DEGREE OF IONISATION /DISSOCIATION (DOD)

We know that electrolytes are two types STRONG and WEAK electrolyte . Strong electrolytes are excellent conductor of electricity due to 100% ionization it means their degree of dissociation is  very high or we can say it approaches unity.

Other hand weak electrolytes are not completely ionized theirfore they are bad conductor of electricity for example CH3COOH , H3PO4 , H3BO3, NH4OH etc.

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Degree of dissociation  may be defined as “fraction of total number of molecules which dissociates into ion”. It is represented by alpha.

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The extent of dissociation of a substance is expressed in terms of degree of dissociation.

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(1) ARRHENIUS THEORY OF DISSOCIATION: the theory deals with behaviour of weak electrolyte in aqueous phase .

The theory state that when a weak electrolyte ( acid ,base and salts ) is added in water , a chemical equilibrium exist between undissociate  molecules and ions produce by dissociation.

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Where Kd is called dissociation/Ionization constant

For strong electrolyte Kd is greater than one and DOD is equal to one .

For weak electrolyte Kd is lower than one and DOD is also lower than one .

Kd is also known as 

For acids  Ka , for bases Kb and for ions it is represented as Kh.

(2) OSTWALD DILUTION LAW OF DISSOCIATION:

Ostwaled dilution law is applicable for only weak electrolyte not for strong electrolyte.

"The rule state that  at infinite dilution a weak electrolyte behave as strong electrolyte due to complete dissociation and it is called Ostwald dilution law "

Consider AB is a weak electrolyte dissociate as below 

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FACTER'S AFFECTING DOD( @)

(1) Nature of electrolyte (solute): DOD of strong electrolyte is greater than weak electrolyte.

(2) Nature of Solvent: DOD also depends upon nature solvent for example.

(i) acetic acid (CH3COOH) behave as weak acid in water (H2O)but it behave as strong acid in liquid Ammonia (NH3)

(ii) HCl is strong acid in water but weak acid in acetic acid (CH3COOH) and HF. and behave non volatile in benzene.

(iii) NaCl is strong electrolyte in water but act as weak electrolyte in liquid ammonia (NH3).

(3) Effect of temperature on DOD :

(i) for endothermic dissociation DOD increases on increasing temperature according to le chateliers principle (LCA).

(ii) for exothermic dissociation DOD  decrease on increasing temperature.

(4) Effect of concentration ( Dilution): when dilution take place concentration of all the species are decrease but here greater concentration decrease in product side so according to Le chateliers principle equilibrium shifted forward direction and finally DOD increases.

ILLUSTRATIVE EXAMPLE (1):

ILLUSTRATIVE EXAMPLE (2):

ILLUSTRATIVE EXAMPLE (3):

ILLUSTRATIVE EXAMPLE (4):

ILLUSTRATIVE EXAMPLE (5):

(5) Common ion effect on DOD: In presence of common ions the DOD of weak electrolyte decrease,this phenomenon is known as common ion effect.

For example when HCl is added to the aqueous solution of CH3COOH  then concentration of common ion (H+) increases so according to Le chateliers principle equilibrium  of weak electrolyte (CH3COOH) shift in backward direction and finally DOD of CH3COOH decrease.

(6) Opposite ion effect on DOD: In the presence of opposite ion (OH-) concentration of H+ decrease and according to Le chateliers principle equilibrium shifted in forward direction  hence DOD of weak electrolyte H2S increases , this phenomenon is known as opposite ion effect.

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