) . In other words, chemical species that differ only

) . In other words, chemical species that differ only by a proton are called conjugate acid – base pairs. XII U8-Ionic XII U8-Ionic - - - - Proton donar (acid) Proton acceptor (Base) Proton acceptor (Base) Proton donar (acid) HCl H O H O Cl  Conjugate acid - base pair Conjugate acid - base pair HCl and Cl - , H O and H O + 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 H O + is the conjugate acid of H O Limitations of Lowry – Bronsted theory i. Substances like BF , AlCl etc., that do not donate protons are known to behave as acids. Evaluate yourself – Write a balanced equation for the dissociation of the following in water and identify the conjugate acid –base pairs. i) NH ii) H SO iii) CH COOH. . . Lewis concept In , Gilbert . N. Lewis proposed a more generalised concept of acids and bases. He considered the electron pair to define a species as an acid (or) a base. According to him, an acid is a species that accepts an electron pair while base is a species that donates an electron pair. We call such species as Lewis acids and bases. A Lewis acid is a positive ion (or) an electron deficient molecule and a Lewis base is a anion (or) neutral molecule with at least one lone pair of electrons. Les us consider the reaction between Boron tri fluoride and ammonia B F F F Boron tri fluoride NH ammonia