Self-dissociation

Data on chemical properties such as self-dissociation constants for sulfuric and dideuterosulfuric acid (60,65,70,71), as well as an excellent graphical representation of physical property data of 100% H2SO4 (72), are available in the Hterature. Critical temperatures of sulfuric acid solutions are presented in Figure 10 (73). 

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... 

H2SO4.Z2H2O, are known with = 1, 2, 3, 4 (mps 8.5", -39.5". -36.4" and -28.3% respectively). Other compounds in the H2O/SO3 system are H2S2O7 (mp 36") and H2S4O13 (mp 4"). Anhydrous H2SO4 is a remarkable compound with an unusually high dielectric constant, and a very high electrical conductivity which results from the ionic self-dissociation (autoprotolysis) of the compound coupled with a proton-switch mechanism for the rapid... 

The other hydrogen halides are less tractable as solvents, as might be expected from their physical properties (p. 813), especially their low bps, short liquid ranges, low dielectric constants and negligible self-dissociation into ions. Nevertheless, they have received some attention, both for comparison with HF and as preparative media with their own special advantages.In particular, because of their low bp and consequent ease of removal, the liquid HX solvent systems have provided convenient routes to BX4, BF3C1 ,... 

Despite these reaction products there is little evidence for an ionic self-dissociation equilibrium in liquid CIF3 such as may be formally represented by 2CIF3 V— CIF2 + C1F4, and the electrical conductivity of the pure liquid (p. 828) is only of the order of 10 ohm cm. The structures of these ions are discussed more fully in subsequent sections. 

Molten I2CI6 has been much less studied as an ionizing solvent because of the high dissociation pressure of CI2 above the melt. The appreciable electrical conductivity may well indicate an ionic self-dissociation equilibrium such as... 

Autoprotolysis of the Solvent. While studying these proton transfers, there is another type that may be discussed at the same time, namely, the self-dissociation of the solvent itself. As is well known, highly purified solvents show at least a small electrical conductivity. In methanol, for example, it is generally recognized that this conductivity arises from the fact that, a certain number of protons havo been transferred according to the process... 

The smallest value of K for any acid in Table 9 is 2.36 X 10-11 for carbonic acid at 0°C. From (132) in this case we find the value J = 0.6708 electron-volt. The values of K for the self-dissociation of water, given in Table 9, are still smaller. To calculate J we use (129) instead of (91), and at 60°C obtain the value J = 1.092 electron-volts. 

When this takes place in the aqueous solution of a salt, such as NH4CI, the process is known as the hydrolysis of the salt. Formerly, this hydrolysis was regarded as due to the self-dissociation of the water... 

The entropy of the (OH)- ion may be obtained from the value of AS° for the self-dissociation of water. Whether this process is written in the form... 

Autoprotolysis of Formic Acid. The self-dissociation of pure formic acid yields, in addition to the formate ion, the positive ion (HCOOII2)+, according to... 

The Raman spectrum of the monohydrate, HN03.H20, shows it to exist as the hydroxoni-um salt, H30+N03 13. Also, according to analyses of the Raman spectrum, nitric acid exists in aq solns either as a pseudo-acid, N02.0H or as a true acid, N03".H+. In 10 molar aq soln, both acids are present in equal amounts, being caused by the self-dissociation of nitrogen pentoxide (NjOj), while in a 6 molar soln, the pseudo acid is present only to the extent of 2%. and the more dilute the soln, the less pseudo acid is present. In very coned solns, the true acid is present only in small quantities (Refs 32 33)... 

A) In addition to the more modem Bronsted and Lewis theories, it is important not to forget the classic Arrhenius theory in its modern form, the so-called solvents theory, where it can be applied, i.e., with solvents that undergo self-dissociation in this form it was originally formulated in 1949 by Jander3 in Germany and is illustrated by the following reaction equations ... 

