1 9.9 The Strengths of Acids and Bases Chapter 9 Acids, Bases, & Salts

2 A strong acid completely ionizes (100%) in aqueous solutions. HCl(g) + H 2 O(l) H 3 O + (aq) + Cl − (aq) A weak acid dissociates only slightly in water to form a few ions in aqueous solutions. H 2 CO 3 (aq) + H 2 O(l) H 3 O + (aq) + HCO 3 − (aq) Strengths of Acids

3 Strong Acids In water, the dissolved molecules of a strong acid dissociate into ions. give large concentrations of H 3 O + and the anion (A - ). Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

4 Weak Acids In weak acids, only a few molecules dissociate. most of the weak acid remains as the undissociated (molecular) form of the acid. the concentrations of the H 3 O + and the anion (A - ) are small. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings HA(aq) + H 2 O(l) H 3 O(aq) + A − (aq)

5 Strong and Weak Acids In an HCl solution, the strong acid HCl dissociates 100%. A solution of the weak acid CH 3 COOH contains mostly molecules and a few ions.

6 Strong acids make up six of all the acids. have weak conjugate bases. Strong Acids

7 Weak Acids Weak acids make up most of the acids. have strong conjugate bases.

8 Comparing Strong and Weak Acids

9 Strong Bases Strong bases are formed from metals of Groups 1A (1) and 2A (2). include LiOH, NaOH, KOH, and Ca(OH) 2. dissociate completely in water. KOH(s) K + (aq) + OH − (aq)

10 Weak Bases Weak bases are most other bases. dissociate only slightly in water. form only a few ions in water. NH 3 (g) + H 2 O(l) NH 4 + (aq) + OH − (aq)

11 Learning Check Identify each of the following as a strong or weak acid or base. A. HBr B. HNO 2 C. NaOH D. H 2 SO 4 E. Cu(OH) 2

12 Solution Identify each of the following as a strong or weak acid or base. A. HBrstrong acid B. HNO 2 weak acid C. NaOHstrong base D. H 2 SO 4 strong acid E. Cu(OH) 2 weak base

13 Learning Check Identify the stronger acid in each pair. 1. HNO 2 or H 2 S 2. HCO 3 − or HBr 3. H 3 PO 4 or H 3 O +

14 Solution Identify the stronger acid in each pair. 1. HNO 2 2. HBr 3. H 3 O +

15 Acid Dissociation Constant This is the equilibrium constant for the dissociation of an acid HA(aq) + H 2 O(l)  H 3 O + (aq) + A - (aq) The equilibrium expression for this reaction is K = The acid dissociation constant, K a, is simply K[H 2 O] = K a = [H 3 O + ][A - ] [HA][H 2 O] [H 3 O + ][A - ] [HA]

16 Base Dissociation Constant This is the equilibrium constant for the dissociation of a base B(aq) + H 2 O(l)  OH - (aq) + HB + (aq) The equilibrium expression for this reaction is K = The base dissociation constant, K b, is simply K[H 2 O] = K b = [OH - ][HB + ] [B][H 2 O] [OH - ][HB + ] [B]

17 Learning Check Write dissociation reactions and expressions for K a or K b for each of the following weak acids and bases: 1. Hydrocyanic acid (HCN) 2. Phosphoric acid (H 3 PO 4 ) (1 st H only) 3. Dihydrogen phosphate ion (H 2 PO 4- ) (1 st H only) 4. Ammonia (NH 3 )

18 Solution 1. HCN  H + + CN - ; K a = 2. H 3 PO 4  H + + H 2 PO 4 - ; K a = 3. H 2 PO 4 -  H + + HPO 4 2- ; K a = 4. NH 3  OH - + NH 4 + ; K b = [H + ][CN - ] [HCN] [H + ][H 2 PO 4 - ] [H 3 PO 4 ] Note that [H 3 O + ] = [H + ] [H + ][HPO 4 2- ] [H 2 PO 4 - ] [OH - ][NH 4 + ] [NH 3 ]