Engineering Chemistry S Datta

IONIC EQUILIBRIUM 185
Common Ion Effect
If we add NaOAc to a solution of AcOH, the ion (OAc ) becomes common. Let us see its
effect on ionisation of AcOH.
AcOH
H + + OAc , NaOAc ⎯⎯→ Na + + OAc
K a
=
+ −
[H ] [OAc ]
[AcOH]
...(3)
Now, NaOAc being a salt dissociates completely in solution. So concentration term [OAc ]
increases in the equation (3), to maintain the value k a
constant, [H + ] will decrease i.e.,
undissociated [AcOH] will increase. As a result, the dissociation of AcOH will be suppressed.
This phenomenon is known as Common ion effect. In general, we can say, that common ion
effect suppresses the dissociation of a weak electrolyte in solution.
ACIDS AND BASES
Arrhenius Theory of Acids and Bases
According to this theory an acid is a substance which when dissolved in water gives H +
ions and a base is a substance which when dissolved in water gives OH – ions. It is well known
that in neutralisation reactions, acids react with alkalies to form salts and water. In the
Arrhenius concept neutralisation may be explained as follows:
(H + + Cl – ) + (Na + + OH – ) ⎯⎯→ NaCl + H 2
O
(2H + + SO = 4
) + ZnO ⎯⎯→ ZnSO 4
+ H 2
O
The indication of neutralisation is:
H + + OH – ⎯⎯→ H 2
O
i.e., the formation of water.
Bronsted and Lowry Concept
Arrhenius theory is suitable in explaining the reactions of acids and bases in aqueous
solutions but it fails to explain the following reactions, specially in non-aqueous solvents. As
for example, NaNH 2
dissolves in liquid ammonia and behaves as NaOH in water as is implied
from the following equations.
2NaNH 2
+ MgSO 4
= Mg(NH 2
) 2
↓ + Na 2
SO 4
(in liq. NH 3
)
2NaOH + MgSO 4
= Mg(OH) 2
↓ + Na 2
SO 4
(in water)
So, behaviours of NH
–
2
and OH – ions are same and OH – is derived from a base, so we
can say NaNH 2
is a base i.e., a substance can behave as an acid or a base by not producing H +
or OH – ions. The definition of acids and base needs to be extended.
Bronsted and Lowry defined: Acid as a species which can donate a proton and base
as a species which can accept a proton. These species may be neutral or charged. Whether the
species will behave as an acid or a base will depend upon the nature of the solvent. The strengths
of the acids or bases may be determined by their tendencies to donate or accept a proton in a
definite solvent. For every acid, there is a base known as conjugate base and for every base
there is an acid known as conjugate acid.
As for example, Cl – is a conjugate base of the acid HCl and HCl is the conjugate acid of
the base Cl – . Some Acid/Base conjugate pairs are shown in the following table.