8.2 Strong and Weak Acids and Bases

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Acids:

• Strong acids/bases: ionize almost to 100% in water, producing hydrogen ions (acids) or hydroxide ions (bases).
• Weak acids/bases: only partly ionize in water.

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• Strong acids have a very large Ka, equilibrium position is far to the right, [HA(aq)]initial  [H (aq)]equilibrium

• Weak acids have a small Ka, equilibrium position is far to the left, [HA(aq)]initial >> [H+ (aq)]equilibrium

• List of Ka values on pg. 496 Table 2 or pg. 726 appendix B5

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• Oxyacids: an acid in which the acidic hydrogen atom is attached to an oxygen atom:

  

• Organic acids: an acids containing a carboxyl group (COOH)

  

Bases

• Strong Bases – hydroxides formed with metals from group I and II on the periodic table. Group
  I bases have a high solubility, and high dissociation. We can say that virtually all of the base
  dissociates to form the ions:

• BIG K

  

• Although group II hydroxides are strong bases, they have low solubility in water:

  • Not a lot dissolves, but what dissolves forms a lot of hydroxides. Interesting dichotimies.

  

• Weak Bases – only partially react to produce hydroxide ions. A good example is ammonia: has
  one lone electron pair capable of forming coordinate bonds.

  

• In general, the reaction of a weak base in water is:

  

• Write the expression for Kb when ammonia reacts with water:

  

• Autoionization of water: the transfer of a hydrogen ion from one water molecule to another:

  • This is called the ion-product constant for water (Kw)

  

• At 25 oC, [H+ (aq)] is 1.0 x 10-7 mol/L and [OH- (aq)] is 1.0 x 10-7 mol/L. Calculate Kw:

  • Kw = )concentration of hydronium)(concentration of hydroxide)
  • 1.0 x 10^-14
  • Always the equilibrium constant for the autoionization of water

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• Three situations:

  • Kw = KaKb

  

• Overall, as the strength of the acid increases, strength of the conjugate base decreases, and
  vice versa:

  1. A strong acid/base will have a very weak conjugate (only forward rx)
  2. A weak acid/base will have a weak conjugate (equilibrium)
  3. A very weak acid/base will have a strong conjugate (no reaction)

If you need concentrations: