Ch17_Lewis_6

Ch17_Lewis_6 - Acid-Base Theories Lewis theory Gilbert N....

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Unformatted text preview: Acid-Base Theories Lewis theory Gilbert N. Lewis 1875-1946 Formation of hydronium ion is also an hydronium is excellent example. • •• •• + • O—H H O—H + H H H ACID BASE 2 Lewis acid = electron pair acceptor (BF 3) • Bronsted -Lowry—applies to water solutions Lewis Acids & Bases Lewis Acids & Bases 1 Lewis Acids & Bases 3 A Lewis acid and base can interact by sharing an electron pair. Coordinate covalent bond Lewis base = electron pair donor (NH 3) 4 Lewis Acids & Bases Other good examples involve metal ions. Co 2+ ACID • •• • O—H Co 2+ H BASE • •• • O—H H 5 Lewis Acids & Bases The combination of metal ions (Lewis acids) with Lewis bases such as H2O and NH 3 ------> COMPLEX IONS All metal ions form complex ions with water —and are of the type [M(H2O)x]n+ where x = 4 and 6. Page 1 6 [Cu(NH3)4]2+ Lewis Acids & Bases 7 8 Lewis Acids & Bases Lewis Acids & Bases Add NH3 to light blue [Cu(H 2O)4]2+ ------> The Fe2+ in heme can interact with O 2 or CO in a Lewis acid-base reaction. [Ni(H2O)6]2+ + 6 NH3 ---> [ Ni(NH3)6 ]2+ light blue Cu(OH) 2 and then deep blue [Cu(NH 3)4]2+ 9 + DMG 10 Lewis Acids & Bases Many complex ions containing water undergo HYDROLYSIS to give acidic solutions. [Cu(H2 O)4]2+ + H 2O ---> [Cu(H2O)3(OH)]+ + H3O+ 11 Lewis Acids & Bases This explains why water solutions of Fe 3+, Al3+, Cu2+, Pb2+, etc. are acidic. 12 Amphoterism of Al(OH)3 This interaction weakens this bond Another H2O Another H2O pulls this H pulls this H away as H+ away as H+ Page 2 Al(OH)3 on right Add NaOH See Kotz/Treichel, page 830 Add HCl Lewis Acids & Bases 13 Neutral Lewis Acid 14 Carbon dioxide is a neutral Lewis acid. This explains AMPHOTERIC nature of This AMPHOTERIC nature some metal hydroxides. Lewis Acids & Bases Many complex ions are very stable. Al(OH)3(s) + 3 H + --> Al3+ + 3 H2 O Cu2+ + 4 NH3 ¸ [Cu(NH 3)4]2+ [Cu(NH Here Al(OH)3 is a Brønsted base. K for the reaction is called Al(OH)3(s) + OH- --> Al(OH)4- Kformation •• O—H•• or a “formation constant” •• Here Al(OH)3 is a Lewis acid. Al3+ -0.75 Lewis Acids & Bases 16 +1.5 Here K = 6.8 x 10 12. Reaction is strongly product-favored. -0.75 Lewis Acids & Bases Formation of complex ions explains why you can dissolve a ppt. by forming a complex ion. Formation of complex ions explains why you can dissolve a ppt. by forming a complex ion. AgCl (s) + 2 NH3 ¸ AgCl(s) ¸ Ag+ + Cl (s) Ag Ag+ Ag(NH3)2+ + ClAg + 2 NH3 --> Ag(NH3)2 Ksp = 1.8 x 10 -10 + Kform = 1.6 x 10 7 ------------------------------------AgCl(s) + 2 NH3 ¸ Ag(NH3)2+ + ClAg AgCl(s) Knet = Ksp • Kform = 2.9 x 10 -3 Page 3 17 15 ...
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