A reversible reaction that takes place in a closed container will eventually reach a state called dynamic equilibrium. This is when the forward and reverse reactions occur at the same rate, so the concentrations of the reactants and products remain constant.
Dynamic equilibrium can only be reached in a closed system. In an open system, gases formed as products can escape and disrupt the equilibrium.
To understand how the forward and reverse reactions reach equilibrium, look at the diagram below.
Initially, only the reactants are present, so the forward reaction is at its highest rate. As the reaction progresses, the concentration of the reactants decreases, causing the rate of the forward reaction to also decrease. On the other hand, the concentration of the products increases, leading to an increase in the rate of the reverse reaction.
Eventually, the rates of the forward and reverse reactions become equal, and at this point, equilibrium is reached.