1 Chapter 10 Acids and Bases 10.3 Strengths of Acids and Bases 10.4 Dissociation Constants

2 Strong acids completely ionize (100%) in aqueous solutions. HCl + H 2 O H 3 O + (aq) + Cl - (aq) (100 % ions)  Strong bases completely (100%) dissociate into ions in aqueous solutions. H 2 O NaOH Na + (aq) + OH - (aq) (100 % ions) Strengths of Acids and Bases

3 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.

4 Only a few acids are strong acids. The conjugate bases of strong acids are weak bases. Strong Acids

5 Most acids are weak acids. The conjugate bases of weak acids are strong bases. Weak Acids

6 Most bases in Groups 1A and 2A are strong bases. They include LiOH, NaOH, KOH, and Ca(OH) 2 Most other bases are weak bases. Strong Bases

7 Strong Acids In water, the dissolved molecules of a strong acid are essentially all separated into ions. The concentrations of H 3 O + and the anion (A - ) are large.

8 Weak Acids In weak acids, The equilibrium favors the undissociated (molecular) form of the acid. The concentrations of the H 3 O + and the anion (A - ) are small.

9 Dissociation Constants In a weak acid, the rate of the dissociation of the acid is equal to the rate of the association. HA + H 2 O H 3 O + + A - The equilibrium expression for a weak acid is K eq = [H 3 O + ][A - ] [HA][H 2 O]

10 Acid Dissociation Constants The [H 2 O] is large and remains unchanged. It is combined with the K eq to write the acid dissociation constant, K a. K a = K eq [H 2 O] = [H 3 O + ][A - ] [HA]

11 Writing K a for a Weak Acid Write the K a for H 2 S. 1. Write the equation for the dissociation of H 2 S. H 2 S + H 2 O H 3 O + + HS - 2. Set up the K a expression K a = [H 3 O + ][HS - ] [H 2 S]