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Lewis Acids and Bases

Although Bronsted-Lowry theory was more general than Arrhenius theory of acids and bases but it failed to explain the acid base reactions which do not involve transfer of proton.

For example, it fails to explain how acidic oxides such as anhydrous CO2, SO2, SO3, etc., can neutralize basic oxides such as CaO, BaO, etc., even in the absence of solvent.

G.N. Lewis (1923) proposed broader and more general definitions of acids and bases, which do not require the presence of protons to explain the acid base behaviour. According to Lewis concept.

An acid is a substance which can accept a pair of electrons.

A base is a substance which can donate a pair of electrons.

Acid-base reactions according to this concept involve donation of electron pair by a base to an acid to form a co-ordinate bond. Lewis bases can be neutral molecules such  as

  etc., having one or more unshared pairs of electrons, or anions such as CN-, OH-, CI-, etc.

Lewis acids are the species having vacant orbitals in the valence shell of one of its atoms. The following species can act as Lewis acids.

(a) Molecules having an atom with incomplete octet. For example, BF3 and A1Cl3

(b) Simple cations. For example, H+, Ag+, etc.

(c) Molecules in which central atom has vacant orbitals and may acquire more than an octet of valance electrons. For example, SiF4.

(d) Molecules containing multiple bonds. For example, CO2, SO2, etc.

LIMITATIONS OF LEWIS CONCEPT

Though Lewis concept of acids and bases is more general than the Arrhenius as well as Bronsted concepts, yet it has several drawbacks as discussed below:

  1. It is too general and includes all the co-ordination compounds and co-ordination reactions.
  2.  It does not help to assign the relative strengths of acids and bases.
  3. It does not explain the behaviour of protonic acids such as HCl, H2SO4, etc., which do not form coordinate bond with bases which is the primary requirement of Lewis theory.
  4. The catalytic activity of many acids is due to H+(aq) ion. Since a Lewis acid need not contain hydrogen, many Lewis acids will not possess catalytic property.
  5. Normally, formation of co-ordination compounds is slow. Therefore, acid-base reactions should also be slow, but in actual practice, acid-base reactions are extremely fast.