ACIDS, BASES AND SALTS
12.1 Lowry and Bronsted Concepts of Acids and Bases
The classification of substances of acids was at first suggested by their
sour taste (Latin acidus = Sour) and alkali (Arabic alkali = ashes
of a plant) were taken as those substances that could reverse or neutralize the
action of acids.
The classical definition of
ACID : as a substance whose water solution
i) turns blue litmus red
ii) neutralizes base
iii) reacts with active metals like Zn, Mg, Cu etc with evolution of H2
iv) having sour taste
v) decomposes carbonates
The following chart shows different types of acid.
BASE : as a substance whose water solution
i) turns red litmus blue
ii) neutralizes acid to produce salts
iii) tastes bitter
iv) soapy to touch
v) reacts with few metals like Zn, Al, Pb etc liberating hydrogen.
Examples : NaOH, KOH, NH4OH
In 1923, J. N. Bronsted and T. M. Lowry proposed definitions of
acids and bases in aqueous as well as non aqueous solutions according to which
An acid is defined as a substance having a tendency to lose or to
donate one or more protons and,
A base is defined as a substance having tendency to accept or add a
proton.
Example :
NH3 (g) + HCl (g)
 NH4+
+ Cl-
Base Acid
Here HCl is proton donor (hence acid) and ammonia is a proton acceptor (hence
base)
Other examples are :
1) NH3 (aq) + H2O
(l)
NH4+
(aq) + OH- (aq)
Base Acid
2) NH3 (aq) + H3O+
(aq)
NH4+
(aq) + H2O (l)
Base Acid
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