The Concept of Chemical Equilibrium

Chemical reactions are everywhere.  Some go in one direction only, the forward reaction and some go in two directions, the forward and backward reaction.  The latter is called reversible reaction and it is indicated by a double arrow.  Example, when a glass of water with cover is placed under the sun, it undergoes reversible reaction, the vaporization and condensation processes.  When the rate of vaporization process equalizes with the rate of condensation process, the equilibrium is reached.

Chemical equilibrium is a dynamic process.  It occurs when the rate of forward reaction and the backward reaction are equal and the concentration of the reactants and products no longer change with time.  Chemical equilibrium reaction involves different substances as reactants and products. Equilibrium involving two phases of the same substance just like the example given above, the vaporization and condensation processes is an example of physical equilibrium, because the changes that occurs are only physical processes.

Example of chemical equilibrium is the reversible reaction of nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4) 

The progress of reaction is easily monitored because there is a corresponding change in the color of the gases, N2O4 is a colorless gas while NO2 has dark brown color.  Let us say for example, an evacuated glass is injected with known amount of N2O4, and suddenly the color changes to dark brown, it means there is already a formation of NO2 gas.  The color intensifies until such time that equilibrium is reached.  Equilibrium is reached when there is no changed in color observed inside the evacuated glass