The Concepts of Acids and Bases

The concepts of acids and bases have been evolving since the seventeenth century. The word “acid” is an English word which means sourness. This was given in the seventeenth century to certain aqueous solutions in consequence of their sour taste. Bases were regarded essentially as antiacids, that is, substances that neutralize acids. Then acids were defined in terms of their characteristic properties in aqueous solution.

Hence, an acid was regarded as a substance whose aqueous solution turns blue litmus red, tastes sour, reacts with active metals to liberate hydrogen, and loses these properties on contact with alkalis. This type of definition is known as classical concept or definition.

By this definition, certain oxides, e.g., CO₂ and SO₃, whose aqueous solutions possess the properties characteristic of solutions of acids were called acids. Later on, a chemist by name Lavoisier attempted to shift the emphasis from properties to chemical composition. He stated that all acids must be made up of oxygen. This idea had to be abandoned after another chemist, Davy, showed that some acids, e.g., HCl do not contain oxygen.

In modern times, the definitions of acids and bases, which are considered true are those given independently by Arrhenius, BrØnsted-Lowry, and Lewis. We shall now consider them individually.

The Arrhenius Concept of Acids

The Arrhenius theory of ionization attributed the characteristic properties of aqueous solutions of acids to the hydrogen ion, H⁺. Thus, an acid was defined as a compound containing hydrogen atoms which can become hydrogen ions when the acid is dissolved in water.

The Arrhenius definition does not include such compounds as CO₂ and SO₃. These oxides are classified as acidic oxides, but not as acids, because they react with water to produce H₂CO₃ and H₂SO₄, which are acids by the Arrhenius definition. Arrhenius assumed that the excess hydrogen ions present in an aqueous solution of an acid are formed by the simple dissociation of some of the acid molecules into ions.

Thus, in the case of hydrogen chloride, it was assumed that some of the HCl molecules dissociate into positive hydrogen ions and negative chloride ions and that these ions exist in the solution in equilibrium with undissociated HCl molecules.

HCl(aq)

H⁺(aq)

Cl^-(aq)

The hydrogen ion, H⁺, is unique among ions - it contains no electrons at all. In fact, it is nothing more than a proton, and its radius is only about 10^-13 cm compared with 10^-8 cm for other simple ions. This means that the hydrogen ion has an extraordinarily high ratio of charge to radius.

Thus, H⁺ is hydrated in aqueous solution, where it is surrounded by polar molecules of H₂O that have unshared electron pairs. It is therefore appropriate to represent a hydrogen ion in aqueous solution as H₃O+, i.e., [H(H₂O)]⁺, rather than simply as H⁺. H⁺(aq) + H₂O(l) → H₃O+(aq)

Since water molecules are associated with one another by hydrogen bonding, each proton is actually hydrated with a variable number of molecules of water. In addition to H₃O⁺ ions, aqueous solutions of acids contain H₅O₂⁺ ions, H₇O₃⁺ ions, etc., and their relative numbers vary with concentration and temperature. The formula H₃O⁺ is used as a convenience to denote that the hydrogen ion is hydrated.

The Arrhenius concept of acids is essentially correct for aqueous solutions if we attribute the characteristic properties of acids to the hydronium ion, H₃O⁺ , rather than the unhydrated proton, H⁺.

Note: the role of the solvent (water) is to (1) provide the dielectric medium which reduces the mutual attraction of oppositely charged ions so that they can exist as separate particles in the solution, and (2) to hydrate the hydrogen ion (this is a chemical reaction)

HCl(g) + H₂O(l)

H₃O⁺(aq)

Cl^-(aq)

Acids can be classified as inorganic, e.g. aqueous H₂SO₄, HCl, and HNO₃; and organic – some of these are also naturally occurring, e.g. Lactic acid (found in soured milk); citric acid (found in fruits such as lime and lemon); acetic or ethanoic acid (present in vinegar); tartaric acid (found in grape fruits); amino acids (found in proteins); ascorbic acid (also called vitamin C - found in orange fruits); and fatty acids (found in fats and oils).