Acid and base

The word acid is derived from Greek word which means ‘sour’. These have generally sour taste and they can change blue litmus into red. On the other hand the bases are the substance which have bitter taste and which can convert red litmus into blue. When bases are dissolved in water these are given a special name alkali.

Concepts of acids and bases

       Following are the three concepts of acids and bases-

    1. Arrhenius concept
    2. Bronsted- Lowry concept
    3. Lewis concept

1. Arrhenius concept

Acid

According to Arrhenius an acid is a substance which dissociates in an aqueous solution to give hydrogen ions (H⁺). For example, HCl, HNO₃, CH₃COOH, H₂SO₄ etc. out of these acids some acids are strong acids while others are weak. The strength of an acid depends upon its degree of dissociation in water. The acids which can completely dissociate into ions on adding with water thus produce a large number of H⁺ ions are called strong acids e.g. HCl, HNO₃ and H₂SO₄ are strong acids because they dissociate into ions completely. On the other hand the acids which partially dissociate into ions on addition with water and give small number of H⁺ ions are called weak acids e.g. CH₃COOH is a weak acid. The dissociation of acids in water can be shown by following equations:

Strong acids:

HNO₃           H⁺ + NO_3^–

H₂SO₄           2H⁺ + SO_4^2-

HCl           H⁺ + Cl_^–

Weak acid:

CH₃COOH            H⁺ + CH₃COO^–

Base

According to Arrhenius a base is a substance which dissociate in an aqueous solution to give hydroxyl ions (OH^–). For example, NaOH, KOH, Ca(OH)₂ are bases because these can dissociate in water to give hydroxyl ions as given below-

Strong bases:

The bases which completely dissociate into water and give a large number of hydroxyl ions (OH^–) are called as strong base. For example, NaOH and KOH.

NaOH           Na⁺ + OH^–

KOH           K⁺ + OH^–

Weak bases:

The bases which partially dissociate into ions on addition of water and give a small number of hydroxyl ions (OH^–) are called weak bases. For example, Ca(OH)₂.

Ca(OH)₂            Ca²⁺ + 2OH^–

2. Bronsted-Lowry concept

According to Bronsted Lowry concept an acid is defined as a substance which has a tendency to donate a proton (H⁺) to any other substance and a base is a substance which has a tendency to accept proton (H⁺) from any other substance. In other words, an acid is proton donor and a base is proton acceptor. In terms of this definition, acids and bases are inter-related with each other as shown below:

Acid            Base + proton

Examples:

HCl + NH₃            NH_4⁺ + Cl ^–
Acid   base                                      

CH₃COOH + NH₃            NH_4⁺ + CH₃COO ^–
Acid       base                                          

In these examples HCl and CH₃COOH lose a proton so these act as acids while NH₃ accepts a proton so it acts as a base.

3. Lewis concept

According to Lewis concept an acid is a substance which can accept a pair of electrons from other substances while a base is a substance which can donate a pair of electrons to other substances. In other words an acid is an electron pair acceptor while a base is an electron pair donor. For example,

NH₃ + BF₃            NH₃-BF₃
base                                  acid   

Test your understanding and answer these questions:

1. Give arhenius concept of acids and base?
2. Give bronsted lowry concept of acid and base.
3. Give Lewis concept of acid and base.