The Lowery and Bronsted in 1923 give more general definition of Arrhenius concept of acids and bases. According to Bronsted-Lowery Concept,
An acid is a substance which can donate proton i.e. H+ while
A base is a substance which can accept a proton i.e. H+
Hence we can say that acid is a proton donor and base is a proton acceptor. For example in the following reaction, HCl loses a proton and is an acid whereas NH3 accepts a proton and hence it is a base.
HCl (aq) + NH3 (aq) ⇌ NH4+ (aq) + Cl– (aq)
The acid-base reactions are regarded as proton transfer reactions.
All the Arrhenius acids are also Bronsted acids but Arrhenius bases are not Bronsted bases. According to Arrhenius concept, an acid is a substance which can give H+ ions. According to Bronsted theory, an acid is substance which can donate a proton. Hence all Arrhenius acids are also Bronsted acids. However, there is a difference in the definition of base. According to Arrhenius concept, a base is a substance which can give OH– ions and therefore, a base must be a source of OH– ions in aqueous medium. But Bronsted theory requires a base to be a proton acceptor. Hence, Arrhenius base may not be a Bronsted base.
