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Chemical Equilibrium


    1.    Describe a reversible reaction.

    2.    Define chemical equilibrium and explain how it is achieved.

    3.    Explain Le Chatelier's principle.

Key Terms

    reversible reaction        chemical equilibrium        Le Chatlier's principle

Notes: (16-1)

Equilibrium is achieved when the forward rate of a reaction is equal to the reverse rate of a reaction.  This very simple principle can be observed in a closed container of liquid.  In the container the liquid has vapor pressure that is influenced by the pressure above the liquid.  In the closed container the pressure above the liquid will raise until it reaches equilibrium vapor pressure.  At that time the amount of molecules leaving the liquid is equal to the amount of molecules entering the solution.  This equilibrium reaction displays the quality of a reversible reaction.

                                water  D  water vapor

A reversible reaction is one where the reactants and products exist in a state of equilibrium.  Although we learned in stoichiometry that the amount of product can be predicted by the reactants, a reaction that goes to completion is very rare.  Most reactions exist in a state of equilibrium.

the creation of products is called the forward reaction
the creation of reactants is called the reverse reaction
in equilibrium the rates are equal (forward = reverse)

The equilibrium of a chemical reaction can be altered by changing the conditions that surround it.  Le Chatlier described this with his principle that stated:

    If a change in conditions is imposed on a system in equilibrium, the equilibrium will shift in the direction that tends to reduce that change in conditions.

Consider the following reaction:

             2 NO2 D N2O4 + i Pressure  + h temperature (nitrogen dioxide becomes dinitrogen tetroxide)

When the reaction is in equilibrium, a ratio is established between the products and the reactants.  According to Le Chatlier we can manipulate the reactions by manipulating the conditions.

removing products drives the forward reaction
removing reactants drives the reverse reaction
increasing the temperature drives the forward reaction









Last modified: April 04, 2003