Ionic Theory | Arrhenius theory of electrolytic dissociation with limitations

In order to explain the properties of electrolysis in solution, Arrhenius proposed the ionic theory.

Postulates of Arrhenius theory:

1. When an electrolyte is dissolved in water, its molecules spontaneously split into positively and negatively charged particles called ions. This process is called ‘dissociation’ or ‘ionisation’ i.e.,
   BA ↔ B- (cation) + A- (anion)
2. In a solution, the total positive charges on all the cations (+ve ions) and negative charges on all the anions (-ve ions) are exactly equal. Hence, the whole solution is electrically neutral.
3. Ions are responsible for the passage of electric current through electrolytes.
4. Ionisation is a reversible process. A state of equlibrium exists between ions and the undissociated molecules in the solution i.e., BA ↔ B+ + A-; K = [B+][A-]/[BA]
5. The degree of ionisation of an electrolyte increases with dilution.
   Note: Degree of dissociation is the fraction of the total amount of an electrolyte which ionises at equilibrium.
6. The properties of electrolytes are the properties of ions present in them i.e., chemical reactions between electrolytes in solutions take place between the ions instead of the molecules.

Merits for Arrhenius theory:

1. The theory satisfactorily explains the phenomenon of electrolysis and faraday’s laws of electrolysis.
2. Experimental observation shows that the conductance of a solution increases upon dilution. This is in agreement with theory as more ions are formed during dilution.
3. The solutions of copper sulphate, cupric nitrate, cupric chloride are all blue because they all contain Cu2+ ions. This confirms that properties of electrolytes are properties of ions present in them.
4. Silver nitrate solutions reacts with solutions of sodium chloride, potassium chloride, barium chloride etc. to develop a crudy white precipitate. This is possible because all these solutions contain chloride ions which combine with Ag+ ions of silver nitrate. Ag+ + Cl- —-> AgCl- (curdy white)
5. Ionic theory explains the constancy in enthalpy of neutralisation of any strong acid with any strong base.
6. Arrhenius theory explains the abnormality in the colligative properties (Eg: Osmotic pressure, elevation in boiling point etc.) of electrolytic solutions, because colligative properties depend on the number of particles in the solution. Due to ionisation, the electrolyte molecule produces two or more number of ions. Hence the colligative properties of such solutions are more than those of solutions of non-electrolytes.

Limitations or Demerits of Arrhenius theory:

1. The x-ray analysis of crystals like sodium chloride shows the existence of ions even in the solid state. Ions are also present in the molten state. However, according to the theory, ions are formed only in solution.
2. The theory does not explain why ionisation takes place at all when an electrolytes is dissolved in water. It does not explain where the required energy for ionisation comes from.
3. Certain electrolytes like HCl (called strong electrolytes) ionise completely when dissolved in water whereas some other electrolytes like acetic acid (called weak electrolytes) ionise only partially. The theory does not distinguish between the two types of electrolytes.