Arrhenious Concept Of Acids And Bases-Overview

ARRHENIUS CONCEPT

In 1884, Swedish chemist  Svante Arrhenius proposed a concept to classify acids and bases

ACID:

A substance that dissociates in an aqueous solution to give hydrogen ions.

\[HA_{(aq)}\; \rightleftharpoons \; H_{(aq)}^{+} \; + \; A_{(aq)}^{-}\]

For examples:
\[HCl\;,\;HNO_{3}\;,\;CH_{3}COOH\;,\;HCN\;,\;HBr\;,\;HI\;,\;H_{2}SO_{4}\;,\;H_{3}PO_{4}\]

Why H⁺ is released?

In all these compounds, H is bounded to more electronegative species, so when the compound dissociates in water, the electronegative species is released as anion and hydrogen as proton H⁺.
For example:
\[HCl \; \rightleftharpoons\; H^{+}\; +\; Cl^{-}\]
\[H_{3}PO_{4} \; \rightleftharpoons\; H^{+}\; +\; H_{2}PO_{4}^{-}\]
\[HClO_{4} \; \rightleftharpoons\; H^{+}\; +\; ClO_{4}^{-}\]

BASE:

A substance that dissociates in an aqueous solution to give hydroxide ions.
\[BOH_{(aq)}\; ^{H_{2}O}\rightleftharpoons \; B_{(aq)}^{+} \; + \; OH_{(aq)}^{-}\]
For example:
\[NaOH\;,\;NH_{4}OH\;,\;Ca(OH)_{2}\;,\;KOH\;,\;etc\]

EXAMPLE:

HCl is acid because it produces H⁺ ions in an aqueous solution.
\[HCl_{aq}\; \rightleftharpoons \; H_{(aq)}^{+}\; +\; Cl_{(aq)}^{-}\]
NaOH is base because it produce OH^- in aqueous solution.
\[NaOH_{aq}\; \rightleftharpoons \; Na_{(aq)}^{+}\; +\; OH_{(aq)}^{-}\]

Note:

Indeed, the properties of the aqueous solution of acids, such as sour taste, are due to H⁺(aq), whereas properties of the aqueous solution of bases are due to OH^-(aq).

Arrhenius Concept: Modern Statement:

ACID:

A substance that increases the concentration of  H⁺ ions when dissolved in water.

BASE:

A substance that increases the concentration of  OH^- ions when dissolved in water.

Limitations of Arrhenius concept:

1. This concept is only applicable in an aqueous medium and refers to the concentration of solvated ions, it does not explain the nature of acids and bases in a non-aqueous medium.
2. Free H⁺ and  OH^- ions do not exist in water rather they are in association with water molecules to form complex ions. Thus H+ forms a hydronium ion.
3. Arrhenius's concept does not explain the reactions in the gas phase where no solvent is present.
   
4. According to this concept, acids and bases are only those compounds that contain hydrogen and hydroxyl ions respectively. It cannot explain the nature of compounds like CO₂ and NH₃ which are acid and base respectively.
5. Not all compounds having an OH group act as bases. For example alcohols like phenol, methyl alcohol.