Basic and Amphoteric Hydroxides

All the alkali and alkaline earth metal hydroxides, except Be(OH)2, are basic. Be(OH)2, Al(OH 3, Sn(OH)2, Pb(OH)2, Cr(OH)3, Cu(OH)2, Zn(OH)2, and Cd(OH 2 are amphoteric. All amphoteric hydroxides are insoluble, but beryllium hydroxide reacts with both acids and bases as follows  

Aluminum hydroxide reacts with both acids and bases in a similar fashion  

So far we have discussed add-base properties in terms of the Brpnsted theory. For example, a Br0nsted base is a substance that must be able to accept protons. By this definition, both the hydroxide ion and ammonia are bases  

In each case, the atom to which the proton becomes attached possesses at least one unshared pair of electrons. This characteristic property of OH , NH3, and other Brdnsted bases suggests a more general definition of acids and bases. 

In 1932 G. N. Lewis defined what we now call Lewis base as a substance that can donate a pair of electrons. A Lewis acid is a substance that can accept a pair of electrons. In the protonation of ammonia, for example, NH3 acts as a Lewis base because it donates a pair of electrons to the proton H, which acts as a Lewis acid by accepting the pair of electrons. A Lewis acid-base reaction, therefore, is one that involves the donation of a pair of electrons from one species to another. 

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Some metal oxides and hydroxides that are relatively insoluble in water dissolve in strongly acidic and strongly basic solutions. These substances, called amphoteric oxides and amphoteric hydroxides, are soluble in strong adds and bases because they themselves are capable of behaving as either an acid or base. Examples of amphoteric substances include the oxides and hydroxides of Al, Cr, Zn, and Sn. ... 

Oxides and hydroxides can he classified as acidic, basic, and amphoteric. 

The carboxylic acid group is widely used to attach organic species to filler surfaces. Anchoring to the filler is thought to proceed by salt formation with mineral fillers or esterification with organic fillers such as cellulosic products. The first type of attachment is effective on fillers with basic and amphoteric surfaces, such as carbonates and hydroxides, but not so useful on acidic surfaces such as found on silicas and silicates. 

FIGURE 16.16 A plot of solubility versus pH shows that Al(OH)3 is an amphoteric hydroxide. Al(OH)3 is essentially insoluble between pH 4 and 10, but it dissolves both in strongly acidic and in strongly basic solutions. 

The hydroxides of metals are basic and they react with acids to form salts (Chapter 8, p. 124). The hydroxides of some metals, however, will also react with strong bases, such as sodium hydroxide, to form soluble salts. Hydroxides of this type are said to be amphoteric. For example,... 

Basolo, F. Theories of Acids, Bases, Amphoteric Hydroxides, and Basic Salts as Applied to the Chemistry of Complex Compounds, in Bailar and Busch s Chemistry of Coordination Compounds, 416-447, Reinhold Publishing Co., New York (1956). 

Hydroxides M(OH) comprise a numerous class of compounds ranging from strongly basic hydroxides of alkaline metals and alkaline earths, to the so-called amphoteric hydroxides (of beryllium, aluminium, zinc and others) and the hydroxides of transition metals, and further to hydroxo-acids formed by non-metals or semi-metals. 

From the table, the bases between water and the hydroxide ion are the weak bases. They are the ones that ionize to 10%. Note that in the case of bases, the other boundary limit is demarcated by the hydroxide ion rather than the hydronium ion. This is so, because the hydronium ion is not a base thus, it cannot form as a boundary for the bases. One the other hand, H2O is both an acid and a base. Thus, it consistently forms as a boundary limit in both the acids and the bases. Compounds that act both as an acid and a base are called amphoteric substances. H2O is an amphoteric substance. Above water in the table, the compounds do not exhibit any observable basic behavior. Water, then, is the very limit of basicity. Notice the arrows pointing downward from the weakest to the strongest bases. 

If both acidic and basic groups are present, the substance may be amphoteric and therefore soluble in both acid and base. Aromatic aminocarboxylic acids are amphoteric, like aliphatic ones, but they do not exist as zwitterions. They are soluble in both dilute hydrochloric acid and sodium hydroxide, but not in bicarbonate solution. Aminosulfonic acids exist as zwitterions they are soluble in alkali but not in acid. 

Compounds M(OH) range from the strongly basic compounds of the alkali and alkaline-earth metals through the so-called amphoteric hydroxides of Be, Zn, Al, etc. and the hydroxides of transition metals to the hydroxy-acids formed by non-metals (B(OH)3) or semi-metals (Te(OH)6). The latter are few in number and are included in other chapters. 

Generally, elements of intermediate electronegativity form amphoteric hydroxides. Those of high and low electronegativity form acidic and basic hydroxides, respectively. 

Except for Be, all the alkaline earth metals are oxidized to oxides in air. The IIA oxides (except BeO) are basic and react with water to give hydroxides. Beryllium hydroxide, Be(OH)2, is quite insoluble in water and is amphoteric. Magnesium hydroxide, Mg(OH)2, is only slightly soluble in water. The hydroxides of Ca, Sr, and Ba are strong bases. 

The metals of the aluminium sub-group are permanent in the air at ordinary temperatures, but when heated in oxygen or the air they become coated with their oxide. The volatility of the metals increases with the atomic weights, and the heavier metals are more easily reduced than those of lower atomic weight. The metals are all malleable, fusible, have small atomic volumes and form hydroxides, M(OH)3, which are typically amphoteric in the first three elements of the sub-group and basic only in the case of thallium. The last four members of the family form alums, and both aluminium and thallium form organo-metallic compounds, resembling zinc in this respect. 

The increase in solubility of Al(OH)3 in a basic medium is the result of the formation of the complex ion Al(OH)4 in which Al(OH)3 acts as the Lewis acid and OH acts as the Lewis base. Other amphoteric hydroxides behave in a similar manner. 

The trivalent state is the characteristic one for all the lanthanides. They form oxides, M203, which resemble the Ca-Ba group oxides and absorb carbon dioxide and water from the air to form carbonates and hydroxides, respectively. The hydroxides, M(OH)3, are definite compounds, having hexagonal structures, and not merely hydrous oxides. The basicities of the hydroxides decrease with increasing atomic number, as would be expected from the decrease in ionic radius. The hydroxides are precipitated from aqueous solutions by ammonia or dilute alkalis as gelatinous precipitates. They are not 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. 

The ore contains aluminium oxide (amphoteric) with large impurities of iron oxide (basic) and silicon dioxide (weakly acidic). The ore is treated with a hot 10% solution of sodium hydroxide, which reacts with the amphoteric aluminium oxide to form a solution of sodium aluminate. Iron oxide does not react as it is a base, and silicon dioxide does not... 

The oxides and hydroxides of the metals of Group 3 and higher tend to be only weakly basic, and most display an amphoteric nature. Most of these compounds are so slightly soluble in water that their acidic or basic character is only obvious in their reactions with strong acids or bases. 

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