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.
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, NH3, an base, accepts a proton from 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+.