Explain the Lewis concept of acids and bases.

  1. No.2. Explain the Lewis concept of acids and bases.

Answer. G.N. Lewis (1923) proposed a more general and broader concept of acids and bases. According to this concept:

An acid is a substance, molecule or ion which can accept a pair of electrons.

While a base is a substance, molecule or km which can donate a pair of electrons

For example, a reaction between ammonia and boron tri-fluoride forms a coordinate covalent bond between ammonia and boron tri-fluoride. Ammonia donates electron pair to boron tri-fluoride.

Example:

NH is base while BFs is acid. The cations (proton itself or metal ions) act as Lewis acids. For example, a reaction between H+ and where H+ acts as an acid and ammonia as a base     

The product of any Lewis acid-base reaction is a single species, called an addict. So, a neutralization reaction according to Lewis concepts is donation and acceptance of an electron pair and adduct contains a coordinate covalent bond.

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.

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

(a)      Molecules in which the central atom has incomplete octet. For example, in BF3, Aids, FeCl3, the -central atom has only six electrons around it, therefore, these can accept an electron pair.

(b)      Simple cations can act as Lewis acids. All cations act as Lewis acids because they are deficient in electrons. However, cations such as Na+, K+, Ca2+ ions, etc., have a very little tendency to accept electrons. While the cations like H+, Ag+ ions, etc., have a greater electron accepting tendency. These are all Lewis acids.

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

(a)      Neutral species having at least one lone pair of electrons. For example, ammonia, amines, alcohols etc., act as Lewis bases because they contain a lone pair of electrons:

NH2. R — NH2R — O — H

(b)      Negatively charged species or anions. For example, chloride, cyanide, hydroxide ions, etc., act as Lewis bases: CN-, Cl-, OH- etc.

  1. No.3. What is auto-ion ization of water? How is it used to establish the pH of water?

Answer. Concentration of hydrogen ion [H+] in pure water is the basis for the pH scale. Water is a weak electrolyte because it ionizes very slightly into ions. This process is called auto-ionization or self ionization;

H2O              H+ + OH-

The equilibrium expression of this reaction may be written as-

Kc = [H+] [OH-]

[H20

As concentration of water H2O is almost constant. The above equation may be written as

Kc [H20] = [H+] [OH]

Taking   Kc [H2O] = Kw

Kw=[H+][OH-]

Kw is ionic product constant of water

One molecule of water produces one H+ ion and one OH- ion on dissociation so

[H+] = [OH-]

Kw = H+] H+] = [H+]2

Kw at 25°C is = 1.0 x 10-14 so 1.0 x 10-14= [H+]2

or        [H+]= 1.0 x 10-7 M at 25°C

[H+] = [OH-] = 1.0 x 10-7 M

 

Values of [H+] -and [OH-] are very small. So, new terms pH and pOH are used to change very small values into simple numbers. pH is the negative logarithm of molar concentration of the hydrogen ions. i.e.

pH= log[H+]

So, according to this scale, pH of water is = -log-(l.0 x 10-7) =7

Similarly pOH = -log (1.0 x 10-7) = 7

pH value normally varies from 0 to 14.

Therefore; pH + pOH = 14

So, the sum of the pH and pOH of the solution is always 14 at 25’C.

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