Solvents self-ionization

Aurothiopropanolsulfonate, 36 18-19 [Au6(nPT) ], 40 444-445 Autoprotolysis, see Solvents, self-ionization Autotrophic bacteria, 36 105 growth, bacterial, 45 359-362 Axial ligands, substitution properties in quad-ruply bridged dinuclear complexes, 40 232-234... 

Because of the unpaired electron this is param netic and brown. Liquid N2O4 has been widely studied as a nonaqueous solvent (self-ionizes to NO and NO3"). Dinitro-gen tetroxide, along with other nitrogen oxides, is a product of combustion engines and is thought to be involved in the depletion of stratospheric ozone. 

If an acid dissolved in a solvent is stronger than the acid formed from the solvent self-ionization then the leveling of acidic properties takes place. It leads to complete solvolysis of the dissolved acid with the formation of an equivalent quantity of acid from the solvent, therefore the amount of the conjugated base equivalent to initial acid concentration is formed and this base does not reveal basic properties at all. For example, in aqueous solution of HCIO4, the following process takes place ... 

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 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)... 

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... 

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. 

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). 

The solvents that are leveling to both acids and bases are self-ionized solvents, e.g., water, ammonia, alcohols, carboxylic acids, nitric... 

Differentiating solvents are solvents in which neither the acidity of acids nor the basicity of bases is limited by the nature of the solvent. These solvents are not self-ionized. The aliphatic hydrocarbons and the halogenated hydrocarbons are such solvents. 

Fortunately, many of the common solvents by themselves are capable of acting as acids and bases. These amphoteric or amphiprotic solvents undergo self-ionization [e.g., Eqs. (1.2) and (1.3)], which can be formulated in a general way as in Eq. (1.4). 

Autoprotolysis constant — The ion-product calculated from the ion activities of the conjugate acidic and basic species of an -> amphiprotic solvent (SH). The chemical equation of such self-ionization reactions can be schematized as 2HS H2S+ + S , where H2S+ is the conjugate cation, S the conjugate anion. The autoprotolysis constant can be formulated as JCauto = [H2S+] ... 

Ion product — A temperature-dependent constant related to pure substances that can dissociate forming ions and remain in equilibrium with them. It is the product of the ion activities raised to the stoichiometric coefficients of such ionic species in former pure substance. Since the concentration of the pure substance is practically a constant, it is not included in this equilibrium expression. Common pure substances characterized by an ion-product constant are -> amphiprotic solvents, and those salts that are partially dissolved in a given solvent. In the latter case, the ion product is synonymous with solubility product. The following table (Table 1) summarizes self-ionization ionic products and - autoprotolysis constants of some - amphiprotic solvents [i]. 

The pure liquid (bp -10°C) is a useful nonaqueous solvent despite its low dielectric constant (—15), and lack of any self-ionization. It is particularly useful as a solvent for superacid systems. 

Some ionizing solvents are of major importance in analytical chemistry whilst others are of peripheral interest. A useful subdivision is into protonic solvents such as water and the common acids, or non-protonic solvents which do not have protons available. Typical of the latter subgroup would be sulphur dioxide and bromine trifluoride. Non-protonic ionizing solvents have little application in chemical analysis and subsequent discussions will be restricted to protonic solvents. Ionizing solvents have one property in common, self-ionization, which reflects their ability to produce ionization of a solute some typical examples are given in table 3.2. Equilibrium constants for these reactions are known as self-ionization constants. 

Increasing basicity or acidity of the solvent displaces the equilibria (3-8) and (3-10) to the right. The addition of these two equations gives a new equilibrium describing the self-ionization (autoprotolysis) of the solvent. 

Self-ionizing solvents possessing both acid and base characteristics e.g. water) are designated amphiprotic solvents, in contrast to aprotic solvents, which do not self-... 

This method is based on the ability of water to dissolve the substances at increased temperature and pressure. The use of water as a solvent is based on self-ionization 2H2O -> H3O " + OH. ... 

The properties of all other hydrogen hahdes are far removed from those of AHF and, therefore, their use as nonaqueous solvents is rather limited. Nevertheless, they are used for physical studies of solutions and for some synthetic purposes, such as in the formation of salts with HCl2 or BCU and related anions. Anhydrous HX can be considered to undergo self-ionization (equation 90) and can therefore be used to perform reactions of an acid-base nature, and solvolytic and redox reactions (equations 91-93). 

V. Outer-Sphere Interactions, Association and Self-ionization of Solvents. ... 

A common feature of self-ionizing solvents are the amphoteric properties of the solvent molecules, since they may act both as EPD and as EPA (27). It appears therefore that outer-sphere effects, similar to those existing between polarized solvent molecules of solvated ions, occur in the pure liquids resulting in mutual polarization of solvent molecules within associated units. From the functional point of view this is described in the following way ... 

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