Water amphoteric character

A water molecule has amphoteric character. This means it can act as both an acid and a base. The autoionization equilibrium process in water. 

The amphoteric character of water (i.e., the ability to act either as an acid or as a base) makes water so special. While this renders the use of water as a solvent in acid-or base-sensitive reactions problematic, the possibility to have the solvent as a reactant in acid- or base-initiated reactions is often desirable. These qualities led chemists to rediscover water as a solvent in organic chemistry. Unfortunately, from a chemical point of view, not all transformations are feasible in aqueous solvent systems. Many reagents decompose when brought into contact with water while many others are insoluble. Consequently, it is not surprising that water has not been a very popular solvent in organic chemistry in the past, but this picture is changing rapidly. 

Drying, the removal of water, is a special problem because water is a unique compound and there is a lot of it around. The features which together make water unique include a high dipole moment and polarisability, its amphoteric character, its ability to act as donor and acceptor in hydrogen bond formation, and its smallness. 

J. Kendall, J. E. Booze, and J. C. Andrews 3 have shown that the formation of hydrates, in the sense of water of crystallization, with the weak acids very seldom occurs, and when hydrates are formed, the acid has the amphoteric character of a phenol. There is also a regular increase in the tendency of an acid to form hydrates, as the strength of the acid increases, until, with the strong acids, well-defined stable hydrates appear. The complexity and stability of the hydrates increase with the strength of the acid. These facts are in harmony with the weak acid nature of water. 

Zinc(II) oxide is insoluble in neutral water, but soluble in dilute acids Zn2+(aq) is stable at low pH values. If an acid solution containing Zn2+ is treated with an alkali, Zn(OH)2 precipitates around neutral pH but redissolves at a pH of about 14, with formation of Zn(OH)J(aq) and Zn(OH)4 (aq). The fact that Zn(II) forms cationic aqueous species in acid solution and anionic species at alkaline pH determines the amphoteric character of ZnO. 

Amphoteric Character of the Hydroxides of Tin and Lead. Dilute 2 cc. of 1N SnCl2 with 10 cc. of water. From a 10-cc. graduate add 6iV NaOH, noting the amount required... 

In conductometric measurements of two-component system where HN03 was one of the components, Usanovich and his co-workers [50-50c] confirmed the view that nitric acid has an amphoteric character-it can be mi acid or a base depending on the other component. Thus, it acts as a base towMds sulphuric acid and as a strong acid towards water. It also acts as an acid towards CH3OOH Mid CCl3COOH and combines with these compounds, but behaves as a base with CH2ClCOOH and CHCl2COOH. I... 

It is generally known that metal oxide surface is covered with hydroxyl groups when oxide is placed in water. The presence of two free electron pairs of oxygen atom and possibility of hydrogen ion dissociation is the evidence of amphoteric character of these groups. On account of this, the most useful parameter in description of the water/metal oxide interface is pH of the solution being in contact with the surface. Adsorption of H" " or OH ions causes protonization or deprotonization of the surface according to the Eqs. (31a) and (31b). 

Cispentacin has an amphoteric character it is readily soluble in water, slightly soluble in methanol, and insoluble in acetone or ethyl acetate. It gives a positive ninhydrin test. Its accurate mass spectrum shows that cispentacin has the molecular formula C6HMN02. The most important spectral data are as follows ... 

In the case of water, the pairs H30 /H20 and H2O/OH must be taken into account due to the amphoteric character of the water. The resulting total amotmt is... 

The amphoteric character of water offers a way to standardize the strength of acids or bases. For that purpose, we consider how an acidic or basic substance reacts with water as a standard reaction partner following the general scheme Ai + H2O Bi + HsO. This equilibrium is characterized by an equilibrium constant K, the acid dissociation constant. Often the common logarithm of Ka, pfCA, is used. The corresponding formulation of the law of mass action is given Eq. (2.22). 

Actually, in aqueous solutions, the differences are theoretical rather than phenomenological. The amphoteric character of water provides an example of this assertion. In the Arrhenius theory, water is amphoteric because it simultaneously liberates one proton and one hydroxide ion per molecule ... 

In the Brpnsted theory, the amphoteric character requires the participation of two water molecules to be explained ... 

The first molecule plays the part of a base, the second that of an acid. Hence, the amphoteric character of water results from a proton exchange between two different couples in which the water molecule is one member of each of them the couples H3O+/H2O and H20/0H. Again, both theories are different in principle, but in the end, the formed species are the same and the participation of two water molecules is not detectable. 

Titanium dioxide (Ti02) surfaces are covered with surface hydroxyls of an amphoteric character [268-270]. Adsorption of water leads to the formation of hydroxyls on the anatase single crystal surface (Fig. 5.3) [270]. 

Johnston, H.L., Cuta, E, and Garrett, A.B. (1933) The solubility of silver oxide in water, in alkali and in alkaline salt solutions. The amphoteric character of silver hydroxide. / Am. Chem. Soc., 55, 2311-2325. 

In between these two oxides comes AI2O3, which has negligible solubility in pure water, but reveals amphoteric character by dissolving in both acids and bases ... 

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