Acids and Bases : Lewis concept of acids and bases

Certain reactions have characteristics of acid-base reactions but do not fit the Bronsted-Lowry concept.An example is the reaction of the basic oxide Na2O with the acidic oxide SO3 to give salt Na2SO4

                    Na2O + SO3 ----------> Na2SO4(s)
G.N lewis who proposed the theory of covalent bonding ,realized that the concept of acids and bases colud be generalized to include  reactions of acidic and basic oxides and many other reactions as well as proton transfer reactions. 

In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons.

A Lewis acid is therefore any substance, such as the H+ ion, that can accept a pair of nonbonding electrons.n other words, a Lewis acid is an electron-pair acceptor

A Lewis base is any substance, such as the OH- ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor.

For example: consider the reaction between the NH3 and HCl in aqueous solution,it consist of reaction of  a proton from H3O+ with NH3

H              +         : NH3  ---------->  NH4+

electron pair          electron pair

accepted                   donor

here the proton accepting an electron pair from NH3 and an H-N bond being formed.The proton is an electron -pair acceptor ,so it is a lewis acid.NH3 which has a lone pair of electron is an electron pair donor and therefore a lewis base.

 Lets see a solved example :

Sample Problem : In the following reaction ,identify the lewis acid and lewis base

Ag +  2 NH3 <--------> Ag(NH3)2 +

Solution:

Step 1 : Look at the lewis structure of NH3, it carres a lone pair of electrons.

: NH3  

Step 2: In the Ag (NH3)2 + silver ion is forming a complex with two NH3 molecules, which means that each NH3 molecule has donates a pair of electrons to Ag+ to form bond with each NH3 molecule.

Ag +               +         2 :NH3    <------------> Ag (:NH3)2 +

Lewis acid        Lewis base

Step 3 : It means that Ag+ is acting as lewis acid by accepting pair of electron from NH3 and NH3 is acting as base by donating a pair of electrons.

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