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 → H3O+ + Cl-
Strong acid and strong bases dissociates completely in water whereas weak acids and weak bases dissociates partly in water.
Strong acids: HCl, H2SO4
Strong bases: NaOH, KOH
Weak acids: CH3COOH
Weak bases: NH3
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 + H2O → 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 [H2O] = [H3O+] [A-]
[HA]
K [H2O] = 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 H3O+ and A- will remain equal as HA is a weak acid
[H3O+] = [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.
NH3 + 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 [H2O] = [NH4+] [OH-]
[NH3]
K [H2O] = Kb = equilibrium constant for weak base
Kb = [NH4+] [OH-]
[NH3]
Weak base ionized partly; so the concentration of NH3will 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
[NH4+] = [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 [H2O] = 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 %
[H3O+] = 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
[H3O+]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