Self ionization

Initially, it was proposed that SO2 underwent self-ioniza-tion according to eq. 9.14. However, this equilibrium requires the separation of doubly charged ions, in contrast to the singly charged ions involved in other self-ionization equilibria described in Sections 9.6-9.11. Two observables suggest that no self-ionization occurs (i) conductance data are not consistent with the presence of ions in liquid SO2 (ii) when labelled SOCI2 is dissolved in liquid SO2, neither 

Liquid NH3 undergoes self-ionization (eq. 9.12), and the small value of (Table 9.4) indicates that the equih-brium lies far over to the left-hand side. The [NH4]+ and [NH2] ions have ionic mobilities approximately equal to those of alkaU metal and halide ions. This contrasts with the situation in water, in which [H30] and [OH] are much more mobile than other singly charged ions. 

Liquid ammonia has been widely studied, and in this section we discuss its properties and the types of reactions that occur in it, making comparisons between Uquid ammonia and water. 

We described above some precipitations that differ in Uquid NH3 and H2O. Equation 9.16 shows a further example the solubility of KCl is 0.04g per lOOg NH3, compared with 34.4 g per 100 g H2O. 

In water, neutralization reactions follow the general reaction 9.17. The solvent-oriented deliniticHi of acids and bases allows us to write an analogous reaction (eq. 9.18) for a neutraUzation process in liquid NH3. 

Bromine trifliioride -Dinitrogen tetraoxide -Fluoiosuliuric acid -Hydrogen fluoride -Sulfuric acid -Sulfur dioxide -Ammonia Wat er - 

Acid -I- Base — Salt -f Water in aqueous solution (8.17) 

in liquid NH3, reaction 9.19 is a neutralization process which may be followed by conductivity or potentiometry, or by the use of an indicator such as phenolphthalein, 9.10. This indicator is colourless but is deprotonated by a strong base such as [NH2] to give a red anion just as it is by [OH] in aqueous solution. 

Liquid NH3 is an ideal solvent for reactions requiring a strong base, since the amide ion is strongly basic. 

As we discussed in Section 9.4, the behaviour of adds is solvent-dependent. In aqueous solution, sulfamic acid, H2NSO2OH, 9.11, behaves as a monobasic acid according to equation 9.20, but in liquid NH3 it can function as a dibasic acid (equation 9.21). 

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Reaction (5.N) describes the nylon salt nylon equilibrium. Reactions (5.0) and (5.P) show proton transfer with water between carboxyl and amine groups. Since proton transfer equilibria are involved, the self-ionization of water, reaction (5.Q), must also be included. Especially in the presence of acidic catalysts, reactions (5.R) and (5.S) are the equilibria of the acid-catalyzed intermediate described in general in reaction (5.G). The main point in including all of these equilibria is to indicate that the precise concentration of A and B... 

Acid-Base Reactions. Anhydrous hydrazine undergoes self-ionization to a slight extent, forming the hydrazinium, N2H5, and the hydrazide, N H7ions ... 

Despite this enormous viscosity, fused H3PO4 (and D3PO4) conduct electricity extremely well and this has been shown to arise from extensive self-ionization (autoprotolysis) coupled with a proton-switch conduction mechanism for the... 

The low conductivities imply almost negligible self-ionization according to the formal scheme ... 

In contrast to this, consider next a solution of sodium acetate. From vSec. 09 we know that in such a solution the thermal agitation raises a certain number of protons from the solvent molecules to the vacant proton levels of the (CH GOO) ions. In the aqueous solution of such a salt, this process is known as the hydrolysis of the salt and is traditionally regarded as a result of the self-ionization of the water. In Fig. 36, however, it is clear that in the proton transfer... 

The studies of Bunton et al (Ref 38) using heavy oxygen (18Q) are particularly noteworthy in elucidating the self-ionization process. With moderately dilute nitric acid they found that nitrations required presence of nitrous acid. Ingold and co-workers (Ref 36c) suggest that the action of nitrous acid is as follows ... 

At the first stoichiometric point of the titration, aii the diprotic acid has been converted to its conjugate base, H A. This amphiprotic anion can react with itseif, analogous to the self-ionization of water ... 

