. 

Figure 9. The V o/pH response of AI2O3 presented in a reduced fashion, to remove the dominant linear response Vo = Vo + 48(pi — 8) (mV). The various electrolytes at 0.1M concentration are 0 NaCl, A NaH2P04/Na2HP04, Merck buffer. Line a is the site-dissociation theory without counterion adsorption, and the other lines include various amounts of counterion adsorption. Reproduced with permission from Ref. (14). Copyright 1983, North Holland.

Nernst s point of entry into ionic and electronic theories in chemistry, then, was electrolysis and solution theory, in the mainstream of the "Ionist" dissociation theory. Indeed, van t Hoff similarly proposed an ionic theory of the polar molecule in 1895, speculating on the binding forces between 0+ and O- ions in the 02 molecule. 114... 

It has been shown that the interpretation of catalytic reactions involving group VIII transition metals in terms of n complex adsorption possesses considerable advantages over classical theories by providing a link between theoretical parameters and chemical properties of aromatic reagents and catalysts. The concept has led to the formulation of a number of reaction mechanisms. In heavy water exchange the dissociative tt complex substitution mechanism appears to predominate it could also play a major role when deuterium gas is used as the second reagent. The dissociative mechanism resolves the main difficulties of the classical associative and dissociative theories, in particular the occurrence... 

Svante August Arrhenius Sweden electrolytic dissociation theory... 

He made major contributions to electrochemistry, thermodynamics, and photochemistry. Nernsfs early studies in electrochemistry were inspired by Arrhenius dissociation theory which first recognized the importance of ions in solution His heat theorem, known as the Third Law of Thermodynamics, was developed in 1906. In 1918 his studies of photochemistry led him to his atom chain reaction theory. In laoer years, he occupied himself with astrophysical theories, a field in w hich the heat theorem had important applications. 

Davies, C. W. (1930), The Conductivity of Solutions and the Modern Dissociation Theory, J. Wiley Sons, New York, NY. 

The most important conclusions of these dynamical studies is that van der Waals clusters behave in a statistical manner and that IVR/VP kinetics are given by standard vibrational relaxation theories (Beswick and Jortner 1981 Jortner et al. 1988 Lin 1980 Mukamel and Jortner 1977) and unimolecular dissociation theories (Forst 1973 Gilbert and Smith 1990 Kelley and Bernstein 1986 Levine and Bernstein 1987 Pritchard 1984 Robinson and Holbrook 1972 Steinfeld et al. 1989). One can even arrive at a prediction for final chromophore product state distributions based on low energy chromophore modes. If rIVR tvp [4EA(Ar)i], a statistical distribution of cluster states is not achieved and vibrational population of the cluster does not reflect an internal equilibrium distribution of vibrational energy between vdW and chromophore states. If tvp rIVR, and internal vibrational equilibrium between the vibrational modes is established, and the relative intensities of the Ar = 0 torsional sequence bands of the bare chromophore following IVR/VP can be accurately calculated. A statisticsl sequential IVR/VP model readily explains the data set (i.e., rates, intensities, final product state distributions) for these clusters. 

The identicalness of the ionization sites in a linear polyelectrolyte (Tanford, 1961) stimulated the interest of Walter and Jacon (1994) in a possible relationship between Kz and M of ionic polysaccharides displaying the characteristic titration curve of a weak, monobasic acid. Without any theoretical assumption, Eq. (S.4) was derived from simple algebra by combining elementary principles of the dissociation theory of weak acids with polymer segment theory ... 

There is nothing new in principle about the use of isotopes as an aid to chemistry. For twenty years the radioactive elements have been used as indicators to study adsorption, solubility, volatility, distribution, and other phenomena of physical chemistry. Distribution of heavy radioactive atoms in plants has been studied through the relative amount of ionization found in the different parts. The ionization theory was supported by dissolving radioactive lead chloride in an aqueous solution of ordinary lead nitrate and then crystallizing out the lead chloride. The radioactive lead was found to be equally distributed between the two salts. In aqueous solution the two different kinds of lead are free to exchange anions, as predicted from the electrolytic dissociation theory. With un-ionized compounds of lead it was found that exchange does not take place. 

For higher ionic strength, e g. highly saline waters the PITZER equation can be used (Pitzer 1973). This semi-empirical model is based also on the DEBYE-HUCKEL equation, but additionally integrates virial equations (vires = Latin for forces), that describe ion interactions (intermolecular forces). Compared with the ion dissociation theory the calculation is much more complicated and requires a... 

