Acids, Bases and Salts: Concepts of acids and Bases

 Concepts of acids and Bases

Arrhenius Concept of Acids and Bases

In 1887, Swedish chemist Savante Arhenius proposed the first successful theory of acers and bases according to this theory:

An acid is a substance that ionizes in water to produce H+ ions and a base is a substance that ionizes in water to produce OH- ions.

For example,

HCl ----> H+      +     Cl-

NaOH ------>  Na+     +     OH-

Some common acids

Name                           Formula             common use

Hydrochloric acid       HCl             cleaning of metals, bricks and  removingscale                                                                   from boiler 

Nitric acis                     HNO3          manufacturing of fertilizers, explosives 

Sulphuric acid            H2SO4          manufacturing of many chemicals,drugs  dyes                                                                  and explosives 

Phosphoric acid       H3PO4             manufacturing of fertilizers, acidulant for food

Some common Bases

Name                                 Formula                  common use

Sodium hydroxide           NaOH                 Soap making, drain cleaner

Potassium hydroxide       KOH                  Making liquid soap, shaving cream 

Calcium hydroxide          Ca(OH)2            Making mortor, plaster, cement 

Magnesium hydroxide    Mg(OH)2           Antacid, laxative.

Limitations of Arhenius concept 

Arhenius theory has its limitations. It applies to aqueous solutions. 

It does not explain: 

Why compounds such as CO2, SO2 etc are acids as there no H in these compounds.  

Why substances like NH3 are bases as there is no OH in it.

Bronsted-Lowery concept of Acids and Bases 

In 1923, J N Bronsted and TM Lowery independently proposed another theory to overcome limitations of Arhenius theory of acids and Bases. This theory is known as Bronsted-Lowery theory

According to this theory, an acid is a pron donor and a base is a proton acceptor.

For example, 

H-Cl       +     H-O-H    ------------>     H3O+        +       Cl-

Acid                Base   

NH3      +       H2O      -------------->   NH4+       +       OH-

Base                Acid

Lewis concept of Acids and Bases 

1923, G. N. Lewis proposed an acid base theory that focuses on reaction. This concept is more general than either the  Arhenius theory or Bronsted-Lowery theory. 

According to this concept,

A Lewis acid is substance that can accept a pair of electron to from a coordinate covalent bond.

A Lewis base s substance that can donate a pair of electron to from a coordinate covalent bond.

For example,

H- Cl             +        H3N: ---------> H4N+ + Cl-

Electron                 Electron 

pair acceptor        pair donor

H- Cl           +             H2O:  ---------> H3O+    +     Cl-

Electron              Electron 

pair acceptor      pair donor

Example 10.2: Classify substances as Lewis acids or Lewis bases