Acids Bases Part III Other Types of Acids

Acids & Bases Part III: Other Types of Acids & Bases Jespersen Chapter 16 Sec 4 & 5 Dr. C. Yau Spring 2013 1

Yet a Different Definitions of Acid/Base A Lewis acid is any ionic or molecular species that can accept a pair of electrons to form a coordinate covalent bond. A Lewis base is any ionic or molecular species that can donate a pair of electrons in the formation of a coordinate covalent bond. Neutralization is the formation of a coordinate covalent bond between e- donor and the eacceptor. Lewis Base Lewis Acid coordinate covalent bond 2

Examples of Lewis Acids and Bases OH + CO 2 HCO 3 e- pair donor acceptor Lewis base Lewis acid Which is the Lewis acid? Lewis base? How are these acids/bases defined? ? Where is the coordinate covalent bond? 3

Examples of Lewis Acids and Bases SO 2 (g) + Ca. O (s) SO 2 (g) + O 2 - Ca. SO 3 (s) SO 32 - Do Practice Exercise 18 & 19 on p. 758 Be sure to practice putting in arrows to show electron pairs flow in the reaction. 4

Brønsted acid-base viewed as Lewis acid-base H 3 O+ + NH 3 H 2 O + NH 4+ To figure out which is the Lewis acid & which is the Lewis base, draw the Lewis structure and put in arrows to show the flow of electrons in the rxn. Note that this is a transfer of a Lewis acid between two Lewis bases. How? 5

Oxides as Acids & Bases This is something you learned back in Gen Chem I: Metal oxides + water basic solutions Na 2 O (s) + H 2 O (l) 2 Na. OH (aq) Nonmetal oxides + water acid solutions SO 3 (g) + H 2 O (l) H 2 SO 4 (aq) N 2 O 5 (g) + H 2 O (l) 2 HNO 3 (aq) CO 2 (g) + H 2 O (l) H 2 CO 3 (aq) 6

Metals as Lewis Acids When metal ions go into solution, they become "hydrated", i. e. they are surrounded by water molecules. The metal ion is acting as a Lewis acid. Mn+ + H 2 O M(H 2 O)n+ The hydrated metal ion is acting as a Brønsted acid. Al(H 2 O)63+ + H 2 O Al(H 2 O)5(OH)2+ (It donated a + H 3 O + proton to water. ) 7

Metals as Lewis Acids Practice writing the rxn of metal ions as Lewis acids: Fe 3+ (aq) + 6 H 2 O (l) ? Hint: As a Lewis acid, it accepts electrons. Now, write the rxn of the hydrated metal ion as a Brønsted acid. Hint: As an Brønsted acid it is a proton donor. 8

Acid Strengths of Metal Ions The smaller metal ions have a higher positive charge density (+ charge is forced into a smaller space). The smaller metal ions are more acidic. Why? ACID STRENGTH 9

Acid Strengths of Metal Ions Group IA metal ions have only a charge 1+ and do not act as acids. Group IIA metal ions have a higher charge (2+) but only Be 2+ is small enough (positive charge density is high enough) to act as an acid. Transition metals, especially ones with 3+ (such as Fe 3+ and Cr 3+) have a tendency to act as an acid. 10

Effect of Oxidation Number We had said that metal oxides in water becomes a base. However… for metal oxides, as the oxidation number increases, the tendency of the metal to act as an acid increases. Al 2 O 3 + 6 H+ 2 Al 3+ + 3 H 2 O (Rxn 1) Al 2 O 3 + 2 OH 2 Al. O 2 - + H 2 O (Rxn 2) Al 3+ is amphoteric. (It is acting as base in Rxn 1, as acid in Rxn 2) Metals with high oxidation numbers becomes acidic. Cr. O 3 has an oxidation number of +6 and is acidic. Cr. O 3 + H 2 O H 2 Cr. O 4 (a strong acid) 11

Identifying Lewis Acids and Bases • Lewis acids What do they do? – Molecules & ions with incomplete valence shells (e. g. BF 3, H+) – Molecules & ions with multiple bonds that can be shifted to accept electrons (e. g. O=C=O) – Molecules or ions with central atoms that can accommodate additional electrons (SO 2 SO 32 -) • Lewis bases – Molecules & ions that have complete valence shells with unshared electrons (e. g. OH-, NH 3) 12

Practice Exercise 18 p. 758 Identify the Lewis acids & bases in each aqueous rxn. Hint: Draw Lewis structures of the reactants. a) NH 3 + H+ NH 4+ b) (CH 3)2 O + BCl 3 (CH 3)2 OBCl 3 c) Ag+ + 2 NH 3 Ag(NH 3)22+ Do Practice Exercise 19 as well 13

Brønsted vs. Lewis Definitions Brønsted Acid: H+ donor (proton donor) Lewis Acid : e- pair acceptor Brønsted Base: H+ acceptor (proton acceptor) Lewis Base: e- pair donor 14