Acid/Base Definitions


There are multiple definitions for acids and bases. In this course, three definitions are covered.

  • Arrhenius Acid/Base
  • Brønsted-Lowry Acid/base
  • Lewis Acid/Base

The following figure illustrates the relationship between these definitions.

Venn diagram of acid/base definitions.

I will be using the following notation and colors to denote an acid or a base throughout.

  • \(\textcolor{green}{\mathrm{HA}}/\textcolor{green}{\mathrm{HB^+}}/\mathrm{H_3O^+}\) = acid
  • \(\textcolor{red}{\mathrm{B}}/\textcolor{red}{\mathrm{A^-}}/\mathrm{OH^-}\) = base


Arrhenius Acid/Base


An Arrhenius acid is is a substance that dissociates in water to produce H+ ions.

\[ \textcolor{green}{\mathrm{HCl}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^+}(aq) + \textcolor{red}{\mathrm{Cl^-}}(aq)\]

or generally

\[ \textcolor{green}{\mathrm{HA}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^+}(aq) + \textcolor{red}{\mathrm{A^-}}(aq)\]

An Arrhenius base is is a substance that dissociates in water to produce OH ions.

\[ \textcolor{red}{\mathrm{NH_3}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{OH^-} + \textcolor{green}{\mathrm{NH_4^+}}(aq)\]

or generally

\[ \textcolor{red}{\mathrm{B}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{OH^-}(aq) + \textcolor{green}{\mathrm{HB^+}}(aq)\]


Bronsted-Lowry Acid/Base


A Brønsted-Lowry acid is a substance that donates a proton (i.e. is a proton donor).

\[ \textcolor{green}{\mathrm{HCl}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^+}(aq) + \textcolor{red}{\mathrm{Cl^-}}(aq)\]

Here, HCl, an acid, donates its proton to water. A general scheme can be written as

\[ \textcolor{green}{\mathrm{HA}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^+}(aq) + \textcolor{red}{\mathrm{A^-}}(aq)\]

A Brønsted-Lowry base is a substance that accepts a proton (i.e. is a proton acceptor).

\[ \textcolor{red}{\mathrm{NH_3}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{OH^-}(aq) + \textcolor{green}{\mathrm{NH_4^+}}(aq)\]

Here, HCl, an acid, donates its proton to water. A general scheme can be written as

\[ \textcolor{red}{\mathrm{B}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{OH^-}(aq) + \textcolor{green}{\mathrm{HB^+}}(aq)\] or

\[ \textcolor{red}{\mathrm{A^-}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{OH^-}(aq) + \textcolor{green}{\mathrm{HA}}(aq)\]


Lewis Acid/Base


A Lewis acid is a substance that accepts a pair of electrons (i.e. a lone-pair acceptor).

\[ \textcolor{green}{\mathrm{HCl}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^+}(aq) + \textcolor{red}{\mathrm{Cl^-}}(aq)\]

Here, HCl dissociates into H+ and Cl. The oxygen on water donates a lone pair of electrons to the free proton and forms a bond to give the hydronium ion, H3O+.

A Lewis base is a substance that donates a lone pair of electrons (i.e. lone-pair donor).

\[ \textcolor{red}{\mathrm{NH_3}}(aq) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{OH^–}(aq) + \textcolor{green}{\mathrm{NH_4^+}}(aq)\]

Here, the lone pair on the nitrogen in ammonia, NH3, is donated to a proton on water forming a bond to give NH4+.


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