Acid, Base and Salt

Arrhenius Concept: The substance that librates H⁺ ion in aqueous solution is acid while bases are those compounds which give OH^- ion in aqueous solution.

Bronsted – Lowry Concept: (Protonic concept.) Acid is a proton donor and proton acceptor is base, but the proton must be solvated. For example,

        B           +     H⁺                                      BH

   Base                    proton                                         acid

   Such related pair of an acid is said to be conjugate to one another. The acid and base differing from each other by the same proton are known as Conjugate Pair:

     NH₃    +  H₂O                                             NH₄⁺   + OH^-

Here, NH₄⁺ is the conjugate acid of base NH₃

Thus we can say that, every acid has its conjugate base and every base has its conjugate acid.

A strong acid has a weak conjugate base and a weak acid has a strong conjugate base.

     Strongest acid: HCLO₄ (per chloric acid)

     Weakest base: CLO₄^- (perchlorate ion)

     Weakest acid: CH₄ (methane)

     Strongest base: CH₃^-

This concept also shows the amphoteric nature of water as it can both accept and give a proton.

H₂O   +   H₂O                                    H₃O⁺   +   OH^-

 Note: All Arrhenius acids are Bronsted acids but all Arrhenius bases are not Bronsted bases.