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Acids & Bases Part III: Other Types of Acids & Bases

This text explains Lewis acid-base definitions and showcases examples of Lewis acids and bases, as well as the role of oxides and metals as acids. It also highlights the difference between Brønsted and Lewis acid-base definitions.

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Acids & Bases Part III: Other Types of Acids & Bases

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  1. Acids & BasesPart III: Other Types of Acids & Bases Jespersen Chapter 16 Sec 4 & 5 Dr. C. Yau Fall 2014 1

  2. 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. Lewis Base Lewis Acid coordinate covalent bond 2 No H+ is involved in this example!

  3. 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?

  4. 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.

  5. Brønsted acid-base viewed as Lewis acid-base 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. H3O+ + NH3 H2O + NH4+ Note that this is a transfer of a Lewis acid between two Lewis bases. How?

  6. 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)

  7. 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.)

  8. 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.

  9. 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

  10. 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.

  11. 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)

  12. Identifying Lewis Acids and Bases What do they do? • Lewis acids • 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)

  13. 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

  14. 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

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