Water self ionic dissociation

In addition, the liquid undergoes self-ionic dissociation to a greater extent than any other nominally covalent pure liquid (cf. BF3.2H2O, p. 198) initial autoprotolysis is followed by rapid loss of water which can then react with a further molecule of HNO3 ... 

Finally, some chemical properties of water, such as the polarity and solva-tochromic indices, are listed in Table 1.3 for 25 °C. The self-ionic-dissociation, of water is the most important chemical property of water. Its equilibrium constant, Ky, ... 

The chemical properties of solvents that are relevant to their dissolution abilities for electrolytes and the ionic dissociation of the latter include their structuredness or self-association and their donor (electron pair donation, basicity) and acceptor (hydrogen bonding ability, acidity) properties as well as their softness. The mutual solubility with other solvents, in particular water, is also of importance as are the windows for making spectroscopic and electrochemical measurements on solutions of ions in the solvents. 

The acid-base behaviour of aqueous solutions has already been discussed (p. 48). The ionic self-dissociation of water is well established (Table 14.8) and can be formally represented as... 

As in water, neutralization in all amphiprotic solvents represents the backward reaction of self-dissociation down to the equilibrium level of the ionic product in the pure solvent. 

All the reactions discussed in the previous section could be described as acid/base phenomena, defining acids and bases quite liberally. The importance of ionic equilibria in aqueous solution was recognised in the 1880s by Arrhenius, who proposed that acids were sources of H+(aq) while bases were sources of OH-(aq), and it was soon realised that this definition was closely related to the self-dissociation of water ... 

This has been called the ionic self-dehydration reaction 29. Only four of the five equilibria, Eqs. (3) to (7) are independent and it has been found convenient to discuss the self-dissociation in terms of Eqs. (3), (5), (6), and (7). Values for the corresponding equilibrium constants are given in Table II. The values at 10° were obtained from a detailed study (4, 0 of the freezing points of solutions of metal hydrogen sulfates, water, and disulfuric acid, each of which represses the selfdissociation equilibria in a different way. Table III gives the concentration of each of the products of the self-dissociation. The total molal concentration of 0.0424 at 10° corresponds to a freezing point of 10.625°... 

The above technique was applied by Harned and his colleagues to determine acidity constants for a variety of weak acids in both water and in water-non-aqueous solvent mixtures [3]. It may also be used to determine the self-dissociation constant of water. In the case of moderately weak acids the extrapolation procedure requires a more careful consideration of the contribution of H to the ionic strength. More details can be found in the monograph by Harned and Owen [3]. 

In many organic reactions such as hydrolysis or certain rearrangements, water is the solvent and catalyst via self-dissociation, and sometimes also a reactant [11, 12]. The advantage of the use of water is that the addition of acids and bases may be avoided. This means that cleaning the effluent is easier and less expensive. The ionic product of water increases with pressure (under supercritical conditions) therefore reaction rates e.g. of acid- or base-catalyzed reactions also increase. On the other hand, the reaction of free radicals, which are undesirable during pyrolysis, decreases with pressure (see Introduction), thus high selectivities can be achieved. 

From the final model calculations - which basically describe the temperature behavior at a constant pressure correctly - a flow analysis at a medium reaction time can be used to analyze the most important reaction steps in order to get a more compact reaction mechanism. The simphfied ionic mechanism at around 350°C is shown in Fig. 7.9. The thickness of arrows symbolize the relative reaction flow of a reaction pathway from the educt point of view. They are calculated from the reaction rates of the elementary reactions. These relative amounts change (slightly) with reaction time. Here, the most important step is the protonation of glycerol. This means that the reaction rates of the ionic reactions strongly depend on the self-dissociation of water. 

S. Arrhenius, the father of the ionic theory, postulated the self-dissociation of water and defined an acid as a substance which increases the hydrogen ion... 

Ionic conduction can be observed in numerous systems liquids showing self-dissociation (e.g., water, hydrogen sulfide), solutions containing ions formed by dissociation of salts (true or real electrolytes, e.g., NaCl) or molecules (potential electrolytes, e.g., HCl) (these systems are frequently called electrolyte, obviously this convenient simplification is misleading), molten salts, ionic liquids, ionic crystals, etc. 

Besides the self-dissociation of water one should take into account the presence of traces of ionic impurities, such as the HCOs ions coming from the dissolution of CO2 in the water used to prepare the solution. Because it is very difficult to make a precise estimation of the contribution of these impurities at high temperature and pressure, it is convenient to measure the conductivity of the water used to prepare the solution under the same conditions as the solution measurements. The solvent conductivity is discounted from the specific conductivity of the solution in order to obtain the real conductivity of the electrolyte. 

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