Some chemical reactions, like phase changes, 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 the temperature and concentration of PCl5 is high. As some PCl3 and Cl2 are formed, they too collide with one another to re-form PCl5 at a relatively slower rate. Over time, the decrease in concentration of PCl5 and the increase in concentration of PCl3 and Cl2 reach a point where the forward reaction rate equals to the reverse reaction rate, resulting in no net changes in concentrations of the reactant PCl5 and that of 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.