Question

Discuss the Lowry – Bronsted concept of acids and bases.

Answer 1

According to Lowry – Bronsted concept, an acid is defined as a substance that has a tendency to donate a proton to another substance and a base is a substance that has a tendency to accept a proton from other substances. When hydrogen chloride is dissolved in water, it donates a proton to the latter. Thus, HCl behaves as acid and H₂O is the base. The proton transfer from the acid to the base can be represented as

\[\ce{HCl + H2O ⇌ H3O^+ + Cl^-}\]

When ammonia is dissolved in water, it accepts a proton from water. In. this case, ammonia (NH₃) acts as a base and H₂O is acid. The reaction is represented as

\[\ce{H2O + NH3 ⇌ NH^+_4 + OH^-}\]

Let us consider the reverse reaction in the following equilibrium

\[\ce{\underset{Proton donar (acid)}{HCl} + \underset{Proton acceptor (Base)}{H2O} ⇌ \underset{Proton donar (acid)}{H3O^+} + \underset{Proton acceptor (Base)}{Cl^-}}\]

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

\[\ce{Acid1 + Base2 ⇌ Acid2 + Base1}\]

The species that remains after the donation of a proton is a base (Base₁)and is called the conjugate base of the Bronsted acid (Acid₁). In other words, chemical species that differ only by a proton are called conjugate acid – base pairs.

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 the conjugate acid of Cl^– Similarly H₃O is the conjugate acid of H₂O. Limitations of Lowry – Bronsted theory. Substances like BF₃, AICl_3, etc., that do not donate protons are known to behave as acids.