Chap19-Part1 - EXAM April 15th 2011 Friday and 17 Review...

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1 EXAM- April 15 th 2011, Friday Chapters 15, 16 and 17 Review session: Tonight 7pm, Chapman 211
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Factors Affecting Solubility: 4) Complexing Agents The presence in solution of a reagent that complexes with one of the dissolved ions will increase the solubility. Example: AgCl( s ) Ag + ( aq ) + Cl - ( aq ) Add ammonia: Ag + ( aq ) + 2 NH 3 ( aq ) Ag(NH 3 ) 2 + ( aq ) The ammonia shifts the 2 nd equilibrium to the right, decreasing Ag + . The decreased Ag + shifts the 1 st equilibrium to the right, increasing the solubility. 2
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For a complex to significantly affect solubility, its stability must be strong enough. The stability is described by the equilibrium constant K f with the complex as product. 4
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Amphoterism Amphoteric compounds are metal hydroxides that are not appreciably soluble in neutral water but are soluble in strongly acidic or basic aqueous solutions. These materials dissolve in acidic solutions because they can act as a base (see above). These materials also dissolve in basic solutions because they can act as an acid. Recall that the adjective amphiprotic is a general term describing any compound that can either donate or accept a proton. 5
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Amphoterism: Metal Hydroxides The effect is usually associated with the formation of complex ions containing several hydroxides (e.g., four) bound to the metal ion. The extent to which a metal hydroxide is amphoteric depends on the particular metal ion . Amphoteric hydroxides are usually derived from aluminum, chromium, zinc or tin . Metal hydroxides derived from calcium or iron are not usually amphoteric. 6
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Example of Amphoterism: Solubility of Al(OH) 3 ( s ) in a Basic Aqueous Solution 7
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Amphoteric Behavior of of Al(OH) 3 ( s ) Some of the equilibria are as follows: 1) Al 3+ ( aq ) + OH - ( aq ) Al(OH) 2+ ( aq ) 2) Al(OH) 2+ ( aq ) + OH - ( aq ) Al(OH) 2 + ( aq ) 3) Al(OH) 2 + ( aq ) + OH - ( aq ) Al(OH) 3 ( s ) 4) Al(OH) 3 ( s ) + OH - ( aq ) Al(OH) 4 - ( aq ) 5) Al(OH) 4 - ( aq ) + OH - ( aq ) Al(OH) 5 2- ( aq ) Note that Al 3+ ( aq ) acts as an acid. This species donates protons. The other aluminum species can act either as acids (donating protons) or as bases (accepting protons). Non-Amphoteric Behavior of of Ca(OH) 2 (s) 1) Ca(OH) 2 ( s ) Ca 2+ ( aq ) + 2 OH - ( aq ) The reaction is irreversible. This compound can act only as a base (accepting protons) and has increased solubility only in acidic environments.
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