Acid, Basic, and Amphoteric Oxides

Before providing an overview of the acid-base properties of metal oxide surfaces, a brief systematic description of their bulk and stmctural properties and surface chemistry is needed. According to basic inorganic chemistry concepts, the oxides of non-metals as well as the oxides of the metals in very high oxidation states are defined as acidic oxides and anhydrides, respectively, the oxides of metals as basic oxides, and the oxides having both acidic and basic characters are denoted as amphoteric. 

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Selected acidity parameters, a, for acidic, basic, and amphoteric oxides"... 

Soluble binary oxides can be classified according to their acid-base character as acidic, basic, and amphoteric, depending on the nature of the resulting solution from their reaction with water. Thus, K20 reacts with water to produce the base KOH as follows ... 

Ellis Wilson (1991, 1992) examined cement formation between a large number of metal oxides and PVPA solutions. They concluded that setting behaviour was to be explained mainly in terms of basicity and reactivity, noting that cements were formed by reactive basic or amphoteric oxides and not by inert or acidic ones (Table 8.3). Using infrared spectroscopy they found that, with one exception, cement formation was associated with salt formation the phosphonic add band at 990 cm diminished as the phosphonate band at 1060 cm" developed. The anomalous result was that the acidic boric oxide formed a cement which, however, was soluble in water. This was the result, not of an add-base readion, but of complex formation. Infrared spectroscopy showed a shift in the P=0 band from 1160 cm" to 1130 cm", indicative of an interaction of the type... 

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. 

Metal oxide catalysts can be classified as oxides of transition elements or as oxides of other typical metals. Typical transition elements include Cr, Fe, Co, Mo, and V, whose oxides catalyze oxidation and reduction reactions by changing the oxidation state of the metal ion. For selective oxidation of hydrocarbons, mixed oxides containing Mo and V are widely used. Oxides of other metals (acidic oxides such as silica and silica-alumina, basic oxides such as CaO and MgO, and amphoteric oxides such as alumina) catalyze acid or base reactions such as alkylation, isomerization, and hydration-dehydration. 

Some of the oxides of the transition metals are amphoteric, (a) What is meant by the term amphoteric (b) Identify five oxides that are amphoteric, five oxides that are basic, and five oxides that are acidic. 

This brief examination of oxides of the third-period elements shows that as the metallic character of the elements decreases from left to right across the period, their oxides change from basic to amphoteric to acidic. Metallic oxides are usually basic, and most oxides of nonmetals are acidic. The intermediate properties of the oxides (as... 

Write the formulas and names of the oxides of the second-period elements (Li to N). Identify the oxides as acidic, basic, or amphoteric. 

It is found that only alkali and alkaline earth hydroxides can promote the reaction that is, acidic and amphoteric oxides are inactive for the production of acrylonitrile. The best performances are obtained with silica-supported hydroxides of Cs, Rb, and K. The activity for the formation of acrylonitrile decreases in the order of Cs = Rb = K > Na > Li, and Ba > Ca Mg. This indicates that the activity is related to the electronegativity (basic property) of metal ions corresponding to the hydroxides supported on silica. 

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. 

Like other metal oxides and hydroxides, amphoteric spedes dissolve in addic solutions because their anions, 0 or OH , react with acids. What makes amphoteric oxides and hydroxides special, though, is that they also dissolve in strongly basic solutions. This behavior results from the formation of complex anions containing several (typically four) hydroxides bound to the metal ion (T FIGURE 17.21) ... 

This brief examination of oxides of the third-period elements shows that as the metallic character of the elements decreases from left to right across the period, their oxides change from basic to amphoteric to acidic. Metallic oxides are usually basic, and most oxides of nomnetals are acidic. The intermediate properties of the oxides (as shown by the amphoteric oxides) are exhibited by elements whose positions are intermediate within the period. Note also that because the metallic character of the elements increases from top to bottom within a group of representative elements, we would expect oxides of elements with higher atomic numbers to be more basic than the lighter elements. This is indeed the case. 

Small, highly charged metal ions, such as Al and Fe, hydrolyze to yield acidic solutions. 11. Most oxides can be classified as acidic, basic, or amphoteric. Metal hydroxides are eitha- basic or amphotaic. 12. Lewis acids accept pairs of electrons and Lewis bases donate pairs of electrons. The term Lewis acid is generally reserved for substances that can accept electron pairs but do not contain ionizable hydrogen atoms. 

Phosphate Bond. Cold-setting bonds for refractories can be formed by reactions between oxides and phosphoric acid, phosphate-phosphoric acid reactions or the direct use of liquid phosphate bonds. The bond is due to the formation of an acid phosphate, with weakly basic or amphoteric oxides. 

All the elements form oxides except the three lightest noble gases, helium, neon, and argon, which form no stable compounds with any element. Oxides are often classified as acidic, basic, or amphoteric on the basis of their acid-base properties in aqueous solution. 

As the elements become less metallic across a period (smaller size and higher IE), their oxides become more acidic. In Period 3, Na20 and MgO are strongly basic, and amphoteric aluminum oxide (AI2O3) reacts with acid or with base ... 

One criterion for classifying an element as a metal, a non-metal or a metalloid is the acid-base behaviour of the oxide. In general metals form basic oxides, non-metals form acidic oxides and metalloids form amphoteric oxides. Basic oxides react with acid, acidic oxides react with bases and amphoteric oxides react with both acids and bases. 

AI2O3, Si02 (impurities), and other acidic or amphoteric oxides are the main structural promoters of iron-based catalyst. In the total surface area of catalyst after reduction, in addition to the surface area of basic oxides and metal iron, it should also include the surface occupied by acidic or amphoteric oxides. Subtracting the surface area of metal iron and basic oxides from the total surface area of catalyst results in the surface area of acidic and amphoteric oxides, also known as acid surface area (/Sa), that is,... 

Write the formulas for the oxides of potassium, gallium, arsenic, and selenium. Classify each of these as acidic, basic, or amphoteric. 

Except for the lighter noble gases, all the elements form oxides. Table 12.2 shows the s- and p-Block elements, indicating the acidity, basicity or amphoteric nature of the oxides. In the amphoteric section the elements underlined are amphoteric in their lower oxidation states and acidic in their higher oxidation states. 

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