Acids, Bases And Salts - Theories

Lowry - Bronsted theory of acids and bases

J.N. Bronsted and T.M. Lowry independently introduced a new concept to overcome the drawbacks of Arrhenius theory:
Species which has capacity to donate protons to other substance is called an acid. Species which has capacity to accept proton from any other substance is called a base. The process of
transfer of a proton from an acid to base is called neutralisation.
                \( HCl + H_2 OH_3 O^ + + Cl^ - \)  

What is the role played by water in the above reaction?
H₂O accepts a proton from HCl hence it is a base according to this theory and HCl is an acid.
              \( NH_3 + H_2 ONH_4 ^ + + OH^ - \)  

What is the role played by H₂O in the above reaction?
Water donates a proton to NH3 thus acting as an acid according to this theory.
The above reactions are reversible. H₃O⁺ and NH4⁺ donate proton to Cl^- and OH^-. So H₃O⁺ and NH₄⁺ are called acids and Cl^- and OH^- are called bases. Each pair made up of an acid and a base related by the transfer of proton is called conjugate acid base pair. A pair of Bronsted acid and a base differ by one proton.
                \( \mathop {HCl}\limits_{Acid_1 } + \mathop {H_2 O}\limits_{Base_2 } \mathop {H_3 O^ + }\limits_{Acid_2 } + \mathop {Cl^ - }\limits_{Base_1 } \)
HCl and Cl-; H₃O+ and H₂O are conjugate acid - base pairs. Have you realised a strong acid has a weak conjugate base and a strong base has a weak conjugate acid.

• Highlights of Conjugate Acid - Base Pairs 
These are the species which differ by one H⁺ ion.
Conjugate acid are the species fromed after adding one H⁺ to a base or removal of one OH^– 

Ex. NH₃ + H⁺  \( \to \)   NH₄⁺ 
       base                   Conjugate acid  
Ex.     NaOH   \( \to \)     Na⁺ + OH^–
         base                  Conjugate acid 

Conjugate base are the species fromed after removal of one H⁺ ion from an acid
Ex. HCl \( \to \) H⁺ +  Cl^– 
        acid     Conjugate base 
Ex. H₂SO₄   \( \to \)  H⁺  +  HSO₄^– 
         acid            Conjugate base 

In a typical acid base reaction
            HX + B  \( \rightleftharpoons \) X^– + HB⁺   



Forward reaction    -   Here HX being a proton donor is an acid
                                   B being a proton acceptor is a base.
 
Backward reaction  -   Here HB⁺ being a proton donor is an acid
                                     X^– being a proton acceptor is a base.