Acids & Bases Part III: Other Types of Acids & Bases Jespersen Chapter 16 Sec 4 & 5 Dr. C. Yau Fall 2014 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 e- acceptor. coordinate covalent bond Lewis Base Lewis Acid 2 No H+ is involved in this example!

Examples of Lewis Acids and Bases OH- + CO2 HCO3- e- pair donor e- pair acceptor Lewis base Lewis acid Which is the Lewis acid? Lewis base? How are these acids/bases defined?? Where is the coordinate covalent bond?

Examples of Lewis Acids and Bases SO2 (g) + CaO (s) CaSO3 (s) SO2 (g) + O2- SO32- Begin by looking for the coordinate covalent bond. Where did the electrons come from? Do Practice Exercise 18 & 19 on p. 758 Be sure to practice putting in arrows to show how electron pairs flow in the reaction.

Brønsted acid-base viewed as Lewis acid-base H3O+ + NH3 H2O + NH4+ 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?

Oxides as Acids & Bases This is something you learned back in Gen Chem I: Metal oxides + water basic solutions Na2O (s) + H2O (l) 2NaOH (aq) Nonmetal oxides + water acid solutions SO3 (g) + H2O (l) H2SO4 (aq) N2O5 (g) + H2O (l) 2 HNO3 (aq) CO2 (g) + H2O (l) H2CO3 (aq)

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+ + H2O M(H2O)n+ The hydrated metal ion is acting as a Brønsted acid. Al(H2O)63+ + H2O Al(H2O)5(OH)2+ + H3O+ (It donated a proton to water.)

Metals as Lewis Acids Practice writing the rxn of metal ions as Lewis acids: Fe3+ (aq) + 6 H2O (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.

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

Acid Strengths of Metal Ions Group IA metal ions have only a charge 1+ and do not act as Lewis acids. Group IIA metal ions have a higher charge (2+) but only Be2+ is small enough (positive charge density is high enough) to act as Lewis acids. Transition metals, especially ones with 3+ (such as Fe3+ and Cr3+) have a tendency to act as Lewis acids. Lewis acids are frequently mentioned in organic chemistry.

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. Al2O3 + 6H+ 2Al3+ + 3H2O (Rxn 1) Al2O3 + 2OH- 2AlO2- + H2O (Rxn 2) Al3+ is amphoteric. (It is acting as base in Rxn 1, as acid in Rxn 2) Metals with high oxidation numbers becomes acidic. CrO3 has an oxidation number of +6 and is acidic. CrO3 + H2O H2CrO4 (a strong acid)

Identifying Lewis Acids and Bases Molecules & ions with incomplete valence shells (e.g. BF3, 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 (SO2 SO32-) Lewis bases Molecules & ions that have complete valence shells with unshared electrons (e.g. OH-, NH3) What do they do? ChemFAQs: How can I tell whether a substance is likely to act as a Lewis acid? How can I tell whether a substance is likely to act as a Lewis base?

Identify the Lewis acids & bases in each aqueous rxn. Practice Exercise 18 p. 758 Identify the Lewis acids & bases in each aqueous rxn. Hint: Draw Lewis structures of the reactants.   a) NH3 + H+ NH4+ b) (CH3)2O + BCl3 (CH3)2OBCl3 c) Ag+ + 2NH3 Ag(NH3)22+ Do Practice Exercise 19 as well

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