Lewis concept of acids and bases

Although Bronsted-Lowry theory was more general than Arrhenius' theory of acids and bases, it failed to explain the acid-base reactions which do not involve transfer of proton. For e.g., it fails to explain how acidic oxide such as anhydrous CO₂, SO₂, SO₃ etc can neutralise basic oxides such as CaO, BaO, etc. even in the absence of a solvent.

Lewis proposed broader and more general definitions of acids and bases which do not require 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 electron.

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 the ones shown below:

These have one or more unshared pair/s of electrons or anions such as CN^-, OH^- 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 e.g., BF₃, AlCl₃ etc.

b) Simple cations for e.g., H⁺, Ag⁺ etc.

Self Ionisation of Water

The conductivity measurements of water indicates that water is a weak electrolyte. Even in pure state water is weakly ionised to give H₃O⁺(aq) and OH^- (aq) ions as shown below.

Applying law of chemical equilibrium,

Since concentration of water is very high (55.5 mol / L) and only a very small fraction of it undergoes ionisation, therefore [H₂O] may be taken as constant and may be combined with K to have another constant K_w.

The constant K_w is called ionic product of water. Its value at 298K is 1.008 x 10^-14 mol²L^-2. In pure water [H₃O⁺] and [OH^-] are equal.

Therefore

K_w = [H₃O⁺] [OH^-]

and [OH^-]= [H₃O⁺] = 1 x 10^-7mol L^-1

In neutral solution [H₃O⁺] = [OH^-]

In acidic solution [H₃O⁺] >[OH^-]

In basic solution [H₃O⁺] < [OH^-]

Expressing Hydronium Ion Concentration - pH Scale

We have seen that concept of ionic product of water enables us to classify solutions as acidic, basic or neutral by specifying H₃O⁺ ion concentration. The pH scale was devised by P.L Sorensen (1909) is defined as negative logarithm of hydronium ion concentration in moles per litre.

For pure water or neutral solutions at 298K [H₃O⁺] is equal to 1x10^-7 mol L^-1.

pH = -log [H₃O⁺] = -log [1 x 10^-7] = 7

For acidic solutions [H₃O⁺] concentration is more than 1x10^-7mol L^-1. Therefore pH of acidic solutions is less than 7. For basic solutions the pH is greater than 7.

pH Value	Indications
0-2	Strongly acidic
2-4	Moderately acidic
4-7	Weakly acidic
7-10	Weakly basic
10-12	Moderatly basic
12-14	Strongly basic