The sequential reactions 4.1 and 4.2 represent the self-dissociation of water as the exchange of a proton between water molecules, where hydration of the proton according to reaction 4.2 is the driving force for its separation (reaction 4.1) although the proton hydration is not limited to one H20 (hydration number 1), nor is the occurrence of unhydrated OH ion realistic, the overall reaction 4.3 is generally written as the simplest form to show the principle of proton acidity. 

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. 

In the anionotropic system of sulphur dioxide (condensed below -10° C s x 16) we are concerned with an oxidotropic solvent with self-dissociation here the 02 ion does not exist in the free state, but occurs as the solvated ion SO3. A comparable situation is found in many systems of halotropy with self-dissociation, e.g., according to the overall reactions 4.30-4.33... 

As would be expected, the larger titration potential ranges offer much more scope for mutually distinguishing between individual acids or bases in amphiprotic solvents, as a consequence of self-dissociation, the potential ranges are rather limited, whereas in the aprotic protophilic solvents and "aprotic inert solvents these ranges are considerably more extensive. 

As most of the solvents in Fig. 4.1 are amphiprotic and it is interesting to know the correct vertical position of their potential ranges compared with that of water, we shall apply eqn. 4.45 to a solvent HS with self-dissociation ... 

Subsequently, Bos and Dahmen used in m-cresol65 (e = 12.29 at 25° C) a potentiometric titration method combined with conductometry. Essential precautions were the preparation of water-free m-cresol (<0.01% of water), the use of a genuine Bronsted base B, e.g., tetramethylguanidine (TMG), and the application of a glass electrode combined with an Ag-AgCl reference electrode filled with a saturated solution of Me4NCl in m-cresol. The ion product of the self-dissociation of m-cresol, Ks, was determined from the part beyond the equivalence point of the potentiometric titration curve of HBr with TMG comparison with titration curves calculated with various Ka values showed the best fit for Ks = 2 10 19... 

Nitrogen trifluoride is a kinetically inert gas, but when sparked or heated can fluorinate substrates and release nitrogen. Sulfur, for example, is quantitatively converted to SF using NF3 at 5 atm (231). Similarly, boron is converted to BF3 (159) and CF3CN to CF4 (232). Nitrogen trifluoride has some advantages over difluorine. It is safer to handle, and double-compartment bombs to avoid self-ignition are not required. However, some self-dissociation has to be measured and corrections made. 

Note added in proof. Marek and Chmelir [40b, c] found that the polymerization of isobutene in heptane by aluminium bromide is greatly accelerated by addition of titanium tetrachloride. They suggested that the polymerization by aluminium bromide only is initiated by a cation such as AlBr2+ which adds to the isobutene and which is formed by self-dissociation of the catalyst. The enhancement of the rate by titanium tetrachloride they attribute to an increase in the concentration of ions by the reaction... 

Two studies of the self-dissociation of D O have also appeared (33,34). 

In water, the hydronium ion concentration arises by the self-dissociation equilibrium (see Section 4.4) ... 

Self-consistent field, spin density, 43 183 Self-dissociation reactions, sulfuric acid and, 1 387-389... 

The next issue that arises from the weak acidity of carbon acids involves the degree of self-dissociation. In Eq. 3, the equihbrium constant is determined by measuring the concentration of the four species in the equation, but this requires that the carbon acid self-dissociates to an extent that a measurable quantity of the carbanion is formed. Again, because carbon acids are generally weak, this requirement often is not met and therefore another type of equilibrium measurement... 

M Ag, in order to calculate the self-dissociation constants. Since alkali oxides are extremely difficult to prepare in the pure state and to handle, it was decided to produce the oxide ion coulometrically. Consequently, a known current of approximately 8 microamp. was run through the cell, the oxygen electrode acting as the cathode. The current was drawn from an electronically controlled constant current device and timed so that the number of equivalents of charge passing could be calculated. This was assumed to be the number of oxide ions produced. The variation in potential with oxide ion followed the Nernst expression very precisely. 

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