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

$$\mathrm{HCl}+\mathrm{H}{\phantom{A}}_2\mathrm{O}\stackrel{-\rightharpoonup }{\leftharpoondown -}\mathrm{H}{\phantom{A}}_3\mathrm{O}{\phantom{A}}^{+}+\mathrm{Cl}{\phantom{A}}^{-}$$

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

$$\mathrm{H}{\phantom{A}}_2\mathrm{O}+\mathrm{NH}{\phantom{A}}_3\stackrel{-\rightharpoonup }{\leftharpoondown -}\mathrm{NH}{\phantom{A}}_4^{+}+\mathrm{OH}{\phantom{A}}^{-}$$

Let us consider the reverse reaction in the following equilibrium

$$\underset{\mathrm{Proton}\mathrm{donar}(\mathrm{acid})}{\mathrm{HCl}}+\underset{\mathrm{Proton}\mathrm{acceptor}(\mathrm{Base})}{\mathrm{H}{\phantom{A}}_2\mathrm{O}}\stackrel{-\rightharpoonup }{\leftharpoondown -}\underset{\mathrm{Proton}\mathrm{donar}(\mathrm{acid})}{\mathrm{H}{\phantom{A}}_3\mathrm{O}{\phantom{A}}^{+}}+\underset{\mathrm{Proton}\mathrm{acceptor}(\mathrm{Base})}{\mathrm{Cl}{\phantom{A}}^{-}}$$

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

$$\mathrm{Acid}{\phantom{A}}_1+\mathrm{Base}{\phantom{A}}_2\stackrel{-\rightharpoonup }{\leftharpoondown -}\mathrm{Acid}{\phantom{A}}_2+\mathrm{Base}{\phantom{A}}_1$$

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.