When water undergoes self-ionization, a range of cationic species are formed, the simplest of which is the hydronium ion, HjO (Clever, 1963). This ion has been detected experimentally by a range of techniques including mass spectrometry (Cunningham, Payzant Kebarle, 1972), as have ions of the type H+ (HaO) with values of n up to 8. Monte-Carlo calculations show that HjO ions exist in hydrated clusters surrounded by three or four water molecules in the hydration shell (Kochanski, 1985). These ions have only a short lifetime, since the proton is highly mobile and may be readily transferred from one water molecule to another. The time taken for such a transfer is typically of the order of 10 s provided that the receiving molecule of water is correctly oriented. 

The molecules of amphiprotic solvents which are the most important will be designated as SH. Self-ionization occurs to a small degree in these solvents according to the equation... 

The activity of the solvent molecule HS in a single-component solvent is constant and is included in Kus. The concentration of ions is mostly quite low. For example, self-ionization occurs in water according to the equation 2H20— H30+ + OH". The conductivity of pure water at 18°C is only 3.8 X 10"8 Q"1 cm-1, yielding a degree of self-ionization of 1.4xl0"19. Thus, one H30+ or OH" ion is present for every 7.2 x 108 molecules of water. Some values of Kus are listed in Table 1.5 and the temperature dependence of the ion product of water Kw is given in Table 1.6. 

Table 1.5 Self-ionization constants of solvents. (According to B. Tremillon)... 

Simple self-ionization cannot be assumed a priori. The situation is sometimes more complicated, e.g. 

If the solvent is not protogenic but protophilic (acetone, dioxan, tetrahydrofuran, dimethylformamide, etc.), self-ionization obviously does not occur. Consequently, the dissolved acids are dissociated to a greater or lesser degree but dissolved bases do not undergo protolysis. Thus, there can exist only strong acids but no strong bases in these solvents. The pH is not defined for a solution that does not contain a dissolved acid (i.e. in the pure solvent or in the solution of a base). The pKA value can be defined but not... 

The evidence for this self-ionization is the conductivity of the anhydride which, though low, exceeds that of acetic acid.204 208 The ionization of acetic anhydride into acetylium and acetate ions is analogous to the ionization of water molecules into protons and... 

It will be seen from these examples that the process of self-ionization in a protonic solvent involves the transfer of a proton from one solvent molecule to another. Thus, the solvent is acting simultaneously as a Lowry-Bronsted acid and as a base. 

Kv is the self-ionization constant for water (Table 3.2) and equation (3.18) reflects the not surprising inverse relation between Ka and Kh. It is only when Ka and Kv for a compound are of different magnitudes that it may be classified as an acid or a base. An example which is difficult to classify is hypoiodous acid (HOI) where K = 2.5 x lO11 mol dm 3 andKh = 3.2 x 10 10 mol dm3. Although Kb has been widely used in the past, it is a quantity which is largely redundant, for Ka (or pKa) may be used to express the strength of bases as well as acids, see Table 3.3. 

Zwitterion self-ionization of amino acid to produce COO" and -NH3+... 

Polarography in pure liquid acids such as MSA is relatively simple because the ions resulting from the self-ionization of the acid provide the conductivity needed, and the... 

Proposition 1 The initiator solutions of aluminium halide, A1X3, in alkyl halide RX contain ions formed by the self-ionization of the aluminium halide ... 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

Acting as an acid pKa of H2O Water is a very weak acid and can undergo self-ionization as follows ... 

In many respects the chlorine oxyfluorides resemble the chlorine fluorides. For example, they exhibit little or no self-ionization, but are amphoteric. With strong Lewis acids or bases they can form stable adducts. The tendency to form adducts was found (64) not to be so much a function of the relative acidity of the parent chlorine oxyfluoride but rather to depend on the structure of the amphoteric molecule and of that of the anion or the cation formed. The preferred structures are the energetically favored tetrahedron and octahedron. Consequently, a trigonal bipyramidal molecule, such as CIF3O (64), exhibits a pronounced tendency to form either a stable pseudotetrahedral cation or a pseudo-octahedral anion ... 

There is evidence that Lewis acids initiate a slow polymerization in some (but not most) systems by a self-ionization process in addition to the coinitiation process [Balogh et al., 1994 Grattan and Plesch, 1980 Masure et al., 1978, 1980], Two mechanisms are possible for self-initiation. One involves bimolecular ionization... 

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