In a reversible reaction when the rate of forward reaction becomes equal to the rate of reverse reaction then it is called chemical equilibrium.
Rf = Rr
In reversible reaction at start the concentration of reactants is maximum while the concentration of products are minimum therefore the rate or forward reaction Rf is maximum and the rate of reverse reaction Rr is minimum. As time passes the concentration of reactant decreases with decrease in rate of forward reaction while the concentration of product increases with increase in rate of reverse reaction.
After certain period of time both rates becomes equal to each other to establish equilibrium.
Chemical equilibrium is dynamic in nature. It means reaction does not stop and proceeded in both the directions with a same rate but no change is occurred in the concentration of reactants and products.
The concentration of a substance in a reaction that is measured in mol/dm is called active mass of that substance, denoted by [ ].
It is the part of total concentration on which the rate of reaction depends.