Theory of Ionisation





Arrhenius (1887) put forward a theory to explain the behaviour of electrolytes in solution. This theory is known as Theory of ionisation or Theory of electrolytic dissociation.

According to this theory:

(i)    When an electrolyte (acid, base or salt) is dissolved in water, it breaks up or dissociates into two types of particles, one carrying positive charge while one carrying negative but equal charge. These charged particles are called ions. The particles carrying positive charge are called cations and those carrying negative charge are called anions.

AB \to A^+ + B^-

 

(ii)     The ions present in solution are constantly re-combing to form neutral molecules which again dissociate. Thus there is a state of equilibrium set up between the ionised and unionised molecules. Thus, AB \leftrightharpoons A^+ + B^-

Applying the law of mass action to the ionic equilibrium,

\dfrac{[A^+][B^-]}{[AB]} = K (Ionisation constant)

 

(iii)     The extent to which an electrolyte dissociates into ions is known as degree of dissociation or ionisation (\alpha) and depends upon the following factors:

  •  Nature of the electrolyte: Strong acids, bases and their salts are almost completely ionised, while the weak acids, bases and their salts are feebly ionised.
  •    Temperature: Ionisation increases with the increase of temperature.  \alpha \propto T
  •     Dilution: Ionisation also increases with the dilution at infinite dilution \alpha is maximum.
  •  Nature of the solvent: It also affects ionisation to a marked degree. It cuts the lines of forces binding the two ions and separates them in solution. This effect of the solvent in measured by its dielectric constant. The dielectric constant of a solvent may be defined as “Its capacity to weaken the force of attraction between the dielectrical charges immersed in that solvent”. Water having a high dielectric constant (82) is a good and strong ionising solvent.
  •    Presence of other ions: Degree of ionisation of weak electrolytes is effected by the presence of other ions   (common ion effect etc.) e.g., solubility of AgCl decreases in presence of NaCl.

 

(iv)  When an electric current is passed through the solution, negative ions (anions) move towards the anode  and positive ions (cations) move towards the cathode.

The movement of the ions is thus responsible for the conduction of electricity through solution and hence explains the phenomenon of electrolysis. The electrical conductivity of a solution depends, therefore, on its power of ionisation. Glucose, urea etc. which do not ionise are regarded as bad conductors of electricity and strong electrolytes e.g., NaCl, HNO_3 etc. which ionise almost completely are considered as good conductors.

 

(v)    The ions behave as molecules in causing osmotic pressure, lowering of vapour pressure, depression of freezing point and elevation of boiling point whereby their abnormal values can be accounted for.

 

(vi)     The properties of electrolytes in solution are the properties of the ions which they produce.



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