-Ionization Constants of Acids and Bases -Strengths of Acids and Bases Chemistry Mrs. Coyle

Nitric Acid, HNO3 O H N O O

Ionization of an Acid in Water (Dissociation) HA(aq) + H2O(l)  H3O+(aq) + A–(aq) or simply HA(aq)  H+(aq) + A–(aq) The first equation shows H2O, which pulls the H+ from the acid.

Ionization of a Strong Acid in Water (Dissociation) HA(aq) + H2O(l)  H3O+(aq) + A–(aq) or simply HA(aq)  H+(aq) + A–(aq) A strong acid almost fully dissociates. Note: the yield sign may be drawn to the right only to show the almost complete ionization of strong acids.

Strong vs Weak Acids

Common Strong Acids H2SO4 (note:HSO4-1 is weak) HNO3 HClO4 HCl, HBr, HI (note: HF is weak) In general organic acids are weak. Rule of thumb for oxyacids: #O’s- #H’s >=2 the acid is strong. (ex: H3PO4 is weak)

Acid Dissociation Constant for a Weak Monoprotic Acid HA(aq) + H2O(l) H3O+(aq) + A–(aq) Ka = [H3O+] [A–] or Ka = [H+] [A–] [HA] [HA] For a weak acid Ka is small. For strong acids Ka cannot be calculated accurately.

Some Values of Ka HF 7.2 x 10-4 (no units) CH3COOH 1.8 x 10 -5 Which of the above weak acids is stronger?

Example: Calculate the hydronium concentration of a 0.5M solution of HF. (Ka = 7.2 x 10-4) Strategy: Write reaction. Write Ka. Because Ka is very small, use the approximation for the denominator that the equilibrium concentration of the acid is about equal to the original concentration 0.5M. Answer: 1.9 x 10 -2 M

Ionization of a Base (Dissociation) B(aq) + H2O(l)  BH+(aq) + OH–(aq) or BOH  B+(aq) + OH–(aq)

Ionization of a Strong Base in Water (Dissociation) NaOH(s)  Na+(aq) + OH–(aq) A strong base almost fully dissociates. The equilibrium shifts to the right. Note: the yield may be drawn to point only to the right to show the almost complete ionization of strong bases.

Common Strong Bases Hydroxides of Group I and Larger Metals of Group II. Mg(OH)2 and Ca(OH)2 are slightly soluble but what small amount dissolves, almost completely ionizes.

Ionization of a Weak Base in Water (Dissociation) B(aq) + H2O(l)  BH+(aq) + OH–(aq) or BOH(aq) B+(aq) + OH–(aq) Kb = [BH +] [OH–] = [B+] [OH–] [B] [BOH] For a weak base Kb is small. For strong bases Kb cannot be calculated accurately.

Ex: Base Dissociation Constant for a Weak Bases NH 3(aq) + H2O(l) NH 4 +(aq) + OH–(aq) Kb = [NH 4 +] [OH–] [NH 3]