# 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+.