Amphoteric hydroxide

Most metals will precipitate as the hydroxide in the presence of concentrated NaOH. Metals forming amphoteric hydroxides, however, remain soluble in concentrated NaOH due to the formation of higher-order hydroxo-complexes. For example, Zn and AP will not precipitate in concentrated NaOH due to the formation of Zn(OH)3 and Al(OH)4. The solubility of AP in concentrated NaOH is used to isolate aluminum from impure bauxite, an ore of AI2O3. The ore is powdered and placed in a solution of concentrated NaOH where the AI2O3 dissolves to form A1(0H)4T Other oxides that may be present in the ore, such as Fe203 and Si02, remain insoluble. After filtering, the filtrate is acidified to recover the aluminum as a precipitate of Al(OH)3. 

Another reason chemists find the above complex ion picture of aqueous solutions useful is that it is easily extended to explain amphoteric behavior. Take the case of chromium hydroxide, Cr(OH)3, a good example of an amphoteric hydroxide. It dissolves very little in water, but is quite soluble both in acid and in base. Presumably it can react with either. How can this behavior be explained in terms of the complex ion picture ... 

This way a neutralization reaction can lead to an enhancement of the solubility of an otherwise insoluble or sparingly soluble material. Because of its ability to dissolve both in acid and alkali, aluminum hydroxide is known as an amphoteric hydroxide. 

Arrhenius acid, Bransted-Lowry acid Arrhenius acid, Bransted-Lowry acid Arrhenius base, Bransted-Lowry base Arrhenius acid, Bransted-Lowry acid amphoteric hydroxide (can act as either an acid or base)... 

In systematically testing the various amphoteric hydroxides for their catalytic effect, alkali was also added. As a result, the hydroxides of the trivalent metals in particular increased considerably in activity. When sodium hydroxide is used... 

The hydroxo complex ions formed in this way have instability constants, just as ammine or other complexes do. These instability constants are somewhat special, in that one of the products of the equilibrium is the insoluble amphoteric hydroxide. Thus, for aluminum hydroxide,... 

The following questions involve the typical amphoteric hydroxides, Pb(OH)2... 

Answer the following questions involving the typical amphoteric hydroxides,... 

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. 

Other amphoteric hydroxides are lead hydroxide (Pb(OH)2) and aluminium hydroxide (Al(OH)3). We can use this sort of behaviour to help identify metal cations, as their hydroxides are soluble in strong bases. 

Amphoteric hydroxide A hydroxide which can behave as an acid (react with an alkali) or a base (react with an acid), for example zinc hydroxide. 

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). 

The transition, the amphoteric hydroxides, can be found in the Periodic Table along a diagonal, because of the radius increasing downwards. Thus HOH, Be(OH)2, Al(OH)3 are... 

Metals capable of forming amphoteric hydroxides produce stable hydroxo-complexes that prevent metal removal... 

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. 

For the compounds with amphoteric properties, the existence of coordination groups Me(OH)4 (tetrahedrons) and Me(OH)g (octahedrons) is characteristic hydroxyl bonds exist between separate groups. Molecular structures are characteristic of the compounds with clearly expressed acidic properties. This means that lattice sites are occupied by molecules bound to each other by hydrogen bonds. Since this leads to the decrease of lattice strength, some acids are liquids under normal conditions, while all the bases and amphoteric hydroxides are solids. 

Amphoteric Hydroxides. A hydroxide, such as zinc hydroxide, which can combine with acids to form salts and also with bases to form salts is called an amphoteric hydroxide. The amphoteric properties of a metal hydroxide are determined by the stability of the hydroxide complex of the metal. 

TIic principal common amphoteric hydroxides and their anions are the following ... 

Ammonia complexes.. EflFect of complex formation on solubility. Cyanide complexes. The cyanide process of treating gold and silver ores. Complex halides and other complexes. Sodium thiosulfate as photographic fixer. Hydroxide complexes. Amphoteric hydroxides. Sulfide complexes. Equilibrium expressions for complex formation. Structural chemistry—tetrahedral, octahedral, square complexes. Existence of isomers. 

Adil-forming and base-forming oxides and hydroxides, in relation to position in the periodic tabic. Amphoteric hydroxides. 

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. 

Metal or Metalloid ions Insoluble Amphoteric Hydroxide Complex Ion Formed in an Excess of a Strong Base... 

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

Table 10-1 contains lists of the common amphoteric hydroxides. Three are hydroxides of metalloids, As, Sb, and Si, which are located along the line that divides metals and nonmetals in the periodic table. 

Chromium(III) hydroxide and lead(II) hydroxide are typical amphoteric hydroxides, (a) Write the formula unit, total ionic, and net ionic equations for the complete reaction of each hydroxide with nitric acid, (b) Write the same kinds of equations for the reaction of each hydroxide with an excess of potassium hydroxide solution. Reference to Table 10-1 may be helpful. 

Amphoteric hydroxides such as Zn(OH)2 also dissolve in excess strong base by forming complex ions (Section 10-6). 

The hydroxides of some metals and some metalloids are amphoteric (Section 10-6). Aqueous NH3 is a weak base (A, = 1.8 X 10 ), so the [OH ] is too low to dissolve amphoteric hydroxides to form hydroxo complexes. 

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