Introduced in 1884 by Swedish chemist Svante Arrhenius, the Arrhenius theory was the first modern theoretical definition of acids and bases. It linked chemical behavior directly to the presence of specific ions formed when a substance dissolves in water.
Core Definitions
- Arrhenius Acid: A substance that dissociates in water to increase the concentration of hydrogen ions (H+).
- Arrhenius Base: A substance that dissociates in water to increase the concentration of hydroxide ions (OH-).
The Role of Aqueous Solutions (Water)
A fundamental requirement of the Arrhenius theory is that the substance must be dissolved in water. The theory relies entirely on the process of electrolytic dissociation—how compounds break apart into free-moving ions in an aqueous environment.
Neutralization According to Arrhenius
Neutralization is strictly defined as the reaction between hydrogen ions from an acid and hydroxide ions from a base to produce water.
H+ (aq) + OH- (aq) → H2O (l)
Example: Hydrochloric Acid and Sodium Hydroxide
When hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) in water, they fully dissociate into their respective ions to form water and a salt.
| Substance | Classification | Behavior in Water |
|---|---|---|
| HCl | Arrhenius Acid | Dissociates into H+ and Cl- ions. |
| NaOH | Arrhenius Base | Dissociates into Na+ and OH- ions. |
| NaCl | Salt | Ionic byproduct formed from the remaining spectator ions (Na+ and Cl-). |
Limitations of the Arrhenius Theory
While groundbreaking for its time, the Arrhenius theory had significant limitations that eventually led to the development of the broader Brønsted–Lowry and Lewis theories:
- Restricted to Water: It only applies to reactions occurring in aqueous solutions. It cannot account for acid-base behavior in gases or non-aqueous solvents (like liquid ammonia or benzene).
- Excludes Certain Bases: It cannot explain why compounds like ammonia (NH3) or sodium carbonate (Na2CO3) display basic properties and turn litmus paper blue, despite lacking a hydroxide group (OH) in their formulas.
- The Reality of the H+ Ion: Free H+ ions (bare protons) cannot exist stably in water. Instead, they immediately bond with water molecules to form hydronium ions (H3O+).
Related topics
Lux-Flood Theory of Acids and Bases
Brønsted–Lowry Theory of Acids and Bases
Lewis Theory of Acids and Bases
Hard Soft Acid Base Principle