Why the equilibrium is established only when the vessel is closed?

Equilibrium is established in a closed vessel because it prevents the exchange of matter with the surroundings, allowing for the reaction components to reach a stable state. In a closed system, the concentrations of reactants and products can change until they reach a point where their rates of formation are equal, maintaining constant concentrations over time. In contrast, an open system allows for the continuous addition or removal of substances, thus preventing the establishment of true equilibrium.