Fig. 4 to Fig. 8 show the severe divergence for activity coefficients such as given here for calcium, chloride, sulfate, sodium and water ions, calculated with different equations. The activity coefficients were calculated applying Eq. 13 to Eq. 17 for the corresponding ion dissociation theories, whereas the values for the PITZER equations were gained using the program PHRQPITZ. The limit of validity of each theory is clearly shown. 

The most common approach used by geochemical modeling codes to describe the water-gas-rock-interaction in aquatic systems is the ion dissociation theory outlined briefly in chapter 1.1.2.6.1. However, reliable results can only be expected up to ionic strengths between 0.5 and 1 mol/L. If the ionic strength is exceeding this level, the ion interaction theory (e.g. PITZER equations, chapter 1.1.2.6.2) may solve the problem and computer codes have to be based on this theory. The species distribution can be calculated from thermodynamic data sets using two different approaches (chapter 2.1.4) ... 

Furthermore, compared to the PHREEQC version from 1995, it was already possible to model kinetically controlled reactions with EQ 3/6. An advantage of EQ 3/6 over the recent PHREEQC version is that it can use both the ion dissociation theory and the PITZER equations for solutions with higher ionic strengths. 

For the species in solution (SOLUTIONSPECIES, Table 23), listed in the top row with current number, solubility constant log k and enthalpy delta h are given in kcal/mol or kJ/mol at a temperature of 25 °C. Using the sub-key-word gamma parameters for the calculation of the activity coefficient y according the WATEQ-DEBYE-HUCKEL ion dissociation theory (compare to chapter 1.1.2.6.1) are given. With the sub-key-word analytical , coefficients At to A5 are defined to calculate the temperature dependence of the solubility-product constant. 

The difficulty in explaining the effects of inorganic solutes on the physical properties of solutions led in 1884 to Arrhenius theory of incomplete and complete dissociation of ionic solutes (electrolytes, ionophores) into cations and anions in solution, which was only very reluctantly accepted by his contemporaries. Arrhenius derived his dissociation theory from comparison of the results obtained by measurements of electroconductivity and osmotic pressure of dilute electrolyte solutions [6]. 

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. 

Whilst for this case, without the aid of the dissociation theory, there is hardly anything to be said, that theory pictures a mutual action of the two acids which explains that the properties of the solutions—conductivity, rate of inversion of cane sugar, c.—are not the mean between those of two solutions containing the acids separately. When one acid is mixed with another, the increased quantity of H-ions causes a displacement of equilibrium which can be followed out numerically. 

The relation so proved is actually that which is to be expected from the dissociation theory. 

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. 

A method of this kind, dating from times when the electrolytic dissociation theory had not become accepted, rests on the following basis. To an electrolyte the relation... 

Dissociation Theory.—According to Ostwald the color changes in phenolphthalein are explained as due to electrolytic dissociation, the negative ion of the salt being colored. In the phenolphthalein itself no dissociation occurs and the compound is thus colorless in neutral or acid solutions. When a salt is formed dissociation takes place and the colored ions produce a colored solution. This does not seem quite satisfactory in the case of the tri-sodium salt (p. 754) which evidently does not dissociate as the solution is colorless. This point is explained by the effect of the excess of alkali in retarding dissociation. 

Raoult established his law by a large number of determinations in which the nature of solvent and solute were varied widely. The most characteristic of his results for dilute solutions are given in the following tables. The law was found to hold good in all cases, except aqueous solutions of salts. We shall see later that the exceptions can be accounted for satisfactorily by the electrolytic dissociation theory. 

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... 

According to the electrolytic dissociation theory, electrolytes in aqueous solution are completely or partially dissociated into ions. Thus the acid HA splits into H+ ions and A ions, BOH into B+ and OH ions, and the salt BA into B+ ions and A ions. Hence equation (1) may be written more appropriately in the following form ... 

Most dissociation constants recorded in the literature have been calculated on the basis of the old dissociation theory of Arrhenius. It is therefore of great practical interest to evaluate these data critically and to see how the thermodynamic dissociation constants may be calculated from them. Only the three most important methods for determining dissociation constants will be discussed. Sufficient information has been given... 

The procedure is based upon the assumption that two different solutions have the same concentration of hydrogen ions if they produce equal color intensities of a given indicator. The simple dissociation theory of indicators provides that... 

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