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Ionization

Definition: A process in which a substance is dissolved in water to produce ions which carries electrical charge is called as ionization.

Example: When strong HCl is dissolved in water it forms H3O+ and Cl- ions.
 
HCl + H2O  →  H
3O+ + Cl-

Strong acid and strong bases dissociates completely in water whereas weak acids and weak bases dissociates partly in water.

Strong acids: HCl, H
2SO4
Strong bases: NaOH, KOH
Weak acids: CH
3COOH
Weak bases: NH
3


Ionization Constant (Dissociation constant):

Ionization of weak acid:
Let we have HA as a weak acid and is dissolve in water to produce H3O+ and A- ions.

HA + H
2O  →  H3O+ + A-

Equilibrium constant:
 (K) =
[H3O+] [A-]
           [HA] [H2O]

Concentration of water is negligible as compare to its original concentration, so considering it as constant:

K [H
2O] = [H3O+] [A-]
                       [HA]

K [H
2O] = Ka = equilibrium constant for weak acids

Ka =
[H3O+] [A-]
              [HA]

Weak acids ionized partly; hence the concentration of HA will remain same as its original concentration, [HA] = Co = Original concentration of HA

Similarly concentration of H
3O+ and A- will remain equal as HA is a weak acid

 [H
3O+] = [A-]

Ka =
[H3O+]2
            Co


Ionization of weak base:
Let we have NH3 as a weak base and is dissolve in water to produce NH4+ and OH- ions.

NH
3 + H2O → NH4+ + OH-

Equilibrium constant (K) = [NH4+][OH-]
                                         [NH3] [H2O]

Concentration of water is negligible as compare to its original concentration, so considering it as constant,

K [H
2O] = [NH4+] [OH-]
                       [NH3]

K [H
2O] = Kb = equilibrium constant for weak base

Kb =
[NH4+] [OH-]
             [NH3]

Weak base ionized partly; so the concentration of NH
3will remain same as its original concentration, [NH3] = Co = Original concentration of NH3

Similarly concentration of NH4+ and OH- will remain equal as NH3 is a weak acid
 
 [NH
4+] = [OH-]

Kb =
[OH-] 2
            Co


Ionization of Water:
Water is a weak electrolyte and ionized partly.

H2O ↔ H+ + OH-

At equilibrium, Keq = [H+][ OH-]
                                     [H2O]

As water ionized partly, the concentration of water is nearly constant.

Keq [H
2O] = Kw = [H+][ OH-]

Kw = equilibrium constant for water or ionic product of water.

Kw = [H
+][ OH-]

Kw is not constant but depends on strength of the solution and temperature. At 25°C, the value of Kw is 1.0 x 10
-14.


Calculation:
Exp 1: Calculate the ionization constant of 0.05M CH3COOH, which ionizes upto 2% at 30 °C.

Reason:
Co= 0.05M
Ionization occurs = 2%


0.05M acetic acid = 100 moles of H3O+
  ?   M acetic acid = 2 %
                [H
3O+] = 0.05 x 2
                                   100
                             = 0.001M

Ka =
[H3O+]2
           Co
     =
(0.001)2
          0.05 
     = 2.0 x 10
-5


Exp 2: If the value of ionization constant of an aqueous solution of 0.02M acetic acid is 1.20 x 10-10, then calculate the concentration of [H3O+].

Reason:
Co = 0.02M
Ka = 1.20 x 10
-10
 [H3O+] = ?
 
Ka =
[H3O+]2
            Co

 [H
3O+]2 = Ka x Co
               = 1.20 x 10
-10 x 0.02
               = 2.4 x 10
-12
 [H3O+]  = 1.56 x 10-6


Exp 3: Calculate the pH of a solution containing 0.02M acetic acid whose ionization constant is 8.0 x 10-6.

Reason:
Co = 0.02M
Ka = 8.0 x 10
-6
pH = ?

Ka =
[H3O+]2

               Co
 

 [H3O+]2 = Ka x Co
               = 8.0 x 10
-6 x 0.02
               = 0.16 x 10
-6
 [H3O+]  = 4.0 x 10-4

pH  = - log [H3O+]

        = - log (4.0 x 10-4)
       = - (-3.38)
       = 3.38

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