Autodissociation

Table 1 Hsts many of acetamide s important physical properties. Acetamide, CH2CONH2, dissolves easily ia water, exhibiting amphoteric behavior. It is slow to hydroly2e unless an acid or base is present. The autodissociation constant is about 3.2 x 10 at 94°C. It combines with acids, eg, HBr, HCl, HNO, to form soHd complexes. The chemistry of metal salts ia acetamide melts has been researched with a view to developing electroplating methods. The hterature of acetamide melts and complexes, their electrochemistry and spectroscopy, has been critically reviewed (9).

According to the Bronsted and Lowry definition, water itself can act as both an acid and a base as the result of autodissociation ... 

Any substance which increases the concentration of hydrogen ions in an aqueous solution above the level provided by the autodissociation of water itself is, by Bronsted and Lowry s definition, an acid. For pure water, the ionic product (Kw) suggests that at equilibrium at 25°C, the following holds true ... 

Because of the definition of acidity and pH, we can conclude that liquids with a pH of less than 7 are acidic, and have excess hydrogen ions, and that pHs greater than 7 are basic, with excess hydroxyl ions. Because of the autodissociation of water, however, the concentrations of hydrogen ions and hydroxyl ions (i.e., pH and pOH) are inextricably linked, but calculable through the above simple formula. 

As with Ps , there is only one bound state of Ps2 but there exist Rydberg series of autodissociating states arising from the attractive interaction between one of the positrons and the residual Ps- (or between one of the electrons and the charge conjugate of Ps ). The positions and widths of several of these states were determined by Ho (1989) using the complex coordinate rotation method. To date Ps2 has not been observed in the laboratory. 

Figure 7.3 Fluorescence excitation (a and b) and emission (c and d) spectra of the 7-amino-4-methylcoumami -DEVI ) complex (spectra a and c) and of 7-amino-4-methylcoumarin, obtained after autodissociation of the 7-amino-4-methylcoumarin-DEVD complex (spectra b and d). DEVD = Asp-Glu-Val-Asp. 7-Ami no-4-methylcoumarin-DEVD complex (a) 7ern = 470 nm and Xgmax = 323 nm. (c) Aex = 260 nm and Xmax = 393 nm. 7-Amino-4-methylcoumarin (b) Xem = 440 nm and Xmax = 350 nm. (d) kex = 300 nm and Amax = 438 nm.

Although for the molecules in Fig. 4 gissociative attachment processes are expected to be fast (<<10 sec), for C2C1 at 0.0 eV (and for a number of other autodissociating long-lived parent negative ions) both the autodissociation and the autodetachment processes are slow, as a result of vibrational redistribution of internal energy ( ). 

Despite the favorable properties of acetonitrile as a solvent, its use for equilibrium acidity measurements has its definite limitations. The pK range that is tolerable is limited at the high end by onset of solvent deprotonation, and at the low end by substrate autodissociation, as has been implicated for HCo(CO)4 [14a] and TpCr(CO)3H [22b]. These limitations can be overcome by the choice of a less polar solvent, e.g. 1,2-dichloroethane (DCE), dichloromethane, or THE. To make reliable, quantitative comparisons of thermodynamic data obtained in different solvents, it is necessary to link the acidity scales and electrode potential references in the different solvents. This has all too often proven to be a far from trivial task. Although, in principle, 1 1 relationship between the acidity scales in different solvents never exists, pK differences between closely related compounds are often almost constant when compared in different solvents. This is because their solvation properties are similar, because of similarities in size and charge distribution. In less... 

Aprotic nonaqueous solutions require a similar approach, but with a different definition of acid and base. The solvent system definition applies to any solvent that can dissociate into a cation and an anion (autodissociation), where the cation resulting from autodissociation of the solvent is the acid and the anion is the base. Solutes that increase the concentration of the cation of the solvent are considered acids and solutes that increase the concentration of the anion are considered bases. 

