Question
Fri January 25, 2013 By:
 

what is LeChatelier's Principle? explain in detail with equations

Expert Reply
Fri January 25, 2013

Le Chatelier's principle describes what happens to a system when something momentarily takes it away from equilibrium. This section focuses on three ways in which we can change the conditions of a chemical reaction at equilibrium:

(1) changing the concentration of one of the components of the reaction

(2) changing the pressure on the system

(3) changing the temperature at which the reaction is run.

Changes in the concentrations of the reactants or products of a reaction shift the position of the equilibrium, but do not change the equilibrium constant for the reaction.

Similarly, a change in the pressure on a gas-phase reaction shifts the position of the equilibrium without changing the magnitude of the equilibrium constant. Changes in the temperature of the system, however, affect the position of the equilibrium by changing the magnitude of the equilibrium constant for the reaction.

Chemical reactions either give off heat to their surroundings or absorb heat from their surroundings. If we consider heat to be one of the reactants or products of a reaction, we can understand the effect of changes in temperature on the equilibrium. Increasing the temperature of a reaction that gives off heat is the same as adding more of one of the products of the reaction. It places a stress on the reaction, which must be alleviated by converting some of the products back to reactants.

The reaction in which NO2 dimerizes to form N2O4 provides an example of the effect of changes in temperature on the equilibrium constant for a reaction. This reaction is exothermic.

2 NO2(g) N2O4(g)   Ho = -57.20 kJ  

Thus, raising the temperature of this system is equivalent to adding excess product to the system. The equilibrium constant therefore decreases with increasing temperature.

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