Ionic equillibrium

This type of equilibrium is observed in substances that undergo ionization easily, or in polar substances in which ionization can be induced. Ionic and polar substances are more easily soluble in polar solvents because of the ease of ionization taking place in the solvent medium. With the dissolution of ionic and polar substances in the solvent, these solutions become rich in mobile charge carriers (ions) and thus can conduct electricity. Substances, which are capable of conducting electricity are called as electrolytes while those substances which are non-conducting are called as non-electrolytes.

Ionization in electrolytes

Strong electrolytes are almost completely dissociated / ionized into the constituent ions in aqueous solution. Even at higher concentrations, very small amount of any strong electrolyte is present in the unionized form. Weak electrolytes are however, partially ionized and a dynamic equilibrium exist between the ionized and unionized forms.

In 1887, S.Arrhenius postulated the first theory of electrolytic ionization. The basic postulates of his theory are:
  • When an electrolyte is dissolved in water, it ionizes to produce ions in the solution. These ions are free to move throughout the bulk of the solution.
  • The total number of positive charges is equal to the total number of negative charges in any solution of any electrolyte. The ions tend to recombine to form the unionized electrolyte. Hence, a dynamic equilibrium exists between the ionized and unionized form of the electrolyte. For example, an electrolyte AB ionizes in the solution to form A+ and B-. The equilibrium established in the solution is:

The ionization constant (K) is then given by,

formula for ionization constant

  • The properties of an electrolyte in the solution are due to the ions it gives in the solution. For example, a solution of copper sulphate is blue due to the presence of Cu2+(aq) ions in it.
  • The fraction of the total number of molecules present as free ions in the solution is known as the degree of ionization (a). This is defined as,
formula for degree of ionization

The degree of ionization at any concentration (C) of AB in moles per litre is denoted as aC.

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