Oxidation number acid strength

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

Strategy The structure can be obtained by removing an oxygen atom from H O, (Figure 21.8). Relative acid strengths can be predicted on the basis of the electronegativity and oxidation number of the central nonmetal atom, following the rules cited above. 

TABLE 10.5 Correlation of Acid Strength and Oxidation Number... 

Self-Test 15.2A How does (a) the acidity of a nitrogen oxuacid and (b) its strength as an oxidizing agent change as the oxidation number of N increases from +1 to +5 ... 

The acid strengths and oxidizing abilities of the halogen oxoacids increase with the oxidation number of the halogen. The hypohalous acids, HXO (halogen oxidation number +1), are prepared by direct reaction of the halogen with water. For example, chlorine gas disproportionates in water to produce hypochlorous acid and hydrochloric acid ... 

The interhalogens have properties intermediate between those of the constituent halogens. Nonmetals form covalent halides metals tend to form ionic halides. The oxoacids of chlorine are all oxidizing agents both acidity and oxidizing strength of oxoacids increase as the oxidation number of the halogen increases. 

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

The interaction between selective metal oxides and molecules to be oxidized is, of course, based on electron-accepting and electron-donating properties, respectively. In this way, Mo6+, Vs+, etc. act as electron acceptors and molecules with 7r-bonds as donors. Ai et al. [5—12] have drawn attention to the fact that this can also be described by acid—base properties. An electron donor molecule like butene is a basic entity interacting with acidic sites on the catalyst. Hence it follows that activity and selectivity depend on the relative acidity and basicity. Mo03, for example, is an acidic oxide, while Bi203 is a basic oxide. Different compositions Bi Mo have different acidities. The rate of oxidation depends on the number of acid sites (=acidity) and the acid strength, viz. 

Effect of Acid Strength. When a higher fatty acid is used instead of acetic acid, the oxidation rate is lower as the carbon number of the... 

The acid strengths and oxidizing abilities of the halogen oxoacids (Table 15.8) increase with the oxidation number of the halogen. The... 

Because dissociation of an oxoacid requires breaking an O — H bond, any factor that weakens the O — H bond or increases its polarity increases the strength of the acid. Two such factors are the electronegativity of Y and the oxidation number of Y in the general reaction... 

For oxoacids that contain the same atom Y but different numbers of oxygen atoms, acid strength increases as the oxidation number of Y increases. The oxidation number of Y increases, in turn, as the number of oxygen atoms increases. This effect is illustrated by the oxoacids of chlorine ... 

In contrast to the amphoteric Cr(OH)3, chromium(II) hydroxide is a typical basic hydroxide. It dissolves in acid, but not in excess base. Conversely, the chromium(VI) compound, Cr02(0H)2, is a strong acid (chromic acid, H2Cr04). Recall from Section 15.15 that acid strength increases with increasing polarity of the O-H bonds, which increases, in turn, with increasing oxidation number of the chromium atom. 

As is seen, the strength of acid decreases as the oxidation number ( charge ) of the central atom decreases the smaller size of sulfur makes H2SO3 a slightly stronger acid than H2Se03. 

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