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:
NH
3 + H
2O NH
4+
+ 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.