Metal hydroxides amphoteric

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. 

Nloha.tes, Niobic acid is amphoteric and can act as an acid radical in several series of compounds, which are referred to as niobates. Niobic acid is soluble in solutions of the hydroxides of alkaH metals to form niobates. Fusion of the anhydrous pentoxide with alkaH metal hydroxides or carbonates also yields niobates. Most niobates are insoluble in water with the exception of those alkaH metal niobates having a base-to-acid ratio greater than one. The most weU-known water-soluble niobates are the 4 3 ad the 7 6 salts (base acid), having empirical formulas MgNb O c, (aq) and M24Nb2202y (aq), respectively. The hexaniobate is hydrolyzed in aqueous solution according to the pH-dependent reversible equiHbria (130), when the pH is ca 9. 

The alkali metal hydroxides are also readily absorb CO2 and H2S to form carbonates (or hydrogencarbonates) and sulfides (or hydrogen-sulfides), and are extensively used to remove mercaptans from petroleum products. Amphoteric oxides such as those of Al, Zn, Sn and Pb react with MOH to form aluminates, zincates, stannates and plumbates, and even SiC>2 (and silicate glasses) are attacked. 

There are certain substances which can act acids as well as bases, and these are called amphoteric. A number of metallic hydroxides are amphoteric examples include aluminum hydroxide, zinc hydroxide, and tin hydroxide. Thus, zinc hydroxide, Zn(OH)2, is capable of reacting with hydrochloric acid to form zinc chloride ... 

Although this behavior has been illustrated starting with the metal ion, analogous equations can be written starting with the metal oxide as well as the hydroxide. Amphoteric behavior is exhibited in other solvents, as will be illustrated later. 

Recall from Section 16.12 that this behavior is typical of amphoteric metal hydroxides. 

Table 10.5 shows the effects of adding a few drops of sodium hydroxide solution to solutions containing various metal ions, and of adding an excess. The colours of the insoluble metal hydroxides can be used to identify the metal cations present in solution. In some cases the precipitate dissolves in excess hydroxide, owing to the amphoteric nature of the metal hydroxide. This amphoteric nature can also be used to help identify metals such as aluminium and zinc. 

The hydroxide Ni(OH)2 may be precipitated from aqueous solutions of Ni11 salts on addition of alkali metal hydroxides, forming a voluminous green gel that crystallizes [Mg(OH)2 structure] on prolonged storage. It is readily soluble in acid and also in aqueous ammonia owing to the formation of ammine complexes. Ni(OH)2 is not amphoteric. 

There are many indications in the literature that surface chemistry plays an important role in photoelectrochemical reactions at extended electrodes and at particles. There are, however, only a few quantitative investigations on this problem (see e.g. [12, 57, 58]), probably due to the lack of sufficiently sensitive methods. In the case of metal oxide particles, the adsorption of H2O plays already an important role. Due to the amphoteric behaviour of most metal hydroxides, two surface equilibria have to be considered [18] ... 

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. 

Plausibly there is a correlation of the solubility of the stable oxide, hydroxide, or oxyhydroxide of a cation with the stability of the first hydrolysis product (Figure 6. 7). Many multivalent hydrous oxides are amphoteric because of the acid-base equilibria involved in the hydrolysis reactions of aquo metal ions. Alkalimetric or acidimetric titration curves for hydrous metal oxides provide a quantitative explanation for the manner in which the chaige of the hydrous oxide depends on the pH of the medium. The amphoteric behavior of solid metal hydroxides becomes evident from such titration curves. From an operational point of view, such hydrous oxides can be compared with amphoteric polyelectrolytes and can be considered hydrated solid electrolytes, fn -... 

As we have seen, whether a particular substance behaves as an acid or as a base depends on its environment. Earlier we described the amphiprotic nature of water. Amphoterism is a more general term that describes the ability of a substance to react either as an acid or as a base. Amphiprotic behavior describes the cases in which substances exhibit amphoterism by accepting and by donating a proton, H+. Several insoluble metal hydroxides are amphoteric that is, they react with acids to form salts and water, but they also dissolve in and react with excess strong bases. 

Aluminum hydroxide is a typical amphoteric metal hydroxide. Its behavior as a base is illustrated by its reaction with nitric acid to form a normal salt. The balanced formula unit, total ionic, and net ionic equations for this reaction are, respectively ... 

Other amphoteric metal hydroxides undergo similar reactions. 

What are amphoteric metal hydroxides (a) Are they bases (b) Write the names and formulas for four amphoteric metal hydroxides. 

A common approach in precipitating metals from wastewater is to precipitate the metals as metal hydro-complexes. Lime (Ca(OH)2) and sodium hydroxide (NaOH) are typically used. Lime is widely used and is the less expensive of the two. The formation of various metal hydro-complexes is dependent on the solution pH. Figure 3 shows the solubilities of several metal hydroxides at different pH in equilibrium with the metal precipitate. Hydro-complexes of metals are amphoteric resulting in a minimum solubility between pH 9 and 11 but are fairly soluble at elevated pHs (> 11) and at low pH or acidic conditions (<6). The... 

We have seen that the alkali and alkaline earth metal hydroxides [except Be(OH)2l are basic in properties. The following hydroxides are amphoteric 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. For example, aluminum hydroxide reacts with both acids and bases ... 

Most oxides can be classified as acidic, basic, or amphoteric. Metal hydroxides are either basic or amphoteric. 

The reagents are strong bases and poorly soluble metal hydroxides. In the above equations, the metal hydroxides function as acids, in agreement with the modern theory of their amphoteric behavior. As far as is presently known, the following metals (arranged in order of increasing valence) form hydroxo salts ... 

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) ... 

The extent to which an insoluble metal hydroxide reacts with either acid or base varies with the particular metal ion involved. Many metal hydroxides—such as Ca(OH)2, Fe(OH)2, and Fe(OH)3—are capable of dissolving in acidic solution but do not react with excess base. These hydroxides are not amphoteric. 

We have seen that the alkali and alkaline earth metal hydroxides [except Be(OH)2] are basic in properties. The following hydroxides are amphoteric Be(OH)2, Al(OH)3,... 

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. 

Some metal hydroxides and oxides that are relatively insoluble in neutral water dissolve in strongly acidic and strongly basic solutions. These substances are soluble in strong acids and bases because they themselves are capable of behaving as either an acid or base they are amphoteric. (Section 16.2) Amphoteric substances include the hydroxides and oxides of Al, Cr, Zn, and Sn. ... 

---