Three important concepts are :
(1) According to Arrhenius concept of acid and bases, an acid is a substance which gives H+ ions in the aqueous solution whereas a base is a substance which gives OH- ions in the aqueous solution.
(2) According to Bronsted-Lowry concept of acids and bases, an acid is a substance which can give a proton and a base is a substance which accepts a proton. A substance which can act both as an acid a well as base in different reactions is called amphoteric. An acid-base pair which differs by a proton is called conjugate acid-base pair.
Conjugate acid « Conjugate base +H+
A few examples of the conjugate acid-base pairs in reactions are given below :
Thus H2O acts both as an acid as well as base. Hence it is amphoteric or amphiprotic.
(3) According to Lewis concept of acid and bases, an acid is a substance which can accept a pair of electrons whereas a base is a substance which can donate a pair of electrons.
Types of Lewis bases – Neutral molecules containing a lone pair of electrons on the central atom like : NH3 , ROH , H2 O : etc. and all negative ions like
F- , Cl- , Br- , I- , OH- , CN- etc.
Types of Lewis acids – (i) Molecules having central atom with incomplete octet e.g. BF3, AlCl3 etc.
(ii) Simple cations e.g. Ag+ , Cu2+ , Fe3+ etc.
(iii) Molecules having central atom with empty d-orbitals e.g. SnCl4 , SiF4 , PF5
(iv) Molecules containing multiple bonds between different atoms e.g. O = C = O.
Note : q Arrhenius theory is valid only for aqueous solutions.
q In simple compounds, the metal atoms (or ion) act as lewis acid while the ligands act as lewis bases.
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