Ionic Equilibrium
Ionic Equilibrium
Ionic Equilibrium
IONIC EQUILIBRIUM
INTRODUCTION
1.Equilibrium is the state at which the
concentration of reactants and
products do not change with time .
2. Equilibrium may also be defined as
state of process in which properties
like temperature,colour,pressure,
Concentration of the system do not
show any change with time.
TYPES OF EQUILIBRIUM
PHYSICAL EQUILIBRIUMIf the
opposing processes involves only physical
changes(melting of ice water)the
equilibrium is called physical equilibrium.
CHEMICAL EQUILIBRIUMIf
opposing processes involves chemical
changes(the reaction between H2 and I2)the
equilibrium is called chemical equilibrium
TYPES OF ELECTROLYTES
STRONG ELECTROLYTES-THE ELECTROLYTES WHICH ARE ALMOST
COMPLETELY IONIZED IN Aq. SOLUTION.
Eg.H2SO4 ,HCl
WEAK ELECTROLYTESTHE
ELECTROLYTES WHICH ARE IONIZED TO
SMALL EXTENT IN Aq. SOLUTION .
Eg.CH3COOH,NH4OH
Ionization of an Electrolyte in
aqueous solution
The molecules of some compounds, when dissolved
in water gets decomposed in two types of charged
particles called ions.The ions which carry positive
charge are called cations . The ions which carry
negetive charge are called Anions.Initially, very few
molecules get decomposed into ions. Gradually, the
concentration of ions increases.When the
concentration of ions becomes too high, the ions
reunite to form molecules again. At certain stage,
Equilibrium is set up.This state of process is called
ionic equilibrium.
Contd.
6.At moderate concentration, only a fraction of the
total amount of electrolyte ionizes.The fraction of
total number of molecules of an electrolyte which
ionizes on dissolution is called degree of
ionization.
AB A+ + BInitially t =0 C
At equilibrium C(1-)
0
C
0
C
Contd.
dissociation constant may be given
K = [A+][B-] =
[AB]
(C * C)
C(1- )
= C2 /(1-
) ....... (i)
For very weak electrolytes,
<<< 1, (1 - ) = 1 .. K = C2
= K/C ....... (ii)
Contd.
= K/C ....... (ii)
For a weak electrolyte degree
of ionization is inversely
proportional to molar conc.
it is a
increases on dilution.
H+
H2O + H2O
OH
+
H3O + OH-
Contd.
Keq for this reaction is
given by
k
= [H+] [ OH-]
[H2O]
k
[ H2O]2= constant
Eq (2) may be written as
Contd.
Eq.2 may be written as
[H3O+] [OH-]= K [H2O]2
=constant x constant
=Kw , called ionic product of water
KW = 1.008 x10-14
Effect of temperature:
The ionic product increases with increase in temperature.
This is because with increase in temperature degree of
H2O + H2O
H3O+ + OH-
[H3O]+ = [OH-]
But at 298 K
Kw= [H3O+] [OH-] =1.0 x10 -14
[H3O+]2 =1.0 x10 14
[H3O+] =1.0 x10 7 mol/l
Thus in pure water at 298 K
[H3O]+ = [OH-] = 1.0 x10 7 mol/l
Contd
If some acid is added to pure water then
[H3O+] >1.0 x10 7 mol/l
Since Kw is constant, thus [OH-] decreases and
may be calculated as
[OH-]
Kw
[H3O+]
Kw
[OH-]
In general,
Concept of pH
The symbol pH has been taken from danish
word potenz the hydrogen ionwhich
means power of hydrogen ion.
The pH of solution may be defined as
negetive logarithm of hydrogen ion
concentration.
pH = -log [ H 3O+ ]
(1)
=log
1
[ H 3O+]
(2)
Contd.
From eq.(1)
log [ H 3O+ ] = -pH
Taking antilog both sides
[ H 3O+ ] = 10 pH
Thus pH may be defined as negative power to which 10
must be raised to equal the hydrogen ion concentration.
pH = -log [ H 3O+ ]
= -log [1x10-7 ]
=-(-7) log10
=+7
pH of Solution
For an acidic solution ---- pH < 7
For an basic solution -----pH > 7
For an neutral solution-----pH = 7
Example:
If the base has an OH- concentration of 0.001M,
find the pH.
Solution:
First find the pOH, (similar to finding the pH,)
then subtract the pOH from 14.
OH- = 0.001M = 10-3;
pOH = -log [OH-]
= -log (10-3)
= +3 = pOH
pH = 14 - pOH; pH = 14 - 3 = 11 = pH