Bronsted – Lowry Theory The Proton – donor – Acceptor System

Bronsted and Lowry in 1923 independently proposed a more general definition of acids and bases. According to them, an acid is defined as any hydrogen containing material (a molecule or a cation or an anion) that can release a proton (H+) to any other substance, whereas a base is any substance (a molecule or a cation or an anion) that can accept a proton from any other substance. In short, an acid is a proton -donor and a base is a proton – acceptor.

Conjugate Acid – Base Pairs :

Consider a reaction

Acid1    Base2     Acid2     Base1

H2O + NH3 <=> H3O+  OH

In this reaction HCl donates a proton to H2O and is, therefore an acid. Water, on the other hand, accepts a proton from HCl, and is, therefore, a base. In the reverse reaction which at equilibrium proceeds at the same rate as the forward reaction, the H3O+ ions donates a proton to Cl ion, hence H3O+, ion is an acid. Cl ion, because it accepts a proton from H3O+ ion, is a base. Acid base pairs such as.

the members of which can be formed from each other mutually by the gain or loss of proton are called conjugate acid – base pairs.

If in the above reaction, the acid HCl is labelled Acid1 and its conjugate base viz. Cl as Base1 and further, if H2O is designated Base2 and its conjugate acid viz. H3O+ as Acid2, the equilibrium can be represented by a general equation.

This is the fundamental equation representing the relationship between an acid and a base on the basis of Bronsted concept. Thus on the basis of this concept Acid1 and Base1 form one conjugate acid-base pair and Acid2 and Base2 form another conjugate acid-base pair

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