Conjugate acid–base pairs are a central idea in IB Chemistry They help explain reversible reactions, buffer systems, pH changes, weak acid behavior, and equilibrium expressions. Once you understand how conjugate pairs work, acid–base reasoning becomes far easier, especially for equilibrium and titration questions.
What Is a Conjugate Acid–Base Pair?
A conjugate acid–base pair consists of two species that differ by one proton (H⁺).
- A base becomes its conjugate acid after gaining a proton.
- An acid becomes its conjugate base after losing a proton.
This concept is based on the Brønsted–Lowry definition, which describes acids as proton donors and bases as proton acceptors.
Examples of Conjugate Acid–Base Pairs
Here are the most common IB-style examples:
1. HCl / Cl⁻
- HCl (acid) donates a proton → Cl⁻ (conjugate base)
2. NH₃ / NH₄⁺
- NH₃ (base) accepts a proton → NH₄⁺ (conjugate acid)
3. H₂CO₃ / HCO₃⁻
- H₂CO₃ loses a proton → HCO₃⁻ (conjugate base)
4. HCO₃⁻ / CO₃²⁻
- HCO₃⁻ can lose another proton → CO₃²⁻
(HCO₃⁻ is amphiprotic because it can act as both acid and base)
A single species can have two conjugate partners if it is amphiprotic.
How Conjugate Pairs Work in Acid–Base Reactions
Consider the reaction:
