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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Kohlraush's Law - Application with example

This law states that, ''at infinite dilution wherein the ionisation of all electrolytes is complete, each ion migrates independently and contributes a definite value to the total equivalent conductance of the electrolyte''. Consider an electrolyte AB in aqueous solution.

KOHLRAUSH'S LAW

 

This law states that, ''at infinite dilution wherein the ionisation of all electrolytes is complete, each ion migrates independently and contributes a definite value to the total equivalent conductance of the electrolyte''. Consider an electrolyte AB in aqueous solution. It dissociates as

AmBn --- --- > mAn+   +  nBm-

Then at infinite dilution, according to Kohlrausch's law, the total equivalent conductance of the electrolyte, λ ∞ = (1/n+ ) λA+ + (1/m- ) λB-  where λ+  and λ- are the cationic and anionic equivalent conductances at infinite dilutions and n+ and m- correspond the valency of cations and anions furnished from each molecule of the electrolyte.


Application of Kohlraush's law : The important use of Kohlraush's law is to deduce the l value of the weak electrolytes correctly by arithmetically combining the l values of strong electrolyte in appropriate manner.

 

For example λ of CH3COOH which is a weak electrolyte is deduced from λ values of NaCl, HCl, and CH3COONa in such a manner that λ of CH3COOH is obtained. Sodium acetate (CH3COONa) is a strong electrolyte and it ionises to acetate (CH3COO- ) and sodium (Na+) ions at all concentrations in water. Applying Kohlraush's law,

Molar conductances at infinite dilution : (m) when an electrolyte dissociates into g+ number of cations and g- number of anions then total molar conductance at infinite dilution m of the electrolyte is given by

A γ+ B γ -  ---- >  g+ A+ + γ - B-

 

The common ion effect


 

When a soluble salt (say A+C- ) is added to a solution of another salt (A+B- ) containing a common ion (A+), the dissociation of AB is suppressed.

AB  < --- > A+ + B-

By the addition of the salt (AC), the concentration of A+ increases. Therefore, according to Le Chatelier's principle, the equilibrium will shift to the left, thereby decreasing the concentration of A+ ions or the degree of dissociation of AB will be reduced.

 

The reduction of the degree of dissociation of a salt by the addition of a common-ion is called the Common-ion effect.




Tags : 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes
Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail

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