Ionization Constants of Acids and Bases

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Ionization Constants of Acids and Bases

• Honors Chemistry
• Mrs. Coyle

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Sulfuric Acid, H2SO4
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Nitric Acid, HNO3
O
H
N
O
O
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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.

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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.
• The equilibrium shifts to the right.
• Its conjugate base is weak (it has a low affinity
  for the proton).
• Note the yield sign may be drawn to the right
  only to show the almost complete ionization of
  strong acids.

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Strong vs Weak Acids
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(No Transcript)
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Common Strong Acids

• H2SO4 (noteHSO4-1 is weak)
• HNO3
• HClO4
• HCl, HBr, HI (note HF is weak)
• In general organic acids are weak.
• Rule of thumb for oxyacids Os- Hs gt2 the
  acid is strong. (ex H3PO4 is weak)

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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.

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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?

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Example

• Calculate the hydronium concentration of a 0.5M
  solution of HF. (Ka 7.2 x 10-4)
• Strategy Write reaction. Set up ICE. Write Ka.
• Because Ka is very small, use approximation for
  denominator that 0.5M-x 0.5M.
• Answer 1.9 x 10 -2 M

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Percent Ionization

• Ionization Amount Ionized x100
• Original Acid

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Ionization of a Base (Dissociation)

• B(aq) H2O(l) ?? BH(aq) OH(aq)
• or
• BOH ? B(aq) OH(aq)

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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.

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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.

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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.

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Ex Base Dissociation Constant for a Weak Bases

• NH 3(aq) H2O(l) NH 4 (aq)
  OH(aq)
• Kb NH 4 OH
• NH 3