Acid-Base Concepts
Arrhenius, Brønsted-Lowry & Lewis Theories | Ionic Equilibrium
1. Arrhenius Concept (1887)
The most basic definition, restricted to aqueous solutions.
- Acid: A substance that dissociates in water to give Hydrogen ions ($H^+$).
Ex: $HCl \rightleftharpoons H^+ + Cl^-$ - Base: A substance that dissociates in water to give Hydroxyl ions ($OH^-$).
Ex: $NaOH \rightleftharpoons Na^+ + OH^-$
Limitations:
- Applicable only in aqueous medium.
- Cannot explain acidic nature of $CO_2, SO_2$ (no H) or basic nature of $NH_3$ (no OH).
2. Brønsted-Lowry Concept (1923)
A broader definition based on proton transfer.
- Acid: Proton ($H^+$) Donor.
- Base: Proton ($H^+$) Acceptor.
Conjugate Acid-Base Pairs
An acid and a base that differ only by a proton ($H^+$) are called a conjugate pair.
$$ \underset{\text{Acid}_1}{HCl} + \underset{\text{Base}_2}{H_2O} \rightleftharpoons \underset{\text{C. Acid}_2}{H_3O^+} + \underset{\text{C. Base}_1}{Cl^-} $$
- $HCl$ loses $H^+$ $\to$ $Cl^-$ (Conjugate Base).
- $H_2O$ gains $H^+$ $\to$ $H_3O^+$ (Conjugate Acid).
Amphoteric Substances: Can act as both acid and base (e.g., $H_2O, HCO_3^-$).
3. Lewis Concept (1923)
The most comprehensive definition, based on electron transfer. It explains reactions without H or OH.
- Lewis Acid: Electron Pair Acceptor (Electrophile).
- Lewis Base: Electron Pair Donor (Nucleophile).
Examples:
- Lewis Acids:
- Electron deficient molecules: $BF_3, AlCl_3$ (Incomplete octet).
- Simple Cations: $Ag^+, H^+, Cu^{2+}$.
- Molecules with multiple bonds to electronegative atoms: $CO_2, SO_3$.
- Lewis Bases:
- Molecules with Lone Pairs: $:NH_3, H_2\ddot{O}:, R-\ddot{O}-H$.
- Simple Anions: $Cl^-, OH^-, CN^-$.
Reaction: $H_3N: \rightarrow BF_3$ (Formation of Coordinate Bond).
4. Summary Comparison
| Concept | Acid Definition | Base Definition | Limitation |
|---|---|---|---|
| Arrhenius | Gives $H^+$ in water | Gives $OH^-$ in water | Aqueous only |
| Brønsted | $H^+$ Donor | $H^+$ Acceptor | Requires protons |
| Lewis | $e^-$ Pair Acceptor | $e^-$ Pair Donor | Explains all, including $BF_3$ |
Practice Quiz
Test your understanding of Acid-Base Theories.
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