Chemical equilibrium occurs when the rates of the forward and reverse reactions in a dynamic system become equal, resulting in constant concentrations of reactants and products despite ongoing processes.
Some chemical reactions are reversible (i.e. the products can react to re-form the reactants), e.g.
Such reactions are denoted by the double harpoon sign , instead of the single arrow sign → for irreversible reactions.
Consider only the presence of PCl5 in a closed, heated reaction vessel at the start of the reaction. Initially, PCl5 molecules collide with each other to form PCl3 and Cl2 at a relatively high rate, as both the temperature and the concentration of PCl5 are high. As some PCl3 and Cl2 are formed, they also begin to collide and re-form PCl5, but at a relatively slower rate. Over time, the decreasing concentration of PCl5 and the increasing concentrations of PCl3 and Cl2 reach a point where the forward reaction rate equals the reverse reaction rate, resulting in no net change in the concentrations of the reactant PCl5 and the products PCl3 and Cl2. We say that the reaction has attained dynamic equilibrium when it reaches such a state. The course of the reaction is represented by the graph below:
It is important to note that a closed system is necessary for a reaction to achieve chemical equilibrium.