Le Chatelier’s Principle

Brief description about Le Chatelier’s Principle.

Le Chatelier’s Principle
According to this principle, if a system at equilibrium is subjected to a
disturbance or stress, then the equilibrium shifts in the direction that tends to
nullify the effect of the disturbance or stress. Let us consider the effects of changes
in temperature, concentration and pressure, on the equilibrium reactions and the
predictions of Le Chatelier’s principle

Effect of change of concentration
Consider the following equilibrium reaction
N2(g) + O2(g) 2NO(g)

At the equilibrium conditions the reaction mixture contains both the reactant and
product molecules, that is, N2, O2 and NO molecules. The concentrations of
reactant and product molecules are constant and remain the same as long as the
equilibrium conditions are maintained the same. If a change is imposed on the
system by purposely adding NO into the reaction mixture then the product
concentration is raised. Since the system possesses equilibrium concentrations
of reactants and products, the excess amount of NO react in the reverse direction
to produce back the reactants and this results in the increase in concentrations of
N2 and O2. Similarly if the concentration of reactants such as N2 and O2 are
purposely raised when the system is already in the state of equilibrium, the excess
concentrations of N2 and O2 favour forward reaction. Concentration of NO is
raised in the reaction mixture.
In general, in a chemical equilibrium increasing the concentrations of the
reactants results in shifting the equilibrium in favour of the products while increasing
the concentrations of the products results in shifting the equilibrium in favour of
the reactants.
Effect of change of temperature
A chemical equilibrium actually involves two opposing reactions. One
favouring the formation of products and the other favouring the formation of
reactants. If the forward reaction in a chemical equilibrium is endothermic
(accompanied by absorption of heat) then the reverse reaction is exothermic
(accompanied by evolution of heat).
Let us consider the example
N2O4(g) 2NO2(g) ; ΔH = +59.0 kJ/mole
In this equilibrium, the reaction of the product formation (NO2) is endothermic
in nature and therefore, the reverse reaction of reactant formation (N2O4) should
be exothermic. If the above equilibrium reaction mixture is heated then its
temperature will be raised. According to Le Chatelier’s principle, the equilibrium
will shift in the direction which tends to undo the effect of heat. Therefore,the
equilibrium will shift towards the formation of NO2 and subsequently dissociation
of N2O4 increases. Therefore, generally, when the temperature is raised in a
chemical equilibrium, among the forward and reverse reactions, the more
endothermic reaction will be favoured. Similarly, if the temperature of the
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equilibrium is decreased i.e., cooled, then the exothermic reaction among the
forward and reverse reaction of the equilibrium will be favoured.
Effect of change of pressure

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