The classic solvent system is water, which undergoes autodissociation ... 

The solvent system approach can also be used with solvents that do not contain hydrogen. For example, BrF3 also undergoes autodissociation ... 

IF5 undergoes autodissociation into IF4 -t IF5 . SbFs acts as an acid and KF acts as a base when dissolved in IF5. Write balanced chemical equations for these reactions. 

Table 6-2 gives some properties of common solvents. The pK,on is the autodissociation constant for the pure solvent, indicating that, among these acids, sulfuric acid dissociates much more readily than any of the others, and that acetonitrile is least likely to autodissociate. The boiling points are given to provide an estimate of the conditions under which each solvent might be used. 

Even in a doped semiconductor, mobile electrons and holes are both present, although one carrier type is predominant. For example, in a sample of silicon doped with arsenic (w-type doping), the concentrations of mobile electrons are slightly less than the concentration of arsenic atoms (usually expressed in terms of atoms/cm ), and the concentrations of mobile holes are extremely low. Interestingly, the concentrations of electrons and holes always follow an equilibrium expression that is entirely analogous to that for the autodissociation of water into H and OH ions (Chapter 18) that is,... 

Warshel and coworkers have employed the empirical valence-bond (EVB) method [49] to simulate FERs for PT [50] and other reactions [51]. The PT step between two water molecules in the mechanism of the reaction catalysed by carbonic anhy-drase was described as an effective two-state problem involving reactant-like (H0H)(0H2) and product-like (HO )(HOH2+) VB structures [50a], Diabatic energy curves for these two VB structures were calibrated to reproduce the experimental free energy change for autodissociation in water, and the mixing of the... 

The initial work of Warshel, Hwang and Aqvist [50a] for PT in water and in carbonic anhydrase (a zinc metalloenzyme) reported FERs simulated in two ways. First, the autodissociation in water catalysed by a metal ion was studied using explicit Zn +, Mg +, Ca + and Na+ cations. Second, the energy of the product diabatic curve (for the ionic VB structure) was shifted vertically in order to change... 

Preliminary experiments confirmed that bisulfite solutions autodissociate into elemental sulfur and sulfate, and that the reverse reaction also proceeds readily (5). However, it was... 

Addition of NH4CI to POCl3 will result in an increase of anion particles formed by the autodissociation of the solvent ... 

The preconditions of another approach to the treatment of acid-base concepts can be found in the classic solvosystem concept described above. Careful reading of this concept shows that the acid-base definition connects the terms acid and base only with the process of autodissociation of a molecular solvent or of one capable of ionization. Nevertheless, it is obvious that acid-base interactions can occur in those solvents, which are not able to form acid and base owing to a dissociation process. Aprotic solvents may serve as a typical example of solvents of such a kind another case of solvents incapable of the acid-base autodissociation takes place if we consider the Lux acid-base equilibria in molten oxygen-free media. Therefore, in relation to any given acid or base there exist two kinds of solvents of differing auto-dissociation ability with the formation of the said acid or base [36, 37, 44, 45]. 

Acid-base processes in molten ionic media are hardly described in the framework of the classic solvosystem concept. The reason for the seeming principal distinction of ionic melts from room-temperature molecular solvents consists in the limitations contained in the solvosystem concept. The main reason is that the division of substances into acids and bases is performed in relation to their reactions with the products of a molecular solvent autodissociation and the degree of this auto-dissociation, in the pure solvent is negligible. On the contrary, an ionic melt is a case of a completely ionized solvent, and this is the fact that does not allow one to apply the solvosystem concept fruitfully for studies of acid-base interactions in this kind of liquid media. 

The Lux acid-base interactions in oxygen-containing ionic melts have been studied more extensively than those in the media of the second kind. The complications are caused, first by the superimposition of the acid-base equilibrium of the melt-solvents themselves (autodissociation), which, according to what we have said in Part 1, belongs to the solvents of the first kind. 

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