# What is the need for so many theories of acids and bases?

What is the need for the Brønsted-Lowry theory and the Lewis theory of acids and bases?Is there any problem with the Arrhenius theory?

## 1 Answer

First came the Arrhenius theory. It defines an acid as a species that dissociates to produce $\ce{H+}$ in solution, and a base as a species that dissociates to produce $\ce{OH-}$ in solution. (Examples: $\ce{HCl}$, $\ce{NaOH}$)

Next came the Brønsted-Lowry theory, which expands the notion of acid and base. It defines an acid as a species that loses $\ce{H+}$ in a reaction, and a base as a species that gains $\ce{H+}$ in a reaction. All Arrhenius acids and bases are Brønsted-Lowry acids and bases; hence the Brønsted-Lowry theory generalizes the concepts of Arrhenius acids and bases and allows the application of these concepts to a larger range of species. (Examples of "new" acids and bases under this theory: $\ce{NH4+}$, $\ce{CH3COO-}$)

Finally came the Lewis theory, which further expands the notion of acid and base. It defines an acid as a species that accepts an electron pair, and a base as a species that donates an electron pair. All Brønsted-Lowry acids and bases are Lewis acids and bases; hence the Lewis theory generalizes the concepts of Brønsted-Lowry acids and bases and allows the application of these concepts to a larger range of species. (Examples of "new" acids and bases under this theory: $\ce{Li+}$, $\ce{CO}$)

Remark: Hopefully the repetition and structure within my answer has been suggestive. The benefits of generalization are obvious: the theory can explain more, and more things can be explained; this is the natural progression of science.

• Nice post, but I'd point out that by working up from the Arrhenius theory through the Brønsted-Lowry theory to the Lewis theory one develops a good knowledge of the subtleties of chemistry. Like in math you don't need a 10 pound theorem to prove a two pound problem. – MaxW Sep 3 '16 at 22:50
• Shouldn't "solution" in Arrhenius theory be qualified by "aqueous" or is it that the dissociation can take place in any solution? – MrAP Aug 10 '17 at 16:16
• @MrAP, yes, an aqueous solution is implied. – a-cyclohexane-molecule Aug 10 '17 at 17:37