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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-
- Arrhenius Theory
- Bronsted-Lowry Theory
- 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.
- This theory limits to water only
- 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
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.
Reference: Essentials of Physical Chemistry by Arun Bahl, B.S. Bahl , G.D. Tuli