Acids and Bases: Ionization Constants
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Predicting Acid-Base Reactions
Ammonia is a weak base and will react with acetic acid (a weak acid) to form ammonium and acetate ions:
NH3 + CH3COOH 
NH4+ + CH3COO-
Will ammonia react with any weak acid? Suppose sodium hydrogen phosphate (Na2HPO4) was added to ammonia. Will the following reaction be product favored?
NH3 + HPO42- NH4+ + PO42-
In order to answer this question, let's look at the reaction of ammonia with acetic acid and determine why it occurs. In any acid-base reaction, an acid reacts with a base to form another acid and base:
The Kb of NH3 is 1.8 x 10-5 and the Ka of acetic acid is 1.8 x 10-5.
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NH3
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CH3COO-
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CH3COOH
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NH4+
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You can calculate the Kb of CH3COO- using the Ka of CH3COOH:
Ka x Kb = Kw
You can calculate the Ka of NH4+ using the Kb of NH3:
Ka x Kb = Kw
Good! This shows that the stronger base reacted with the stronger acid. This is always true of acid-base reactions. We can also use the ionization constants to show quantitatively that this is a product favored reaction:
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Since the equilibrium constant for the reaction of ammonia and acetic acid is greater than one, it is a product favored reaction.
The Ka of HPO42- is 3.6 x 10-13. Will the reaction between NH3 and HPO42- be product or reactant favored?
| Product Favored | Reactant Favored |
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Remember the favored reaction will be between the strongest acid and the strongest base.
Good! The ionization constant for HPO42- is 3.6 x 10-13 while the ionization constant for NH4+ is 5.6 x 10-10. Therefore NH4+ is the stronger acid. The ionization constant for NH3 is 1.8 x 10-5 while the ionization constant for PO43- is 2.8 x 10-2. Therefore PO43- is the stronger base. The favored reaction will be between the stronger acid and the stronger base. Therefore, the reaction between NH4+ and PO43- will be favored.
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