Conjugate Acid & Base Pair
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Topic Summary & Highlights
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Core Concept
A conjugate acid-base pair consists of two species that differ by one proton ($H^+$).
The acid donates a proton ($H^+$) to become its conjugate base.
The base accepts a proton ($H^+$) to become its conjugate acid.
General Representation:
$\text{Acid} \rightleftharpoons \text{Conjugate Base} + H^+$
$\text{Base} + H^+ \rightleftharpoons \text{Conjugate Acid}$
Practice Tips
Conjugate pairs differ by one proton ($H^+$).
Strong acids have weak conjugate bases, and strong bases have weak conjugate acids.
Understanding conjugate pairs helps predict the direction of acid-base reactions and the behavior of buffer systems.
Examples of Conjugate Pairs
| Acid | Conjugate Base | Base | Conjugate Acid |
|---|---|---|---|
| HCl | Cl⁻ | NH₃ | NH₄⁺ |
| H₂O | OH⁻ | OH⁻ | H₂O |
| H₃PO₄ | H₂PO₄⁻ | CO₃²⁻ | HCO₃⁻ |
| HNO₃ | NO₃⁻ | CH₃COO⁻ | CH₃COOH |
How to Identify Conjugate Pairs
Start with a Reaction: For example:
$HCl + H_2O \rightleftharpoons H_3O^+ + Cl^-$
Identify the Acid and Base:
HCl: Donates $H^+$, so it’s the acid.
$H_2O$: Accepts $H^+$, so it’s the base.
Determine the Conjugates:
$Cl^−$: Formed when HCl loses $H^+$, so it’s the conjugate base.
$H_3O^+$: Formed when $H_2O$ gains $H^+$, so it’s the conjugate acid.
Relationship Between Acidity and Basicity
Strength of Acids and Bases:
A strong acid has a weak conjugate base (e.g., HCl and Cl^−).
A strong base has a weak conjugate acid (e.g., $OH^−$ and $H_2O$).
Weak acids and weak bases form conjugates that are relatively stronger.
Amphiprotic Substances:
Some species, like $H_2O$, can act as both an acid and a base:
As an acid: $H_2O \rightarrow OH^-$
As a base: $H_2O + H^+ \rightarrow H_3O^+$