Acids and Bases:

The earliest criteria for the characterization of acids and bases were the experimentally observed properties of aqueous solutions. An acid* was defined as a substance whose water solution tastes sour, turns blue litmus red, neutralizes bases and so on. A substance was a base if its aqueous solution tasted bitter, turns red litmus blue, neutralizes acids and so on. Faraday termed acids, bases and salts as electrolytes and Liebig proposed that acids are compounds containing hydrogen that can be replaced by metals.

Different concepts have been put forth by different investigators to characterize acids and bases but the following are the three important modern concepts of acids and bases:

(1)  Arrhenius concept

 According to Arrhenius concept all substances which give H⁺ ions when dissolved in water are called acids while those which ionize in water to furnish OH^- ions are called bases.

            HA ↔  H⁺ + A^- (Acid)

            BOH ↔ B⁺ + OH^- (Base)

Thus, HCl is an acid because it gives H+ ions in water. similarly, NaOH is a base as it yields OH^- ions in water.

            HCl ↔ H⁺ + Cl^-

            NaOH ↔  Na⁺ + OH^-

Some acids and bases ionize completely in solutions and are called string acids and bases. Others are dissociated to a limited extent in solutions and are termed weak acids and bases. HCl, HNO₃, H₂SO₄, HCIO₄, etc., are examples of strong acids and NaOH, KOH, (CH₃)₄NOH are strong bases. Every hydrogen compound cannot be regarded as an acid, e.g., CH₄ is not an acid. Similarly, CH₃OH, C₂H₅OH, etc., have OH groups but they are not bases.

Actually free H+ ions do not exist in water. they combine with solvent molecules, i.e., have strong tendency to get hydrated.

                    HX + H₂O ↔ H₃O⁺ + X^-

                                                   (Hydronium ion)

The proton in aqueous solution is generally represented as H+ (aq). It is now known that almost all the ion are hydrated to more or less extent and it is customary to put (aq) after each ion.

The oxides of many non-metals react with water to form acids and are called acidic oxides or acid anhydrides.

            CO₂ + H₂O  H₂CO₃ ↔ 2H⁺(aq) +  (aq)

            N₂O₅ + H₂O  2HNO₃ ↔ 2H⁺(aq) +  (aq)

Many oxides of metals dissolve in water to form hydroxides. Such oxides are termed basic oxides.

Na₂O + H₂O →  2NaOH  ↔  2Na⁺(aq) + 2OH^- (aq)

The substance like NH₃ and N₂H₄ act as bases as they react with water to produce OH^- ions.

NH₃ + H₂O →  NH₄OH  ↔  NH⁺₄ (aq) + OH^- (aq)

The reaction between an acid and a base is termed neutralization. According to Arrhenius concept, the neutralization in aqueous solution involves the reaction between H⁺ and OH^- ions or hydronium and OH^-. This can be represented as

        H₃O⁺ + OH^- ↔  2H₂O