Arrhenius concept of acids and bases:
In 1884, Ar Svante Arrhenius, a Swedish chemist proposed a new theory of acids and bases known as Arrhenius concept of acids and bases.
According to Arrhenius concept:
An acid is a substance which dissociates in an aqueous solution to give hydrogen ions. The acidic properties of a substance in aqueous solution are due to the presence of hydrogen ions i.e. H+ (aq). Acids like HCl, H2SO4, HNO3 etc are strong acids as they are almost completely ionized in aqueous solution while the acids like HCN, CH3COOH are weak acids as they are weakly ionized in aqueous solution. Hence the ionization of an acid, HX in water can be represented by the equation:
HX (aq) ————> H+ (aq) + X– (aq)
HX (aq) + H2O (l) ————-> H3O+ (aq) + X– (aq)
According to Arrhenius concept:
A base is a substance which dissociates in an aqueous solution to give hydroxyl ions. The basic properties of a substance in aqueous solution are due to the presence of hydroxyl ions i.e. OH– (aq). Bases like NaOH, KOH etc are strong bases as they are almost completely ionized in aqueous solution while the bases like NH4OH, Ca(OH)2 are weak bases as they are weakly ionized in aqueous solution. Hence the ionization of a base, MOH can be represented by the equation:
MOH (aq) —————> M+ (aq) + OH–
Hence we can conclude that according to Arrhenius concept, acids give H+ ions and bases give OH– ions in water.
The strength of an acid or a base depends upon the degree of ionization to give H+ ions or OH– ions respectively.
Greater the number of H+ ions or OH– ions given by an acid or base in water, greater will be the strength of the acid or base.
