In this article we shall study the application of Le-Chatelier’s principle in Haber’s process, contact proces, etc.

Statement of Le-Chatelier’s Principle:

This principle is given by, a French chemist Le-Chatelier in 1888. It states that “If an external stress is applied to a reacting system at equilibrium, the system will adjust itself in such a way that the effect of the stress is nullified”.

Application of Le-Chatelier’s Principle to Haber’s process (Synthesis of Ammonia):

Ammonia is manufactured by using Haber’s process. In this reaction Nitrogen and Hydrogen in ratio 1:3 by volume are made to react at 773 K and 200 atm. Pressure.

The chemical reaction is

N_2(g)  +   3H_2(g)   ⇌   2 NH_3(g)  + 96.3 kJ

1 Vol         3 Vol              2 Vol

4 Vol                2 Vol

From this reaction it is clear that, the reaction is exothermic and accompanied by the decrease in volume.

Effect of Concentration:

By the law of mass action; the increase in concentration of one of the reactant will shift equilibrium towards right.  And here increase in concentration of hydrogen (as more moles of it are used) in preference to Nitrogen has more effect.

Effect of Pressure:

Above reaction indicate that formation of ammonia takes place with decrease in volume.  Hence increase in pressure will favour forward reaction. Optimum pressure for maximum yield of ammonia is about 200 atm.

Effect of Temperature:

The reaction is exothermic, so lowering the temperature will favour forward reaction. But decrease in temperature results in the decrease in the rate of reaction. Hence temperature of 773K is maintained and iron is used as catalyst.

Application of Le-Chatelier’s Principle to Contact process (Synthesis of Sulphur Trioxide):

H₂SO₄ is manufactured by contact process. In this reaction SO₂ is oxidized to SO₃. Sulphur trioxide is further used for manufacturing of sulphuric acid.

2SO_2(g) + O_2(g)   ⇌   2 SO_3(g) +  189 kJ

2 Vol           1 Vol          2 Vol

From this reaction it is clear that, the reaction is exothermic and accompanied by the decrease in volume.

Effect of Concentration:

By the law of mass action; increase in concentration of one of the reactant will shift equilibrium towards right.  And here increase in concentration of sulphur dioxide in preference to oxygen has more effect.

Effect of Pressure:

Above reaction indicate that formation of sulphur trioxide takes place with decrease in volume.  Hence increase in pressure will favour forward reaction. Optimum pressure for maximum yield of sulphur trioxide is about 1.5 atm to 1.7 atm

Effect of Temperature:

The reaction is exothermic, so lowering the temperature will favour forward reaction. But decrease in temperature results in the decrease in the rate of reaction. Hence temperature of 723 K is maintained and vanadium pentoxide is used as catalyst.

Application of Le-Chatelier’s Principle to  Manufacture of Ozone:

Ozone is manufactured by passing silent electric discharge through pure oxygen.

3O_2(g)   ⇌  2 O_3(g) –  288.56 kJ

3 Vol               2 Vol

From this reaction it is clear that, the reaction is endothermic and accompanied by the decrease in volume.

Effect of Concentration:

By the law of mass action; increase in concentration of one of the reactant will shift equilibrium towards right.  And here increase in concentration of oxygen increases the rate of forward reaction.

Effect of Pressure:

Above reaction indicate that formation of ozone takes place with decrease in volume.  Hence increase in pressure will favour forward reaction.

Effect of Temperature:

The reaction is endothermic, so increasing the temperature will favour forward reaction. Due to increase in temperature heat will be absorbed by the reaction.

Application of Le-Chatelier’s Principle to  Manufacture of Nitric oxide:

The reaction is

N_2(g) +  O_2(g)   ⇌  2 NO_(g) –  181 kJ

1Vol  + 1 Vol        →       2 Vol

From this reaction it is clear that, the reaction is endothermic and accompanied by no change in volume.

Effect of Concentration:

By the law of mass action; increase in concentration of one of the reactant will shift equilibrium towards right.  And here increase in concentration of nitrogen or oxygen increases the rate of forward reaction.

Effect of Pressure:

Above reaction indicate that formation of nitric oxide takes place with no change in volume.  Hence pressure has no effect on the equilibrium.

Effect of Temperature:

The reaction is endothermic, so increasing the temperature will favour forward reaction. Due to increase in temperature heat will be absorbed by the reaction.

Application of Le-Chatelier’s Principle to Manufacture of Nitrogen dioxide:

The reaction is

2NO_(g) +  O_2(g)   ⇌  2 NO_2(g) +  116.4 kJ

2Vol  + 1 Vol        →       2 Vol

From this reaction it is clear that, the reaction is exothermic and accompanied by decrease in volume.

Effect of Concentration:

By the law of mass action; increase in concentration of one of the reactant will shift equilibrium towards right.  And here increase in concentration of nitrogen or oxygen increases the rate of forward reaction. Due to use of more number of moles, the increase in concentration of nitrogen oxide has a prominent effect.

Effect of Pressure:

Above reaction indicate that formation of nitrogen dioxide takes place with decrease in volume.  Hence increase in pressure favour forward reaction.

Effect of Temperature:

The reaction is exothermic, so decreasing the temperature will favour forward reaction. Due to the decrease in temperature heat will be removed from the reaction.

Application of Le-Chatelier’s Principle to Dissociation of Phosphorous Pentachloride:

The reaction is

PCL_5(g)   ⇌  PCl_3(g)  +  Cl_2(g)    –  62.8kJ

1Vol  → 1 Vol      +    1 Vol

From this reaction it is clear that, the reaction is endothermic and accompanied by increase in volume.

Effect of Concentration:

By the law of mass action; increase in concentration of one of the reactant will shift equilibrium towards right.  And here increase in concentration of phosphorous pentachloride increases the rate of forward reaction.

Effect of Pressure:

Above reaction indicate that formation of nitrogen dioxide takes place with increase in volume.  Hence decrease in pressure favour forward reaction.

Effect of temperature:

The reaction is endothermic, so increasing the temperature will favour forward reaction. Due to the increase in temperature heat will be absorbed by the reaction.

Applications of Le-Chatelier’s Principle to Physical Equilibria:

Melting of Ice:

The reaction is

_Ice(s)   ⇌  Water_(l)     –  6.01 kJ

more vol  → less vol

From this reaction, it is clear that the reaction is endothermic and accompanied by decrease in volume.

Effect of Pressure:

Above reaction indicate that formation of liquid water takes place with decrease in volume.  Hence increase in pressure favour forward reaction.

Effect of Temperature:

The reaction is endothermic, so increasing the temperature will favour forward reaction. Due to the increase in temperature heat will be absorbed by the reaction.

Boiling of Water:

The reaction is

_Water(l)   ⇌  Water vapours_(g)     –  40.84 kJ

less vol  → more vol

From this reaction, it is clear that the reaction is endothermic and accompanied by increase in volume.

Effect of Pressure:

Above reaction indicate that formation of liquid water takes place with increase in volume.  Hence increase in pressure favour forward reaction. (Principle of pressure cooker).

Effect of Temperature:

The reaction is endothermic, so increasing the temperature will favour forward reaction. Due to the increase in temperature heat will be absorbed by the reaction.

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