Ch17_Lewis_6 - Acid-Base Theories •...

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