] . We use both H and H O + to mean the same. Similarly a base is a substance that dissociates to give hydroxyl ions in water. For example, substances like NaOH, Ca(OH) etc., are bases.
Ca(OH) Ca (aq)+2OH (aq) H O + ⇀ ↽ XII U8-Ionic XII U8-Ionic - - - - Limitations of Arrhenius concept i. Arrhenius theory does not explain the behaviour of acids and bases in non aqueous solvents such as acetone, Tetrahydrofuran etc... ii. This theory does not account for the basic nature of the substances like ammonia ( NH ) which do not possess hydroxyl group.
Evaluate yourself – Classify the following as acid (or) base using Arrhenius concept i)HNO ii) Ba(OH) iii) H PO iv) CH COOH . . Lowry – Bronsted Theory (Proton Theory) In , Lowry and Bronsted suggested a more general definition of acids and bases. According to their 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.
In other words, an acid is a proton donor and a base is a proton acceptor. When hydrogen chloride is dissolved in water, it donates a proton to the later. Thus, HCl behaves as an acid and H O is base. The proton transfer from the acid to base can be represented as HCl+H O H O +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 H O+NH NH +OH Let us consider the reverse reaction in the following equilibrium HCl H O H O Proton donor (acid) Proton acceptor (Base) Pr oton donor (acid) Proton acceptor (Base) Cl - H O + donates a proton to Cl - to form HCl i.e., the products also behave as acid and base. In general, Lowry – Bronsted (acid – base) reaction is represented as Acid +Base Acid +Base 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