Water self-dissociation constant

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]. 

The right-hand side product is equal to the self-dissociation constant for water, K ... 

Now, the self-dissociation constant of water is expressed in terms of pAf , which is defined as. 

The autoprotolysis (self dissociation) constant of water (Equation 4-4a) is given by... 

IONIZATION AND SELF-DIFFUSION. Water molecules readily dissociate to form protons and hydroxide ions, and the dissociation constant is ... 

Surfactants having an appropriate hydrophobic/hydrophilic balance (sodium bis(-2-ethylhexyl)sufosuccinate, or AOT, for example) undergo concentration-dependent self association in apolar solvents to form reversed or inverted micelles (Fig. 33) [256-262]. Reversed micelles are capable of solubilizing a large number of water molecules (AOT reversed micelles in hexane are able to take up 60 water molecules per surfactant molecule, for example). Reversed-micelle-entrapped water pools are unique they differ significantly from bulk water. At relatively small water-to-surfactant ratios (w = 8-10, where w = [H20]/[Surfactant]), all of the water molecules are strongly bound to the surfactant headgroups. Substrate solubilization in the restricted water pools of reversed micelles results in altered dissociation constants [256, 257, 263-265], reactivities [256, 258, 266], and reaction products [267]. 

The data on the rate constants are given in Table 7.15. Based on the experimental data on kd and water exchange rate constant, it is surmised that self dissociation of the complex does... 

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 effects of meso substituents on the acidities of benzimidazoles have been examined by potentiometric titration of a series of 2-substituted benzimidazoles in acetonitrile. The 2-substituents appear to exert primarily inductive effects on the acidity centre, with the pAg dependent on glass electrodes are inaccurate at pH values of > 11 in water (these compounds are very weak acids). As acetonitrile has a self-protolysis constant, pAg, of 33.3 (cf. water, pAg 14) it becomes possible to study the acidic properties of such slightly dissociated compounds. In... 

The resulting equilibrium constant is called the ionization constant, or dissociation constant, or self-ionization constant, or ion product of water, and is symbolized byAfw... 

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. 

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 self-dissociation of water and the proton/hydroxide mobilities in water Water is amphoteric at 298.15 K and 1 atm, the ionization constant (pKw,) of water is 14.004. The proton and hydroxide concentrations are so small that the water activity is almost unity. The standard enthalpy of self-dissociation is 55.81 kj mol the heat capacity -215 J and the standard volume of self-dissociation is approximately-20 cm mol T Ultrafast mid-infrared spectroscopic measurements have suggested that the first step of the autodissociation of water proceeds through an excited vibrational state of the OH bond, probably with v = 2. 

There is an alternative explanation for the conductivity enhancement of semiconductors when doped. We consider an analogue taken fi-om chemical equilibrium. The intrinsic conductivity of semiconductors can be considered to be analogous to the intrinsic conductivity of pure water given by its self dissociation. In dissociation, the ions HsO and OH are formed, however in a very low concentration. By apphcation of the law of mass action it follows that the product of their concentrations is a constant in equilibrium ... 

The value of Kv>, at STP is 1.0x10 ". The self-ionization constant of water, Ky, is thus just a special case of an acid dissociation constant. 

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