Understanding Ionic Equilibrium: Ionisation and Dissociation

Ionic equilibrium is the study of dynamic equilibrium established between ions and undissociated molecules in weak electrolytes. Strong electrolytes dissociate completely, so no equilibrium exists for them.Let us understand how, in ionic equilibrium, ionisation and dissociation work. Read on to learn how electrolytes of a solution are affected.

What is Ionic Equilibrium?

Ionic equilibrium is observed in weak electrolytes (weak acids, weak bases, sparingly soluble salts). These ionise or dissociate only partially in solution. They establish a dynamic equilibrium between ions and undissociated molecules.

Knowing the Difference of Ionisation vs. Dissociation

Ionization:

When in a solvent, a neutral molecule forms ions (the solvent is generally water)
For example, in water, the hydrogen chloride gas is ionised

HCl (g) + H₂O (l) ⇌ H₃O⁺ (aq) + Cl⁻ (aq)

Dissociation:

When in a solvent, there is already the presence of separated, already existing ions for an ionic compound (the solvent is generally water).
For example, in water, the solid sodium chloride is ionised.

NaCl (s) → Na⁺ (aq) + Cl⁻ (aq)

Types of Electrolytes

When talking about ionisation and dissociation, it is important that we know about the types of electrolytes.

Strong electrolytes → These types undergo complete dissociation (E.g., NaCl, HNO₃, H₂SO₄).
Weak electrolytes → These types undergo partial ionisation, which establishes equilibrium (E.g., CH₃COOH, NH₄OH).

Ionisation Constant in Ionic Equilibrium

It is quite essential that we know about the ionisation constant (K), as in weak electrolytes, equilibrium can be expressed using these constants.

For a weak acid (e.g., HA):
HA ⇌ H⁺ + A⁻
The ionisation constant for a weak acid:
For weak base (e.g., BOH):
BOH ⇌ B⁺ + OH⁻
The ionisation constant for a weak base:

Factors which Affect Ionisation

Strong vs weak → Nature of electrolyte
Polar solvents like water. promote ionisation → Nature of solvent
Increases the Ionisation → Temperature
Dilution increases ionisation for weak electrolytes (as explained by Ostwald’s dilution law). Strong electrolytes remain fully dissociated, so dilution has no effect on their degree of ionisation.

Details At A Glance

Property	Details
Process	Ionisation (formation of ions) & Dissociation (separation of ions)
Strong electrolyte	Complete dissociation
Weak electrolyte	Partial ionisation → equilibrium
Constants	Ka for acids, Kb for bases
Important law	Ostwald’s Dilution Law

Neutrality in Ionic Equilibrium

In ionic equilibrium, the principle of electrical neutrality is always maintained: the total positive charge = total negative charge in solution.

Σ (positive charges) = Σ(negative charges)

Summing Up

We have understood that in any reaction, ionisation forms ions, whereas it is quite the opposite in dissociation, as it separates the pre-existing ions. In order to know the degree of ionisation, various factors affect it, like the solution’s concentration, the type of solvent, and the temperature. In order to measure its strength, Ka and Kb are the constants.