Ionic Equilibrium - Postulates of Arrhenius Theory of Ionization | Chemistry Class 12

Chemistry | Notes
Ionic Equilibrium - Postulates of Arrhenius Theory of Ionization

Class : 12

In 1887, Svante Arrhenius, put forward a theory to explain the properties of electrolytes in solution. This theory is known as Arrhenius theory of ionization or electrolytic dissociation after his name. The main points of the modern form of the theory are given below.

1. When an electrolyte (acid, base or salt) is dissolved in water (or any other polar solvent) it yields two kinds of electrically charged particles which are known as ions. The ions carrying positive charge are called cations and those carrying negative charge are known as anions. 

AB ------>  A(+)    +    (B-)

               Cation        Anion 

According to modern theory, it is assumed that even solid electrolyte is composed of ions which are held together by electrostatic lines of forces. In the process of dissolution, these lines of forces are cut off and the ions separate. 

2. In solution, the ions tend to recombine to form unionized molecules of the electrolyte. Thus, there exists a dynamic equilibrium between ions and the unionized molecules of the electrolyte. Such a dynamic equilibrium is described by a constant called ionization constant. For example, when an electrolyte AB ionizes in solution to form A+ and B- ions, then the equilibrium 

AB <----------->A(+)     +     (B-) 

is established in the solution. The ionization constant for this equilibrium can be expressed as 

K   =  ( [A+]  [B-] ) / [AB] 

3. In a solution of an electrolyte the total number of positive charges is equal to the total number of negative charges. The solution is, therefore, electrically neutral. 

4. The ions move toward oppositely charged electrodes on passing electric current through the solution. This movement of ions in solution is responsible for the flow of electric current and thus only those substances which ionize in solution will be conductors of electricity. 

5. The properties of an electrolyte in solution are due to the ions that it gives in the solution. 

6. Each ion that is formed as a result of ionization produces the same effect on osmotic pressure and other colligative properties as an unionized molecule does. In other words, each ion behaves osmotically as a molecule. 

7. At moderate concentration only a fraction of the total number of molecules ionizes. The fraction of the total number of molecules present as free ions in the solution is known as degree of ionization (α). The degree of ionization (α) is given by the relation 

α = (Number of molecules ionized into ions ) / (Total number of molecules dissolved)

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