1st Lecture on Ionic Equilibria Chemistry Part I12th Standard by Rizwana Mohammed

1. The Malegaon High School & Jr. College
Malegaon, (Nashik), 423203
1st Lecture on Ionic Equilibria
Chemistry Part I, 12th Science
By
Rizwana Mohammad

2. Ionic Equilibria
The equilibrium between ions and unionized molecules in solution
is called ionic equilibrium.
Electrolyte:
An electrolyte is a substance that produces an electrically
conducting solution when dissolved in a polar solvent.
Type of electrolyte:
Electrolytes are classified into strong and weak electrolytes.
Strong electrolyte:
The electrolytes ionizing completely or almost completely are
strong electrolytes.
e.g. Strong acid, strong bases and Salts.
Weak electrolyte:
The electrolytes which dissociate to a smaller extent in aqueous
solution are weak electrolytes.
e.g. Weak acid, weak bases.

3. Dissociation:
The spontaneous splitting of an electrolyte into positive
and negative ions in aqueous medium is called Dissociation.
Degree of dissociation: It is defined as, a fraction of total
number of moles of the electrolyte that dissociates into its
ions when the equilibrium is attained.
𝛼 =
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 dissociated
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠
Percent dissociation = 𝛼 × 100

4. Acids and Bases:
Arrhenius theory of acids and bases:
Acid: Acid a Substance which contain hydrogen and gives rise to H
ions in aqueous solution.
e.g. HCl(aq)
𝑤𝑎𝑡𝑒𝑟
H(aq) + Cl(aq)
CH3COOH(aq) ⇌ CH3COO(aq) + H(aq)
Base: Base is a substance that contains OH group and produces
hydroxide ions (OH ) in aqueous solution.
e.g. NaOH(aq) Na(aq) + OH(aq)
NH4OH(aq) ⇌ NH4(aq) + OH(aq)

5. Bronsted-Lowry Theory:
This theory is also known as proton transfer theory.
Acid: Acid is a substance that donates a proton (H ) to another
substance.
Base: Base is a substance that accepts a proton (H ) from another
substance.
e.g. HCl + NH3 ⇌ NH4 + Cl
acid 1 base 2 acid 2 base 1
A pair of an acid and a base differing by a proton is said to be a
conjugate acid-base pair.
HCl(aq) + H2O(l) ⇌ H3O(aq) +Cl(aq)
acid 1 base 2 acid 2 base 1
Conjugate acid base pair 1
Conjugate acid base pair 2

6. Lewis theory:
Acid: Any Species that accepts a share in an electron pair is called
Lewis acid.
Base: Any species that donates a share in an electron pair is called
Lewis base.
e.g. H
N
H
H:+H
+
H
H
N HH
H
N
H
H:+B
F
F
F
H
H
N HB
F
F
F
acid base
acid base
H

7. Amphoteric nature of water:
Water has ability to act as an acid as well as a base. Such
behaviour is known as amphoteric nature of water.
e.g. H2O(l) + NH3(aq) ⇌ OH(aq) + NH4(aq)
acid
H2O(l) + HCl(aq) ⇌H3O(aq) + Cl(aq)
base

8. Ionization of acids and bases:
Acids and bases are classified as strong acids and strong bases,
weak acids and weak bases of their extent of dissociation.
Strong acids and bases are almost completely dissociated in water.
e.g. HCl(aq) H(aq) + Cl(aq)
NaOH(aq) Na(aq) + OH(aq)
Weak acids and bases are partially dissociated in water.
e.g. CH3COOH(aq) ⇌ CH3COO(aq) + H(aq)
NH4OH(aq) ⇌ NH4(aq) + OH(aq)

9. Dissociation constant of weak acids and weak bases:
The dissociation of a weak acid HA in water is expressed as
HA(aq) ⇌ H(aq) + A(aq)
The equilibrium constant called acid dissociation constant for this
equilibrium is:
𝐾 𝑎 =
𝐻 [𝐴 ]
[𝐻𝐴]
Similarly the dissociation of weak base BOH in water is represented as:
BOH(aq) ⇌ B(aq) + OH(aq)
Base dissociation constant is
𝐾 𝑏 =
𝐵 [𝑂𝐻 ]
[𝐵𝑂𝐻]
Thus, the dissociation constant of a weak acid or a weak base is
defined as “the equilibrium constant for dissociation equilibrium of weak
acid or weak base respectively.”

10. Ostwald’s dilute law:
Arrhenius concept of acids and bases was expressed
quantitatively by F.W. Ostwald in the form of the dilution Law.
a- Weak Acid:
Consider an equilibrium of weak acid HA,
HA(aq) ⇌ H(aq) + A(aq)
The acid dissociation constant is given by
𝐾 𝑎 =
[𝐻 ][𝐴 ]
[𝐻𝐴]
…1
HA(aq) ⇌ H(aq) + A(aq)
Initially 1 0 0
Amount present at 𝑒𝑞 𝑚
𝑚𝑜𝑙 1 − 𝛼 α α
Concentration at eqm
mol dm-3
1 − α
𝑉
α
𝑉
α
𝑉

11. Substituting the values in equation 1
𝐾 𝑎 =
α
𝑉
α
𝑉
(1−α
𝑉)
=
α2
1−α 𝑉
…2
If C is the initial concentration of an acid in mol dm-3 and V is
the volume in dm3 mol-1, then 𝐶 =
1
𝑉
Therefore 𝐾 𝑎 =
α2
𝐶
1−α
…3
For a weak acid HA, α is very small, 1 − 𝛼 ≅ 1
therefore 𝐾 𝑎 =
α2
𝑉
and 𝐾 𝑎 = α2 𝐶
α = 𝐾 𝑎 𝑉 and α = 𝐾 𝑎
𝐶 …4
Equation 4 implies that the degree of dissociation of a weak acid is
inversely proportional to the square root of its concentration or
directly proportional to the square root of volume of the solution
containing 1 mole of weak acid.

12. b- Weak base:
Consider 1 mole of weak base BOH dissolved in V dm3 of
solution. The base dissociates partially as
BOH(aq) ⇌ B(aq) + OH(aq)
The base dissociation constant is
𝐾 𝑏 =
𝐵 [𝑂𝐻 ]
[𝐵𝑂𝐻]
…1
BOH(aq) ⇌ B(aq) + OH(aq)
Initially 1 0 0
Amount present at
equilibrium
1 − 𝛼 α α
Concentration at eqm 1 − α
𝑉
α
𝑉
α
𝑉

13. Substituting in eqn …1
𝐾 𝑏 =
α
𝑉 (α
𝑉)
(1−α
𝑉)
𝐾 𝑏 =
α2
𝐶
1−α
α = 𝐾 𝑏 𝑉, α =
𝐾 𝑏
𝐶
The degree of dissociation of a weak base is inversely proportional
to square root of its concentration and is directly proportional to
the square root of volume of the solution containing 1 mole of
weak base.