Element oxide, acid-base behavior

Both our original prediction about the effect of ionization energy on acid-base behavior and the trend which we have observed in the first three elements lead us to expect that the hydroxide or oxide of silicon should not be basic, but perhaps should be weakly acidic. This is in fact observed. Silicon dioxide, Si02, can exist as a hydrated solid containing variable amounts of water,... 

The pattern of oxidation states correlates with the pattern of acid-base behavior of d-metal oxides. Although most d-metal oxides are basic, the oxides of a given element show a shift toward acidic character with increasing oxidation number, just as the oxoacids do (recall Section 10.10). The family of chromium oxides is a good... 

These atomic properties have a profound effect on many macroscopic properties, including metallic behavior, acid-base behavior of oxides, ionic behavior, and magnetic behavior of the elements and their compounds. 

Acid-Base Behavior of the Element Oxides Metals are also distinguished from nonmetals by the acid-base behavior of their oxides in water ... 

Figure 8.17 The trend in acid-base behavior of eiement oxides. The trend in acid-base behavior for some common oxides of Group 5A(15) and Period 3 elements is shown as a gradation in color (red = acidic blue = basic). Note that the metals form basic oxides and the non-metals form acidic oxides. Aluminum forms an oxide (purple) that can act as an acid or as a base. Thus, as atomic size increases, ionization energy decreases, and oxide basicity increases.

Remember, though, that some elements don t lit these categories as graphite, non-metallic carbon is a good electrical conductor the nonmetal iodine is shiny metallic gallium melts in your hand mercury is a liquid and iron is brittle. Despite such exceptions, in this discussion, we U make several generalizations about metallic behavior and its application to the acid-base behavior of oxides. 

Figure 8.16 Acid-base behavior of some element oxides.

In Figure 8.16, the acid-base behavior of some common oxides of elements in Group 5A(15) and Period 3 is shown with a gradient from blue (basic) to red (acidic) ... 

With the exception of OF2, the oxides of the elements in groups 14 to 17 react with water to give oxoacids, as shown above for SO3. OF2 reacts with water, but the reaction gives a binary acid (HF) and not the oxoacid (HOF). That the acid-base behavior of OF2 is different from the oxides of other p-block elements comes as no surprise OF2 is not an oxide. It is a fluoride with oxygen having an oxidation state of +2. 

The ionic-potential (Ip) concept discussed in Chap. 3 best describes element behavior at intermediate pH s. Most of the metal oxides and hydroxide minerals formed by cations with Ip values between about 3 and 8.2 become significantly soluble in acid waters, where the high H+ concentrations can break metal-0 or metal-OH bonds to form water and release metal cations to solution. Some of these otherwise insoluble metal oxides and hydroxides (for example, those of Al and Fe " ) are also solubilized by high pH s. In other words, the mobilities of these metal cations depend on the acid-base properties of the water, as well as on their ionic potentials. The rate of chemical weathering is also greatly accelerated in strongly acid waters. 

The catalytic behavior of vanadium-based catalysts can be generally modified with the addition of a second element which acts as promoter. In this paper we present a comparative study on the catalytic properties in the ODH of propane of undoped and Me-doped Al203-supported vanadium catalysts, in which metal oxides with redox and/or acid-base properties (K, Bi, Mo, P) have been used as potential promoters. 

The basic—acidic behavior of the oxides of the elements is a good indicator of the metaUic—nonmetallic character of the elements. Oxides are classified as basic or acidic depending on their reactions with acids and bases. A basic oxide is an oxide that reacts with acids. Most metal oxides are basic. An acidic oxide is an oxide that reacts with bases. Most nomnetal oxides are acidic oxides. An amphoteric oxide is an oxide that has both basic and acidic properties. 

Consistent with the position of the metal-nonmetal line (and the corresponding acid-base character of metal and nonmetal oxides), boron oxide is an acid anhydride, whereas the oxides of the heavier elements progress from amphoteric to basic in behavior. Boron oxide, then, reacts with water, as shown in Equation (14.2), to produce boric acid, B(OH)3 or H3BO3 ... 

Figure 2.5 summarizes the acid-base properties and possible condensation behaviors in solution for all elements of the periodic table. It is possible to predict and understand the main trends on the basis of their electronegativity and their oxidation level. The next chapters are concerned with the specific behavior differences for elements within one class. 

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