Acid and Base-

There are three concepts of acids and bases-

1. Arrhenius Concept- According to this concept, an acid is a compound that releases H⁺ ions in water ; and a base is a compound that releases OH^- ions in water. For example, HCl is an Arrhenius acid and NaOH is an Arrhenius base.

HCl   H⁺   +  Cl^-

NaOH   Na⁺   +  OH^-

 

2. Bronared- Lowry concept- According to this, an acid is any molecule or ion that can donate a proton (H⁺ ) and a base is any molecule or ion that can accept a proton. eg, HCl and NH3. HCl gas reacts with ammonia to form solid NH4Cl.

                               H

    ..                         I

H-N-H  +  H-Cl  [H-N-H]⁺  +  Cl^-

    I                          I

   H                         H

HCl is a proton donor and hence a Bronsted acid, while NH3 is a proton acceptor and a Bronsted base.

 

3. Lewis Concept- According to this, an acid is an electron-pair acceptor and a base is an electron-pair donor. eg, betwen H ⁺ and NH3, proton (H⁺ ) is a Lewis acid as it can accept an electron pair. Ammonia molecule has an electron pair which it can donate and is a Lewis base. The Lewis reaction between H⁺  and NH3 can be writen as-

                                   H

                  ..                I

H⁺  +      H-N-H     [H-N-H]⁺

Lewis         I                 I        Complex

acid           H               H

                Lewis

                base