Conjugate Acid–Base Pairs
Quick Notes
- A conjugate acid–base pair differs by one proton (H⁺).
- When an acid donates a proton, it forms its conjugate base.
- When a base accepts a proton, it forms its conjugate acid.
- In any Brønsted–Lowry reaction, there are always two conjugate pairs.
- Example: HCl + H₂O ⇌ Cl⁻ + H₃O⁺
HCl and Cl⁻ are one pair; H₂O and H₃O⁺ are another.
Full Notes
What Are Conjugate Acid–Base Pairs?
In a reaction, when an acid donates a H⁺ ion, a negatively charged ion is left over. If this negative ion accepts a proton, it would re-form the original acid and therefore would act as a base.
Equally, if a base accepts a proton it could then later release the proton, acting as an acid.
These linked species are called conjugate pairs.
An acid dissociates, forming its conjugate base. In reverse, a base can accept a proton (H⁺ ion) and form its conjugate acid.
Definition
A conjugate acid–base pair consists of two species that differ by a single proton (H⁺):
- An acid loses H⁺ and forms its conjugate base.
- A base gains H⁺ and forms its conjugate acid.
Identifying Conjugate Pairs
Acid–base reactions follow the general reaction:
Acid + Base ⇌ Conjugate Base + Conjugate Acid
Example H₂O + NH₃ ⇌ OH⁻ + NH₄⁺
- NH₃ is a base and NH₄⁺ is its conjugate acid.
- H₂O is an acid and OH⁻ is its conjugate base.
How to Deduce a Conjugate
To find the conjugate base of an acid, remove one H⁺
For Example H₂SO₄ → HSO₄⁻ (conjugate base)
To find the conjugate acid of a base, add one H⁺:
For Example CO₃²⁻ → HCO₃⁻ (conjugate acid)
Common Examples of Conjugate Acid–Base Pairs
| Acid | Conjugate Base |
|---|---|
| HCl | Cl⁻ |
| HNO₃ | NO₃⁻ |
| H₂SO₄ | HSO₄⁻ |
| HCO₃⁻ | CO₃²⁻ |
| Base | Conjugate Acid |
|---|---|
| NH₃ | NH₄⁺ |
| OH⁻ | H₂O |
| CO₃²⁻ | HCO₃⁻ |
| HSO₄⁻ | H₂SO₄ |
Important Note
In acid–base equilibrium, identifying conjugate pairs helps predict reaction direction and relative strengths:
- A strong acid has a weak conjugate base.
- A weak acid has a relatively stronger conjugate base.
Summary
- A conjugate acid–base pair differs by one proton.
- An acid forms its conjugate base by losing H⁺.
- A base forms its conjugate acid by gaining H⁺.
- Strong acids have weak conjugate bases; weak acids have stronger conjugate bases.
Linked Course Question
What are the conjugate acids of the polyatomic anions listed in 2.1?
The conjugate acid of a polyatomic anion is formed by adding a proton (H⁺) to the ion. Each H⁺ added increases its charge by +1.
- OH⁻ (hydroxide) → H₂O (water)
- NO₃⁻ (nitrate) → HNO₃ (nitric acid)
- HCO₃⁻ (hydrogencarbonate) → H₂CO₃ (carbonic acid)
- CO₃²⁻ (carbonate) → HCO₃⁻ (hydrogencarbonate)
- SO₄²⁻ (sulfate) → HSO₄⁻ (hydrogensulfate)
- PO₄³⁻ (phosphate) → HPO₄²⁻ (hydrogenphosphate)
Note: NH₄⁺ (ammonium) is already the conjugate acid of NH₃ (ammonia), so it doesn’t form a further conjugate acid.