1. The Definition
- The Brønsted-Lowry theory implies that acid-base reactions are reversible.
- When an acid acts as a proton donor, it forms a new species that has the potential to accept that proton back.
Conjugate Acid-Base Pair
A pair of species that differ by exactly one proton (
).
- The acid has the proton.
- The base lacks the proton.
2. The General Equation
- Pair 1:
and - Pair 2:
and
Summary
3. Examples
Example 1: Methanoic Acid + Water
- Acid:
(Donates H) Conj Base 1: - Base:
(Accepts H) Conj Acid 2:
Example 2: Ammonia + Water
- Base:
(Accepts H) Conj Acid 1: - Acid:
(Donates H) Conj Base 2:
4. The Inverse Relationship of Strength
The "See-Saw" Rule
- The Stronger the Acid
The Weaker its Conjugate Base. - The Weaker the Acid
the Stronger its Conjugate Base.
Why does this matter?
- It explains why reactions proceed in a certain direction.
- Equilibrium always favours the formation of the weaker acid and base (the more stable species).
Case A: Strong Acid (
- Since
is a very strong acid (desperate to donate proton), is an incredibly weak base (has zero desire to accept the proton back). - Result: The reaction goes to completion.
Case B: Weak Acid (Ethanoic Acid)
- Since
is a weak acid, is a reasonably strong conjugate base. - The
actively attacks protons to reform the acid. - Result: The reaction is reversible and equilibrium lies to the left.
5. Amphoteric Nature of Water
Notice in the examples above that water can be part of two different pairs depending on what it reacts with.
1. Water as a Base
- Conjugate Acid of water is Hydronium (
).
2. Water as an Acid
- Conjugate Base of water is Hydroxide (
).