There are several so-called theories of acids and bases, but they are not really theories but merely different definitions of what we choose to call an acid or a base. Since it is only a matter of definition, no theory is more right or wrong than any other, and we use the most convenient theory for a particular chemical situation. So before we talk of strength of acids and bases, we need to know several theories.
Modern Concepts of Acids and Bases :
Following are the important modern concept of acids and bases:
Arrhenius Concept – The Water Ion System
According to this concept, an acid is any hydrogen containing compound which gives H+ ions in aqueous solution and a base which gives OH– ions in aqueous solution. The HCl is an acid and NaOH is a base and the neutralisation process can be represented by a reaction involving the combination of H+ and OH– ions to form H2O.
NaOH <=> Na+ + OH–
H+ + OH– –> H2O
(i) Since the reaction representing neutralisation process involves the combination of H+ and OH– ions, the approximately constant molar heat of neutralisation would be expected. Thus the constant heat of neutralisation of a strong acid by a strong base is readily understandable in terms of this concept.
(ii) This concept has offered a means of correlating catalytic behaviour with the concentration of the H+ion.
(i) The acid or base property of a substance was not inherent in it, but was dependent on the presence of water. According to this concept, HCl is regarded as an acid only when dissolved in H2O and not in some other solvent such as C6H6 or when it exists in the gaseous form.
(ii) The neutralisation process is limited to those reactions which can occur in aqueous solution only, although reactions involving salt formation do occur in many other solvents and even in the absence of solvents.
(iii) It cannot explain the acidic character of certain salts such as AlCl3 in aqueous solution.