Lecture Notes­ – Chapter 18.1     Std 9b & 9c


The Equilibrium Condition  -                           Std – 9b

          In general, equilibrium simply means that any two processes are equally balanced.    (see illustrations on page 516).  These could be evaporation and condensation of liquids (like in a terrarium), or chemical reactions like the the reaction


                   NO2   +  NO2    --->   N2O4                       

                   reddish-brown gas                   colorless gas 

          check this out on the web:

                   Demonstration of LeChâtelier’s Principle using nitrogen dioxide


Chemical Equilibrium – Dynamic Condition    Std – 9b   

          There are 3 conditions that we use in working with K:  (1)  concentrations of products or reactants;  (2)  pressure;  and  (3)  temperature.   Even if all conditions are constant, products and reactants are constantly moving back and forth.  If conditions don’t change and we are at equilibrium, we mean that the NET CONCENTRATIONS of products and reactants remain constant.  The back and forth from products to reactants is equal.  If conditions do change, concentrations of products and reactants will change.

          Example 1 – In a beehive, you have the same number of bees, but some are outside and some are inside.  Initially the number of bees inside and outside don’t change.  But at different times of day, different temperatures, or if the beehive is disturbed by a bear à  equilibrium changes

          Example 2 – 2 islands connected by bridge – if cars going each way across the bridge are the same, you have equal numbers of cars on each side at any given time.


Equilibrium Constant – (for concentrations of GASES and solutions only)       Std – 9c

          Two factors can change equilibrium without changing the value of K:  (1) concentrations of reactants (or products);  and  (2)  pressure.  These conditions can change, but the value of the equilibrium constant K does not change.   Every reaction has a different K .   The condition that can change the value of K is temperature . 


          Here is a generalized formula that defines K for the reaction to its right:


                            [C]c [D]d                                      <---

                 K  = ----------------                    aA + bB  --->  cC  +  dD

                            [A]a [B]b



          *   [A]  means concentration of A  (A, B, C, and D are elements like He  or a compounds like  NH3 ) 


            *  Please understand that concentrations can change without affecting the value of K,  but the TEMPERATURE CANNOT CHANGE.  Check this applet: Equilib. Applet - interactive – make constant and recalculate with different concentrations (move slider). 


                   Example 1:    What is the expression for K for this reaction:


                                       N2  +  3 Cl2  --->  2 NCl3 




                             K  = --------------

                             [N2] [Cl2]3  


                   Example 2:   What is the expression for K for this reaction:


                                      2 H2  +  O2  --->  2 H2O



                             K  = --------------

                             [H2]2 [O2]