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-Ionization Constants of Acids and Bases -Strengths of Acids and Bases

-Ionization Constants of Acids and Bases -Strengths of Acids and Bases. Chemistry Mrs. Coyle. Nitric Acid, HNO 3. O. H. N. O. O. Ionization of an Acid in Water (Dissociation). HA( aq ) + H 2 O( l )  H 3 O + ( aq ) + A – ( aq ) or simply HA( aq )  H + ( aq ) + A – ( aq )

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-Ionization Constants of Acids and Bases -Strengths of Acids and Bases

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

  2. Nitric Acid, HNO3 O H N O O

  3. 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.

  4. 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.

  5. Strong vs Weak Acids

  6. Common Strong Acids • H2SO4 (note:HSO4-1is 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)

  7. 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.

  8. 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?

  9. 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

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

  11. 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.

  12. 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.

  13. 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.

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

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