Acids and Bases: Ionization Constants
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Ionization Constants for Conjugate Pairs
Recall that every acid has a conjugate base:
The
Ka of hypochlorous acid (HOCl) 3.0 x 10-8. The Ka
of hypoiodous acid (HOI) is 2.3 x 10-11. Which is the stronger
acid?
| HOCl | HOI |
 |
|
Remember
that a larger Ka means more ionization.
Good!
The
conjugate bases of HOCl and HOI are OCl- and OI-
respectively. Which conjugate base is stronger?
| OCl- | OI- |
|
|
One way to approach this question is to look at the chemical equations:
Since the Ka for the first equation is larger than the Ka for the second equation, the ionization of HOCl is a more product favored reaction than the ionization of HOI. The back reaction shows the basic nature of OCl- and OI- (it shows them accepting a proton). Which back reaction will occur to a greater extent?
Good! If the ionization of HOCl is more favored than the ionization of HOI, than OCl- will be less likely to accept a proton, making it the weaker base.
This example shows that the stronger the acid, the weaker its conjugate base. Another way of saying this is that the strength of an acid and its conjugate base are inversely proportional. Since Ka and Kb are quantitative measurements of strength, you could also express this relationship mathematically:
Ka x Kb = CONSTANT
Fill
in the following table to determine the value of this constant.
