7.10.1 Arrhenius Concept of Acids and Bases

According to Arrhenius theory, acids are substances that dissociates in water to give hydrogen ions H+(aq) and bases are substances that produce hydroxyl ions OH-(aq). The ionization of an acid HX (aq) can be represented by the following equations:

HX (aq) à H+(aq) + X-(aq)


HX (aq) + H2O(l) à H3O+(aq) + X-(aq)

A bare proton, H+  is very reactive and cannot exist freely in aqueous solutions. Thus, it bonds to the oxygen atom of a solvent water molecule to give trigonal pyramidal hydronium ion, H3O+{[H (H2O)]+} (see box). In this chapter we shall use H+(aq) and H3O+(aq) interchangeably to mean the same i.e., a hydrated proton.

Similarly, a base molecule like MOH ionizes in aqueous solution according to the equation:

MOH(aq) à  M+(aq) + OH-(aq)

The hydroxyl ion also exists in the hydrated form in the aqueous solution. Arrhenius concept of acid and base, however, suffers from the limitation of being applicable only to aqueous solutions and also, does not account for the basicity of substances like, ammonia which do not possess a hydroxyl group.



Hydronium and Hydroxyl Ions

Hydrogen ion by itself is a bare proton with very small size (~10-15 m radius) and intense electric field, binds itself with the water molecule at one of the two available lone pairs on it giving H3O+.

This species has been detected in many compounds (e.g., H3O+Cl-) in the solid state. In aqueous solution the hydronium ion is further hydrated to give species like H5O2+, H7O3+ and H9O4+.

Similarly, the hydroxyl ion is hydrated to give several ionic species like H3O2-, H5O3-and H7O4- etc.

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