# Concept of Acids and Bases

According to Arrhenius, an electrolyte which furnishes hydrogen ions $(H^+)$ in aqueous solution is called acid and that which furnishes hydroxyl ions $(OH^-)$ in aqueous solution is called base.

Bronsted and Lowry (1923) put forward a new concept. According to them a substance which furnishes protons is called acid (proton donor) and a substance which accepts the protons is called base (proton acceptor).

E.g.

$\underset{B}{\text{Base}} + \underset{H^+}{\text{Proton}} \leftrightharpoons \underset{BH^+}{\text{Acid}}$

Hence, such related pair of an acid and a base is said to be conjugate to one another, Some common exapmles of conjugate acids and bases are given below:

Conjugate Acids

It is interesting to note that H2Ocan function as an acid as well as a base according to the conditions i.e., water is amphoteric in nature.

Water donates proton so it is Bronsted acid.

$\underset{\text{Base}}{BH_3} + \underset{\text{Acid}}{H_2O} =\underset{ \text{Acid}}{NH^+_4} + \underset{\text{Base}}OH^-$

Water accepts protons so it is Bronsted base.

$\underset{\text{Acid}}{HCl} + \underset{\text{Base}}{H_2O} = \underset{\text{Acid}}{H_3O^+} + \underset{\text{Base}}{Cl^-}$

(i)     The acids and bases have been defined in terms of the substances themselves, and not in terms of their ionisation in aqueous solution.

(ii)     Acidic properties are indicated by the ability of a substance to release protons while that of a base to capture protons.

(iii)     Acidic and basic behaviour does not depend upon any solvent

Limitations: In this concept too much emphasis is given to proton exchange. There are many reactions with acid-base characteristics in which no protons are involved

E.g.

$\underset{\text{Acid}_1}{SO_2} + \underset{\text{Base}_2}{SO_2} \leftrightharpoons \underset{\text{Acid}_2}{SO^{2+}} + \underset{\text{Base}_1}{SO^{2-}_3}$

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