Question

State the Arrhenius concept of acids and bases and discuss the limitations.

Answer 1

Arrhenius theory: A Swedish chemist Svante Arrhenius defined acids and bases as;

Acid: An acid is a substance that furnishes Hydrogen ions as the only positive ion when dissolve in water.

• For example, $\mathrm{HCl}$(Hydrochloric acid) when dissolved in water furnishes Hydrogen ions.

$$\begin{gathered} \mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right) \\ (\mathrm{Hydrochloric}\mathrm{acid})(\mathrm{Hydrogen}\mathrm{ion})(\mathrm{Chloride}\mathrm{ion}) \end{gathered}$$.

• Hydrogen ion further combines with a water molecule to give Hydronium ion.

$$\begin{gathered} {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}_3{\mathrm{O}}^{+}\left(\mathrm{aq}\right) \\ (\mathrm{Hydrogen}\mathrm{ion})\left(\mathrm{Water}\right)(\mathrm{Hydronium}\mathrm{ion}) \end{gathered}$$

Base: A base is a substance that furnishes Hydroxyl ions when dissolved in water.

• For example:$\mathrm{NaOH}$(Sodium hydroxide) when dissolved in water furnishes hydroxide $\left({\mathrm{OH}}^{-}\right)$ ion.

$$\begin{gathered} \mathrm{NaOH}\left(\mathrm{aq}\right)\to {\mathrm{Na}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right) \\ (\mathrm{Sodium}\mathrm{hydroxide})(\mathrm{Sodium}\mathrm{ion})(\mathrm{Hydroxyl}\mathrm{ion}) \end{gathered}$$

Limitations of the Arrhenius Theory:

• In non-aqueous solutions, the theory fails to explain the acidic or basic nature of the substance.
• It fails to explain the acidic or basic nature of a substance that does not contains ${\mathrm{H}}^{+}\mathrm{or}{\mathrm{OH}}^{-}$ions.
• Example: $\mathrm{Sulphur}\mathrm{dioxide}\left({\mathrm{SO}}_2\right),\mathrm{Carbon}\mathrm{dioxide}\left({\mathrm{CO}}_2\right),\mathrm{Sulphur}\mathrm{trioxide}\left({\mathrm{SO}}_3\right),\mathrm{Ammonia}\left({\mathrm{NH}}_3\right)$