Q:25. Lewis concept of Acids and Bases:

Ans: The Arrhenius and Lowry- Bronsted concepts of acids and bases are limited to substances which contain protons. G.N Lewis (1923) proposed a more general and broader concept of acids and bases. According to this concept,.

Lewis acid: An acid is a substance (molecule or ion) which can accept a pair of electrons while a base is a substance (molecule or ion) which can donate a pair of electrons.

A             +             :B            →       A ← B

(Lewis acid)     (Lewis Base)

In Other words, acids are electron pair acceptors while bases are electron pair donors. Thus, it is evident that any substance which has an unshared pair of electrons can act as a Lewis base while a substance which has an empty orbital that can accommodate a pair of electrons acts as Lewis acid. In fact, there is a donation of a pair of electrons from Lewis base to Lewis acid to form a coordinate bond between the two.

Q:26. Write down the characteristics of Lewis acids and bases.

Ans: Lewis bases: According to Lewis concept, the following species can act as Lewis bases.

i. Neutral specie having at least one lone pair of electrons.

For example, ammonia, amines, alcohols etc. act as Lewis bases because they contain a pair or electrons:

:NH3,     -:NH,   R-O:-H

ii. Negatively charged species or anions:

For example, chloride, cyanide, hydroxide ions etc. act as Lewis bases:

CN, Cl, OHetc.

Lewis acids: According to Lewis concept, the following species can act as Lewis acids:

i. Molecules in which the central atom has incomplete octet:

For example, in BF3, AlCl3, FeCl3. The central atom has only six electrons around it as shown below:

ii. Simple cations can act as Lewis acids: All cations are expected to act as Lewis acids since they have deficiency in electrons. However, cations such as Na, K, Ca2+ ions etc. have very little tendency to accept electrons. While the cations like H+, Ag+ ion etc. have a greater electron acceptation tendency and therefore act as Lewis acids.

2NH3           +       Ag+              →     [H3N→ Ag ← NH3]

Lewis base        Lewis acid

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