Chemical equilibrium is defined as the state where the concentrations of reactants and products remain constant with time. Since no changes occur in the concentrations of reactants or products in a system at equilibrium, it appears that the system has stopped reacting. However, on a molecular level, a dynamic situation exists where the reactants are being converted to products and the products are being converted to reactants at the same rate.
Consider a hypothetical reaction where A(g) B(g) and the reaction is first order in A. The graph of Concentration vs Time is shown below.
As the reaction proceeds, a decrease in the concentration of the reactant(s) can be measured as well as an increase in the concentration of the product(s). As shown in the graph, there is a decrease in [A] and an increase in [B]. Eventually, as indicated by the dashed vertical line, a condition of static concentrations is reached. For a system to be in equilibrium means that the concentrations of all chemical species do not change with time provided the temperature, volume, and pressure remain constant. It is important to note that the concentration of A never goes to zero and there are no further apparent changes in the system.
A dynamic equilibrium occurs because as the forward reaction proceeds the concentration of A decreases causing the forward reaction to slow down. Initially, no B exists and the reverse reaction can not occur. As the forward reaction proceeds the concentration of B increases causing the rate of the reverse reaction to increase. Once the concentrations of A and B reach levels where the forward rate equals the reverse rate the system reaches equilibrium as the graph of Rate vs Time indicates.
Once equilibrium is reached, it is maintained only if all relevant factors remain the same. A change in reactant or product concentrations, temperature, pressure, or volume will disturb the equilibrium causing the system to undergo additional net changes to establish a new equilibrium.