Introduced independently in 1923 by Danish chemist Johannes Nicolaus Brønsted and English chemist Thomas Martin Lowry, the Brønsted–Lowry theory fundamentally expanded our understanding of acids and bases beyond the limitations of the Arrhenius definition.
Core Definitions
- Brønsted–Lowry Acid: A substance that can donate a proton (hydrogen ion, H+) to another substance. (Proton Donor)
- Brønsted–Lowry Base: A substance that can accept a proton (hydrogen ion, H+) from another substance. (Proton Acceptor)
The Role of Protons
In this theory, the behavior of acids and bases is entirely centered on the transfer of a proton (H+). Because a hydrogen atom consists of just one proton and one electron, losing that electron to become an H+ ion means it is literally just a bare proton.
Conjugate Acid-Base Pairs
A key feature of the Brønsted–Lowry theory is the concept of conjugate acid-base pairs. When an acid loses a proton, it forms its conjugate base. Conversely, when a base accepts a proton, it forms its conjugate acid.
Acid + Base ↔ Conjugate Base + Conjugate Acid
Example: Hydrochloric Acid and Water
When hydrochloric acid (HCl) dissolves in water, the HCl molecule donates a proton to a water molecule.
| Reactant/Product | Role | Description |
|---|---|---|
| HCl | Brønsted–Lowry Acid | Donates an H+ ion. |
| H2O | Brønsted–Lowry Base | Accepts the H+ ion. |
| Cl- | Conjugate Base | What remains after HCl donates its proton. |
| H3O+ (Hydronium) | Conjugate Acid | What forms after H2O accepts the proton. |
Amphoteric Substances
One of the major advantages of this theory is its ability to explain amphoteric (or amphiprotic) substances—substances that can act as either an acid or a base depending on the reaction environment.
Water (H2O) is the most common example:
- Acting as a Base: In the HCl example above, water accepts a proton.
- Acting as an Acid: When reacting with ammonia (NH3), water donates a proton:
NH3 (aq) + H2O (l) ↔ NH4+ (aq) + OH- (aq)Here, H2O is the acid and NH3 is the base.
Why it Surpassed the Arrhenius Theory
- Not Limited to Water: The Arrhenius theory required acids and bases to be dissolved in aqueous solutions. The Brønsted–Lowry theory applies to reactions in gas phases, non-aqueous solvents, and solutions.
- Broader Definition of Bases: Arrhenius bases had to produce hydroxide ions (OH-) in solution. The Brønsted–Lowry definition explains why substances like ammonia (NH3) act as bases even though they do not contain OH group in their formulas.
Related topics
Lux-Flood Theory of Acids and Bases
Arrhenius Theory of Acids and Bases
Lewis Theory of Acids and Bases
Hard Soft Acid Base Principle