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Introduction to Equilibrium Expression

The chemical equilibrium is the state in a reversible reaction which is characterized by the constancy of the some features like density, concentration, pressure and temperature. So the concentration of the product and reactant do not change with time. it is a dynamic process not a static one that is in a chemical equilibrium the product are constantly changing in t the reactant and the reactant are changing in to the product with the same rate.

Equilibrium Expression

An equilibrium expression is the expression representing as the quotient taking numerator as the product of active masses of the product raised to the power in which they appear in the stoichiometric equation and the denominator and the product of molar mass or active mass of reactants raised to the power in which they appear in the stoichiometric equation following example will clear the concept. Let a reversible reaction as

                                            aA+Bb ═ cC+dD

                     The equilibrium expression for above reaction can be written as       

                                    [C]c [D]d

                                 KC =   -------------------               where KC= equilibrium constant

                                               [A]a [B]b

       Equilibrium expression are worked out for some equilibrium                       

                                CaCO3= CaO+ CO2

                                       (S)                 (S)             (g)

                             [CO2][CaO] 

                          KC =    -------------------               where KC= equilibrium constant

                                        [CaCO3]

If reaction involves the dissociation of substance in to ions the equilibrium constant is called the ionic constant and the equilibrium expression is represented as:                              

                               CH3COOH  = CH3COO-+ H+                                                       

                                         [CH3COO-][H+]

                        KC =    -------------------------------               where KC= ionic constant

                                          [CH3COOH]  

If the reaction involves only gaseous products and the reactant then the equilibrium expression can also be expressed in terms of the partial; pressure of the gas

                                   N2+3H2═ 2NH3

                                   (g)   (g)      (g)

                                                      [NH3]2

                                                      KC =    -------------------

                                                                     [N2] [H2]3

Since the above reaction involves all the gaseous products the expression for equilibrium can also be expressed in terms of partial pressure of the gases as                      

                      [PNH3]2

        KP =    -------------------                  Kp= Equilibrium constant in terms of partial pressures

                          [PN2] [PH2]3