Arrhenius Theory of Ionization of Electrolytes

In 1887 S- Arrhenius proposed a theory to explain the conductance of electrolytes in aqueous solvent. His postulates were:

1) A molecule of electrolyte when dissolved, dissociates into ions. Neutral molecules dissociates into charged species (positively charged cations and negatively charged anions). The process is electrolyte dissociation or ionization, eg Na⁺(aq) + Cl^-(aq)

2) Complete ionization of electrolyte does not occur. Fraction of molecules is dissociated. Both dissociated and dissociated molecules are present.

MA ⇌ M⁺ + A^-

3) Electrolytic solution conduct electric current due to movement of cations and anions.

4) Conductance of electrolytic solution depends on number of cations and anions present in a solution.

5) Greater the number of dissociated ions, greater is the conductance. Strong electrolytes have high degree of dissociation. Weak electrolytes have less degree of dissociation.

6) The ions produced by dissociation behave alike towards colligative properties of solution (which depends on number of ions in this case).

7) The properties of electrolytes are "the properties of ions”.

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Degree of Dissociation and its Factors

It is the ratio of no of molecules present in dissociated form to the total number of molecules of electrolytes. It is represented by "α”.

α = molecule dissociated/ total number of molecules

also α = Ʌ/ Ʌ_o

here Ʌ = equivalent conductance and Ʌ_o equivalence conductance at infinite dilution.

Factors of degree of dissociation

Degree of dissociation depends on various factors:

1) Nature of electrolyte:

Strong electrolytes ionize completely e.g. acids, bases salts. Weak electrolytes ionized partially e.g. organic acids.

2) Nature of solvent:

Solvents of high dielectric constant value has high degree of dissociation (e.g water, polar). Solvents of low dielectric constant value have small degree of dissociation (benzene).

3) Presence of common ion / effect:

Degree of dissociation decreases in the presence of another electrolyte which contains a common ion. One electrolyte does not affect the degree of dissociation of another electrolyte if and only if common ion is present.

e.g. If NH₄OH will be added in NH₄Cl then degree of dissociation of NH₄Cl will decrease. If NaCl will be added in KNO₃ then degree of dissociation will not change.

4) Concentrations:

Increase in dilution increases the degree of dissociation. Addition of solvent shifts the equilibrium to right.

5) Temperature:

Degree of dissociation will increase with increase in temperature.

By Arrhenius theory of ionization, there occurs an equilibrium between dissociated and dissociated ions. Law of mass action con be observed in it. Thus Ostwald derive a method called Ostwald dilution law for its verification.

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