Complex ions amphoteric hydroxides

The solubility of metal salts is also affected by the presence of certain Lewis bases that react with metal ions to form stable complex ions. Complex-ion formation in aqueous solution involves the displacement by Lewis bases (such as NH3 and CN ) of water molecules attached to the metal ion. The extent to which such complex formation occurs is expressed quantitatively by the formation constant for the complex ion. Amphoteric oxides and hydroxides are those that are only slightly soluble in water but dissolve on addition of either acid or hase. 

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 ... 

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 zinc ion Zn(H20)4 is a rolciless ion formed by solution of zinc in acid. It is poisonous to uian. and to bacteria, and is used as a disinfectant. It forms tetracomdmated complexes readily, such as Zn(NH d4+ b Zn(CN)r, and Zr (OH)4" ". The while precipitate of zinc hydroxide, Zn(OH)2, whidi foirn. when ammonium hydroxide is added to a solution containing zinc ion, dissolves ill excess ammonium hydroxide, forming the zinc ammonia complex. The zinc hydroxide complex, Zn(OH), which is called zincate ion, is similarly formed on solution of zinc hydroxide in an excess of strong base zinc hydroxide is amphoteric. 

Of the three oxides only ZnO is amphoteric, the other two are purely basic. Thus zinc forms the most stable complex anion with oxygen, and mercury the most stable complex ions with the halogens. The hydroxides of zinc and cadmium are obtained as crystalline precipitates on the addition of alkali to solutions of the nitrates a hydroxide of mercury is unknown. 

Tin(II) oxide is amphoteric. Write balanced chemical equations for its reactions with an aqueous solution of hydrochloric acid and with an aqueous solution of sodium hydroxide. (Note The hydroxide complex ion of tin(II) is [Sn(OH)3]-.)... 

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

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

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. 

Beryllium hydroxide is amphoteric and this sets it apart from the hydroxides of the other group 2 metals which are basic. In the presence of excess [OH] , Be(OH)2 behaves as a Lewis acid (equation 11.24), forming the tetrahedral complex ion 11.7, but Be(OH)2 also reacts with acids, e.g. reaction 11.25. 

When metal oxide surfaces are contacted with aqueous solutions, the surface-site complexes form among surface sites and all solutes in the solution, including protons and hydroxide ions. The solute—surface site complexes form just as complexes form among solute molecules and ions in a bulk solution. And as wifli solution complexes, the amphoteric nature of the solute—surface site complexes imparts a significant pH dependence to the relative abundance of each dififerent type of solute— surface site complex. 

The ions Cr and behave in a manner similar to Fe except that, with them, hydroxide complex-ion formation can continue until complex anions are produced. Cr(OH)3 and Al(OH)3, as we previously noted, are soluble in alkaline as well as acidic solutions they are amphoteric. 

It is readily dehydrated on warming, to give the black oxide CuO. It dissolves in excess of concentrated alkali to form blue hydroxo-cuprate(II) ions, of variable composition it is therefore slightly amphoteric. If aqueous ammonia is used to precipitate the hydroxide, the latter dissolves in excess ammonia to give the deep blue ammino complexes, for example [Cu(NH3)4(H20)2] ... 

Cadmium is rapidly oxidized by hot dilute nitric acid with the simultaneous generation of various oxides of nitrogen. Unlike the ziac ion, the cadmium ion is not markedly amphoteric, and therefore cadmium hydroxide [21041-95-2] Cd(OH)2, is virtually iasoluble ia alkaline media. However, the cadmium ion forms stable complexes with ammonia as well as with cyanide and haUde ions. The metal is not attacked by aqueous solutions of alkaU hydroxide. 

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