ACIDS AND BASES

Introduction to Acids and Bases

We encounter acids and bases in our daily life. For example, excess acid production in the stomach leads to acidity .Antacids are bases that reduce acidity.

The three main important theories in Chemistry which define acids and bases are-

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

1. Arrhenius Theory of Acids and Bases

   According to Arrhenius (1884), an acid is a compound that release H+ ions in water and a base is a compound that releases OH ions in water. E.g. 

HCl in water written as HCl(aq) undergoes ionization as follows

HCl (aq) → H+(aq) + Cl(aq)

Similarly,ionization of NaOH in water is as follows

  NaOH (aq) → Na+(aq) + OH(aq)

 Limitations: This theory proved to be useful but with a few limitations.

  1. This theory limits to water only
  2. Substances do not contain OH but still are bases.

2. Bronsted-Lowry Theory of Acids and Bases

According to Bronsted-Lowry theory (J.N.Bronsted and J.M.Lowry) an acid is a substance that can donate H+ ion and a base is a substance that can accept H+ ion.`

E.g. HCl + H2O → H3O+ + Cl

NH3+ H2O → NH4+ + OH

Bronsted Lowry theory is most widely used in common.

Using curved arrow notation the below equations show how HCl by donating H+ is acting as Bronsted acid while NH3 by accepting H+ is acting as Bronsted base.

Image Courtesy

https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Basics_of_General_Organic_and_Biological_Chemistry_%28Ball_et_al.%29/10%3A_Acids_and_Bases/10.02%3A_Brnsted-Lowry_Definition_of_Acids_and_Bases

3. Lewis Theory

Lewis Theory (G.N.Lewis,1930) proposed that –an acid is an electron pair acceptor and a base is electron pair donor. From the image below we can understand that BF3 is electron acceptor as it is accepting electron pairs from fluoride ion and is hence ,a Lewis acid whereas NH3is electron donor as nitrogen is donating electron pair to H+ and is hence a Lewis base.

Imagecourtesy:https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Book%3A_Introduction_to_Inorganic_Chemistry_%28Wikibook%29/03%3A_Acid-Base_Chemistry/3.02%3A_Brnsted_and_Lewis_Acids_and_Bases

classification of acids
The three theories about Acid
classification of bases
The three theories about Bases

Image courtesy https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Map%3A_Inorganic_Chemistry_%28Housecroft%29/07%3A_Acids_bases_and_ions_in_aqueous_solution/7.01%3A_Introduction/7.1A%3A_Acid-Base_Theories_and_Concepts

Reference: Essentials of Physical Chemistry by Arun Bahl, B.S. Bahl , G.D. Tuli

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