Acid dissociation constant Ka

1
Slide 1
Acid dissociation constant Ka

• HA(aq) H(aq) A-(aq)
• Ka HA-
• HA
• We can write the expression for any acid.
• Strong acids dissociate completely.
• Equilibrium far to right.
• Conjugate base must be weak.

2
Slide 2
Acid dissociation constant Ka

• The equilibrium constant for the general
  equation.
• HA(aq) H2O(l) H3O(aq) A-(aq)
• Ka H3OA- HA
• H3O is often written H ignoring the water in
  equation (it is implied).
• We can write the expression for any acid.
• Strong acids dissociate completely.
• Equilibrium far to right.
• Conjugate base must be weak.

3
Slide 3
Strong Acids

• HBr, HI, HCl, HNO3, H2SO4, HClO4
• ALWAYS WRITE THE MAJOR SPECIES
• Completely dissociated
• H HA
• OH- is going to be small because of equilibrium
• 10-14 HOH-
• If HAlt 10-7 water contributes H

4
Types of Acids
Slide 4

• Polyprotic Acids- more than 1 acidic hydrogen (
  diprotic, triprotic)
• Polyprotic acids dissociate in a stepwise manner
• Each step is characterized by its own
  dissociation constant, Ka1, Ka2, etc.

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For Bases
Slide 5

• You can use the same equation for finding the
  constant for bases (Kb)
• B(aq) H2O(l) ? BH(aq) OH- (aq)
• Kb BHOH- B

6
Slide 6
Back to Pairs

• Strong acids
• Ka is large
• H is equal to HA
• A- is a weaker base than water

• Weak acids
• Ka is small
• H lt HA
• A- is a stronger base than water

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Polyprotic acids cont.
Slide 7

• H2CO3 H2O ? H3O HCO3-
• Ka1 4.3 x 10-7
• Ka1 H3O HCO3-
• H2CO3
• HCO3- H2O ? H3O CO3-2
• 
  
• Ka2
  5.6 x 10-11
• Ka1 H3O CO3-2
• HCO3-

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Homework

• 15.21, 75, 80, 86, 87