Conjugate acid–base pairs are a central idea in IB Chemistry Topic 8 (Acids and Bases). 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:
HCl + H₂O → H₃O⁺ + Cl⁻
Here we identify two conjugate pairs:
- HCl (acid) ↔ Cl⁻ (conjugate base)
- H₂O (base) ↔ H₃O⁺ (conjugate acid)
When an acid donates H⁺, it becomes its conjugate base.
When a base accepts H⁺, it becomes its conjugate acid.
This reversible relationship is central to understanding equilibrium.
Acids and Their Conjugate Bases
Strong acids have weak conjugate bases.
Example:
- HCl is strong → Cl⁻ is very weak
- HNO₃ is strong → NO₃⁻ is very weak
- H₂SO₄ is strong → HSO₄⁻ is weak
Why?
Because if an acid donates a proton easily, the conjugate base has very little tendency to re-accept it.
Bases and Their Conjugate Acids
Strong bases have weak conjugate acids.
Example:
- OH⁻ is strong → H₂O is a very weak acid
- O²⁻ is strong → OH⁻ is weak as an acid
A strong base accepts protons easily, so its conjugate acid is not good at donating them.
Conjugate Pairs in Weak Acid and Weak Base Systems
Weak acids and bases establish equilibrium, making conjugate pairs especially important.
For a weak acid HA:
HA ⇌ H⁺ + A⁻
- HA = weak acid
- A⁻ = conjugate base
Because the acid is weak, both species exist in significant amounts.
For a weak base B:
B + H₂O ⇌ BH⁺ + OH⁻
- B = weak base
- BH⁺ = conjugate acid
The balance between these species controls pH.
Conjugate Pairs in Buffer Solutions
Buffers rely entirely on conjugate pairs.
A buffer contains:
- A weak acid and its conjugate base
or - A weak base and its conjugate acid
When acid is added, the conjugate base neutralizes it.
When base is added, the conjugate acid neutralizes it.
Understanding conjugate pairs is essential for predicting buffer behavior.
Why Conjugate Pairs Matter in IB Chemistry
They help explain:
- Acid and base strength
- Reversible reactions
- pH changes
- Buffer action
- Equilibrium constant expressions (Ka and Kb)
- Why some species are amphiprotic
- Bronsted–Lowry mechanisms
They also appear frequently in Paper 2 and Paper 3 data-analysis questions.
Common IB Misconceptions
“Conjugate means similar structure.”
No—conjugates differ by exactly one proton.
“Water is always neutral.”
Water can be a base or acid depending on the reaction.
“Stronger conjugate base means stronger acid.”
It's the opposite. Strong acids produce weak conjugate bases.
FAQs
How do you identify a conjugate acid–base pair quickly?
Look for two species that differ by exactly one H⁺.
Can a species act as both acid and base?
Yes. Amphiprotic species like HCO₃⁻ and H₂O can either donate or accept protons.
Do conjugate pairs exist in strong acids?
Yes, but the conjugate base of a strong acid is extremely weak.
Conclusion
A conjugate acid–base pair consists of two species that differ by a single proton. The acid donates H⁺ to become its conjugate base, while the base accepts H⁺ to become its conjugate acid. This relationship explains equilibrium behavior, acid–base strength, buffer systems, and many calculations in IB Chemistry.
