Introduction

The equilibria involving ions are called ionic equilibrium. When common salt or sodium chloride is dissolved in water, it can conduct electricity! Conductance of electricity increases with increasing concentration of common salt. On the basis of conductance of their solutions substance were classified by Faraday as electrolytes and non-electrolytes.

An electrolyte is a substance or compound, which conducts electricity in molten aqueous state. On the contrary, the compound does not conduct electricity in molten or aqueous state are called non-electrolytes. Arrhenius gave the concept that an electrolyte conducts electricity due to presence of ions after dissociation of the substances. The extents of dissociation of different electrolytes are different.
 

The fraction of total number of molecules that dissociate into ions is called its degree of dissociation or degree of ionisation and generally denoted by α.

On the basis of degree of ionisation Faraday categorised the electrolytes as ‘strong electrolytes’ and ‘weak electrolytes’.

A strong electrolyte dissociates almost completely into its ions in aqueous solution and is a vey good conductor of electricity. The examples of strong electrolytes are NaOH, KOH, HCl, NaCl, etc.

HCl+ H₂O → H₃O⁺ + Cl^-

A weak electrolyte is a substance that dissociates to a small extent in aqueous solution and hence conducts electricity to a little extent. NH4Cl and CH3COOH are the examples of weak electrolyte.

CH₃COOH →  CH₃COO^- + H₃O⁺

In general, ionisation of a weak electrolyte, AB is represented as follows
 

AB (s) + aq → A⁺(aq) + B^-(aq)


This type of equilibrium is known as ionic equilibrium
 

Ionisation of Weak Acids and Bases

Ionisation of a weak acid HA can be represented as

HA + H₂O → H₃O⁺ + A^-

If C is concentration of acid in mole per litre and α is degree of dissociation

As the degree of dissociation of a weak acid is very small we can take 1- α≈1

K_a = α²C, or 

Ionisation of a weak base BOH can be represented as

BOH + H₂O → B⁺(aq) + OH^-(aq)
 

When a is small, 1-α≈1

Hence K_b= α²C

Protic Acids and its Types

Protic acids
Acids having ionisable hydrogen are known as protic acids.
Protic acids can be classified as monoprotic, and polyprotic acids

Examples:

Monoprotic acids: HCl, CH₃COOH, HNO₃. These acids contain only one ionisable hydrogen.

Polyprotic acids: H₂SO₄, (COOH)₂, H₂CO₃, H₃PO₄ H₃A SO₄, etc. Polyprotic acids produce more than one hydrogen ion on dissociation. They ionise in steps.

For example the dibasic acid H2X may ionise in two steps as follows:

H₂X (aq) → H⁺(aq) + HX^-(aq)

HX^-(aq)  → H⁺(aq) + X^-(aq)

Their ionisation constants and called the first and second ionisation constant respectively and given by the equations



The overall equilibrium is obtained by adding the stepwise dissociation equilibria.
Thus for the above process the overall equilibrium will be

H₂X (aq)  →  2H⁺ (aq) + X^2- (aq)

How does dissociation affect ionic equilibrium in solution?

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