Group amphoteric oxides

Lead(II) oxide is the most basic oxide formed by a Group IV element. It dissolves easily in acids to give lead(II) salts but it also dissolves slowly in alkalis to give hydroxoplumbates(II) and must, therefore, be classed as an amphoteric oxide, for example ... 

Nitrogen is unusual in forming so many oxides. The acidity of the Group V oxides falls from phosphorus, whose oxides are acidic, through arsenic and antimony whose oxides are amphoteric, to the basic oxide ofbismuth. This change is in accordance with the change from the non-metallic element, phosphorus, to the essentially metallic element, bismuth. The +5 oxides are found, in each case, to be more acidic than the corresponding + 3 oxides. 

The product of the second reaction is sodium aluminate, which contains the alumi-nate ion, Al(OH)4. Other main-group elements that form amphoteric oxides are shown in Fig. 10.7. The acidic, amphoteric, or basic character of the oxides of the d-block metals depends on their oxidation state (Fig. 10.8 also see Chapter 16). 

Boron, a metalloid with largely nonmetallic properties, has acidic oxides. Aluminum, its metallic neighbor, has amphoteric oxides (like its diagonal neighbor in Group 2, beryllium). The oxides of both elements are important in their own right, as sources of the elements, and as the starting point for the manufacture of other compounds. 

B Aluminum forms an amphoteric oxide in which it has the oxidation state +3 therefore, aluminum is the element. 14.3B Hydrogen is a nonmetal and a diatomic gas at room temperature. It has an intermediate electronegativity (x — 2.2), so it forms covalent bonds with nonmetals and forms anions in combination with metals. In contrast, Group 1 elements are solid metals that have low electronegativities and form cations in combination with nonmetals. 

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

Bulk alumina and india are isostructural, with a linear structure OMOMO, while B2O3 molecule is V-shaped. The Ga203 can present the both types of isomers, the V-shaped structure being a little more stable than the linear one [1], These very different structural features (shape, electronegativities, etc.) of group lllA oxides may help explain their specific properties that fail to strictly follow any simple rule. Their amphoteric character (except for boria) that is not easy to evaluate, has been confirmed and quantified by the experimental microcalorimetric results. 

Virtually all amphoteric oxides are converted to monomeric anions in sufficiently strong basic media. Rieger and co-workers studied strongly basic oxovanadium(IV) solutions by ESR, optical and Raman spectroscopy.472 Raman absorption at 987 cm-1 confirms the presence of the vanadyl group in solution, and from ESR spectra and titration, the authors concluded that vanadium(IV) exists as a monomer [VO(OH)3] for pHs 12. 

Condensation between phenol and selenium oxychloride in ether or chloroform solution produces two isomeric selenonium chlorides, [(HO.CeH4)3Se]Cl, each containing chlorine precipitable as silver chloride and replaceable by other acid radicals. The three phenolic hydroxyl groups of the complex cation impart acidic properties to the chlorides, causing them to be soluble in aqueous caustic alkali. From such solutions carbonic or acetic acid precipitates the amphoteric oxide [(H0.C6H4)8Se]20, which redissolves in alkalis and reacts with acids to give a bromide, nitrate, sulphate and chloroplatinate. The following scheme shows the compounds obtained ... 

Most metallic main-group elements form basic oxides and most nonmetallic elements form acidic oxides. Elements close to the diagonal frontier between metals and nonmetals form amphoteric oxides, as do some of the d-block elements. 

FIGURE J.3 The location of acidic, amphoteric, and basic oxides in the main groups of the periodic table. Metals form basic oxides, nonmetals form acidic oxides. The diagonal band of amphoteric oxides closely matches the diagonal band of metalloids (recall Fig. B.18). 

FIGURE 14.6 Formulas, acid-base properties, and the covalent-ionic character of the oxides of main-group elements in their highest oxidation states. Basic oxides are shown in blue, acidic oxides are shown in red, and amphoteric oxides are shown in violet. 

The change in the basic and acidic properties here shown for the oxides of main group elements which have the highest oxidation state number. The oxides in the blue colored regions are basic (metallic) oxides and the oxides in the red colored regions are acidic (nonmetallic) oxides. The oxides in both blue and red colored regions are amphoteric oxides (the oxides of amphoteric metals). 

In conclusion, it should be noted that the Barton-Zard condensation formally might be considered as a tandem SnH-Sn ipso process (see Section III.B.3). However, the corresponding crH-complcx is stabilized here not via elimination of any auxiliary leaving group or oxidation but by means of an intramolecular nucleophilic attack on an isonitrile fragment. This is due to the amphoteric nature of isocyanoacetate ion acting at first as a nucleophile and then as an electrophile. 

There is a much more extensive chemistry of the +4 oxides of the Group IVA elements than there is for the +2 oxides. In general, the E02 compounds are acidic or amphoteric oxides, and they show this characteristic by forming oxyanions. This type of behavior has also been illustrated for C02 by the reaction... 

The comparison with amphoteric oxides [57-59] is also instructive. In an early review, Snoeyink and Weber [60] compared the surface functional groups on carbons and silicas but failed to point out the resulting differences in the symmetries" of their electrokinetic behavior. For amphoteric oxides, the symmetry (see Fig. 4a) is a consequence of the following equilibrium [57,61-66] ... 

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). 

Amino acids. A compound that contains at least one amino group and at least one carboxyl group. (25.3) Amorphous solid. A solid that lacks a regular three-dimensional arrangement of atoms or molecules. (11.7) Amphoteric oxide. An oxide that exhibits both acidic and basic properties. (8.6)... 

The Group 2A oxides are very basic (except for amphoteric BeO) and react with acidic oxides to form salts, such as sulfites and carbonates for example,... 

In general, acidic property decreases in the order of Group A > B > C > C, and Group A > B > D. Oxides of Groups A and A are acidic oxides, oxides of Groups B, B C, and C are amphoteric oxides, and oxides of Group D are basic oxides. 

It is concluded that the incorporation of a small amount of alkali or alkaline earth oxide, V20g, amphoteric oxide, or oxide of heavy metal into silica gel induces a marked increase in the activity. This finding suggests that the proton-abstraction from a methyl group of acetaldehyde can be promoted by active sites with a relatively weak base, arising from V2O5 and amphoteric oxides. On the other hand, the formation of acrolein is accompanied by two sides reactions (1) formation of CO2 and methanol by Equations (4) and (5) which is promoted mainly by acid-base dual functions, and (2) polymerization of acrolein to unidentified polymers, which is promoted by strongly acidic sites. 

Lead oxides (and hydroxide) are amphoteric, and react with aqueous acids and alkalis, but lead(II) oxide is the strongest base of the Group 4 oxides ... 

Across a period, oxides change from basic to amphoteric to acidic. Going down a group, the oxides become more basic. 

With respect to hydroxyl groups, the situation on oxide surfaces is more comphcated than that on the surface of zeolites. Some examples of the spectral signature of hydroxyl groups on oxide surfaces were already provided in Table 2.2. The M-O bond in metal oxides may possess an ionic, a covalent, or a mixed character, depending on the oxide. Consequendy, the oxides demonstrate acidic, basic, or amphoteric properties. 

Zwitterionic surfactants have positive and negative charges on the head group. Amphoteric surfactants have a head group with a pH-dependent charge. The amine oxide shown in Fig. 3 is zwitterionic at high pH, but becomes cationic as protonation occurs at low pH. Because amphoteric surfactants are generally zwitterionic at some pH. and zwitterionic surfactants are often amphoteric, in practice, the terms zwitterionic and amphoteric are used as synonyms, and the term ampholytic is used to describe both surfactant types. 

---