According to Lowry – Bronsted concept, an acid is defined as a substance that has a tendency to donate a proton to another substance and base is a substance that has a tendency to accept a proton from other substance. When hydrogen chloride is dissolved in water, it donates a proton to the later. Thus, HCl behaves as an acid and H2O is base. The proton transfer from the acid to base can be represented as
HCl + H2O ⇌ H3O+ + Cl-
When ammonia is dissolved in water, it accepts a proton from water. In. this case, ammonia (NH ) acts as a base and H O is acid. The reaction is represented as
H2O + NH3 ⇌ NH4+ + OH-
Let us consider the reverse reaction in the following equilibrium
H3O+ donates a proton to Cl- to form HCI i.e., the products also behave as acid and base. In general, Lowry – Bronsted (acid – base) reaction is represented as
Acid1 + Base2 ⇌ Acid2 + Base1
The species that remains after the donation of a proton is a base (Base )and is called the conjugate base of the Bronsted acid (Acid ). In other words, chemical species that differ only by a proton are called conjugate acid – base pairs. Conjugate acid – base pair
HCl and Cl- , H2O and H3O are two conjugate acid – base pairs. i.e., Cl- is the conjugate base of the acid HCl (or) HCl is conjugate acid of Cl- Similarly H3O is the conjugate acid of H2O . Limitations of Lowry – Bronsted theory. Substances like BF3 , AICl3 etc., that do not donate protons are known to behave as acids.
