Application of Le-Chatelier's principle

The Le-Chateliers principle has a great significance for the chemical, physical systems and in day to day life in a state of equilibrium.

(1) The applications to chemical equilibrium

(i) Synthesis of ammonia (Haber's process)

     (exothermic)

(a) High pressure ( Δn <0)

(b) Low temperature

(c) Excess of N₂ and H₂

(d) Removal of NH₃ favours forward reaction.

(ii) Formation of sulphur trioxide

   (exothermic)

(a) High pressure ( Δn<0)

(b) Low temperature

(c) Excess of SO₂ and O₂, favours the reaction in forward direction.

(iii) Synthesis of nitric oxide

     (endothermic )

(a) High temperature

(b) Excess of N₂ and O₂

(c) Since reaction takes place without change in volume i.e., Δn=0, pressure has no effect on equilibrium.

(iv) Formation of nitrogen dioxide

(a) High pressure

(b)Low temperature

(c) Excess of NO and O₂ favors the reaction in forward direction.

(v) Dissociation of the phosphorus pentachloride

(a) The low pressure or high volume of container, Δn>0 

(b) High temperature
(c) Excess of PCl₅

(2) The applications to physical equilibrium

(i) Melting of the ice (Ice - water system)

(In this reaction volume is decreased from the 1.09 cc to 1.01 cc per gm.)

(a) At the high temperature more water is formed as it absorbs the heat.

(b) At the high pressure more water is formed as it is accompanied by the decrease in volume.

(c) At the higher pressure, melting point of the ice is lowered, while the boiling point of water is increased.

(ii) Melting of sulphur :  

(This reaction accompanies increase in volume.)

(a) At the high temperature, more liquid sulphur is formed.
         (b) At the higher pressure, less sulphur will melt as melting increases the volume.

(c) At the higher pressure, melting point of sulphur gets increased.

(iii) The boiling of water (water- water vapour system)

(It is accompanied by the absorption of heat and increase in volume.)

(a) At the high temperature more vapours are formed.

(b) At the higher pressure, vapours will be converted into the liquid as it decreases volume.

(c) At the higher pressure, boiling point of water is increased (it is principle of the pressure cooker).

(iv) Solubility of salts: If solubility of a salt is accompanied by the absorption process of the heat, solubility of it increases with the rise in temperature; for example NH₄Cl, K₂SO₄, KNO₃ etc.

On the other hand if it is accompanied by evolution of heat, solubility decreases with increase in temperature; for example CaCl₂, Ca(OH)₂, NaOH, KOH etc.

The relation which we can found between the vapour density and the  Degree of dissociation

In the below reversible chemical equation.

Initial mol                1                     0

At equilibrium  (1-x)                 yx        x = degree of dissociation

Number of moles of A and B at equilibrium = 1-x+yx = 1+x(y-1)

If initial volume of 1 mole of A is V, then volume of equilibrium mixture of A and B is, = [1+x(y-1)]V

Molar density before dissociation, 

Molar density after dissociation process ;   ;

y is the number of moles of products from one mole of reactant. D/d is also called Van't Hoff factor.

In terms of molecular mass,    

Where M = Initial molecular mass,  

 m = molecular mass at equilibrium 

Thus for the equilibria

Also D * 2= Molecular weight (theoretical value)

D * 2 = Molecular weight (abnormal value) of the mixture.

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