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

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