JEE Main & Advanced Chemistry Equilibrium / साम्यावस्था Arrhenius Theory Of Electrolytic Dissociation

(1) Postulates of Arrhenius theory

(i) In aqueous solution, the molecules of an electrolyte undergo spontaneous dissociation to form positive and negative ions.

(ii) \[\text{Degree of ionization (}\alpha \text{)}\]

\[=\frac{\text{Number of dissociated molecules}}{\text{Total number of molecules of electrolyte before dissociation}}\]

(iii) At moderate concentrations, there exists an equilibrium between the ions and undissociated molecules, such as,\[NaOH\] \[\rightleftharpoons \]  \[N{{a}^{+}}\] \[+\,O{{H}^{-}}\]; \[KCl\] \[\rightleftharpoons \]  \[{{K}^{+}}\] \[+\,C{{l}^{-}}\]

This equilibrium state is called ionic equilibrium.

(iv) Each ion behaves osmotically as a molecule.

(2) Factors affecting degree of ionisation

(i) At normal dilution, value of \[\alpha \] is nearly 1 for strong electrolytes, while it is very less than 1 for weak electrolytes.

(ii) Higher the dielectric constant of a solvent more is its ionising power. Water is the most powerful ionising solvent as its dielectric constant is highest.

(iii)  \[\alpha \ \text{ }\propto \frac{\text{1}}{\text{Con}\text{. of solution}}\]  \[\propto \frac{\text{1}}{\text{ wt}\text{. of solution}}\]

\[\propto \] Dilution of solution \[\propto \] Amount of solvent

(iv) Degree of ionisation of an electrolyte in solution increases with rise in temperature.

(v) Presence of common ion : The degree of ionisation of an electrolyte decreases in the presence of a strong electrolyte having a common ion.