Compounds amphoteric oxides

The data given in Tables 1.9 and 1.10 have been based on the assumption that metal cations are the sole species formed, but at higher pH values oxides, hydrated oxides or hydroxides may be formed, and the relevant half reactions will be of the form shown in equations 2(a) and 2(b) (Table 1.7). In these circumstances the a + will be governed by the solubility product of the solid compound and the pH of the solution. At higher pH values the solid compound may become unstable with respect to metal anions (equations 3(a) and 3(b), Table 1.7), and metals like aluminium, zinc, tin and lead, which form amphoteric oxides, corrode in alkaline solutions. It is evident, therefore, that the equilibrium between a metal and an aqueous solution is far more complex than that illustrated in Tables 1.9 and 1.10. Nevertheless, as will be discussed subsequently, a similar thermodynamic approach is possible. 

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. 

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 ... 

The product in this case is sodium aluminate, a compound that contains the aluminate ion, [Al(OH)4]. Because aluminum oxide reacts with both acids and bases, it is classified as amphoteric. Other main-group elements that form amphoteric oxides are shown in Fig. J.3. As you can see, these elements lie in a diagonal band across the table from beryllium to polonium. The acidic, amphoteric, or basic character of the oxides of the d-block metals depends on their oxidation state (see Chapter 16). 

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... 

Germanium, tin, and lead all form compounds representing oxidation state j-4, which is shown also by carbon and silicon. They form a second series of compounds representing oxidation state -f2, which is the most important oxidation state for lead, and is less important for germanium than the higher oxidation state. The hydroxides of these elements tend to be amphoteric. The acidic character is more pronounced for the quadripositive state than for the bipositive state, and is most pronounced for germanium, decreasing to tie and lead. 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

After much testing, chemists developed a compound that stabilizes paper diethyl-zinc [Zn(C2H5)2]. Diethylzinc is volatile so it can be sprayed onto books. It reacts with water to form zinc oxide (ZnO) and gaseous ethane (C2Hg). (a) Write an equation for this reaction, (b) ZnO is an amphoteric oxide. What is its reaction with H+ ions ... 

In principle, amphoteric oxides, such as AI2O3 and BeO can he used to prepare buffer solutions because they possess both acidic and basic properties (see Section 15.11). Explain why these compounds are of little practical use as buffer components. 

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)... 

Fe, or Ni. However, incorporation of a small amount of P20g into an acidic oxide such as WO3 or M0O3 strongly decreases the activity. Pure amphoteric oxides show a relatively high activity. Typically acidic oxides, such as oxides of Si-Al, B, B-P, Mo, W-Mo, and W-V, and heteropoly compounds are inactive, although pure WO3 is relatively active. 

It is concluded that acidic oxides that possess a certain extent of basic property are favorable for promoting the condensation reaction a more basic property is required than in the case of the reaction with carboxylic acid. The acidic oxides such as H-zeolite, Si-Al, B, Mo-P, and W-P oxides, and heteropoly compounds are lacking in a basic property as catalyst for the reaction. On the other hand, alkali and alkaline earth metal oxides and amphoteric oxides combined with K2O are lacking in acidic property. 

With few exceptions, the metal oxides are ionic solids and react with water to form aqueous ions, the nonmetal oxides are network covalent solids that react with water to make covalent compounds, and the amphoteric oxides of the metalloids form oligomeric polar-covalent solids. Similar relationships hold for the hydrides and fluorides of each element, with the metal forming an ionic solid and the non-metal forming a network covalent solid, although the actual demarcation line varies somewhat depending on the anion. 

Intermediate results are possible for the two cases. Where both dissociations are feasible, such compounds are termed amphoteric oxides. 

Zinc-coated steel and monolithic zinc objects corrode similarly a useful reference on the mechanism of corrosion of a galvanized coating is that of Daesen [3]. In dry air, a film of zinc oxide initially forms however, the presence of moisture and carbon dioxide changes the corrosion product to a basic zinc carbonate film. Similarly, a dilute presence of sulfur compounds can result in a production of a basic sulfate film. Zinc oxide, basic zinc carbonate, and basic zinc sulfate are stable protective layers if left undisturbed however, the pH of the environment can interfere with these layers, resulting in the formation of more soluble products. Because zinc forms an amphoteric oxide, both strong alkaline and acid conditions interfere with the formation of these protective layers. Attack is most severe at pH values below 6 and above 12.5. Within this range, corrosion is relatively slow. 

Fig.6.1 depicts the Ebsworth diagram of N and P in acidic solution as well as N in basic solution. The top two elements in this group N and P are typical non-metals. The metallic character, which appears in the heavier elements, increases down the group, although the conductivity of solid Bi is not high. The typical oxidation numbers of all the members of the group are +3 and +5 but the stability of the +3 state in Bi is greater than the +S state (inert pair effect). The chemistry of N and P is dominated by covalent bond formation. On the other hand, ionic compounds of Bi(III) are the common bismuth compounds. The oxides of N and P are acidic in nature (except the neutral N2O and NO), the amphoteric nature becomes apparent in the oxides of the heavier elements. 

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