Equilibrium is attained exclusively in closed systems, which do not permit any exchange of matter or energy with their surroundings.

To elaborate, a closed system is defined as one in which neither matter nor energy can enter or exit. This stands in contrast to an open system, where both matter and energy can be exchanged with the surroundings, and an isolated system, where neither matter nor energy can be exchanged at all. In the realm of chemical reactions, achieving equilibrium means that the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products over time.

In an open system, the addition or removal of reactants, products, or energy can alter the position of the equilibrium, as described by Le Chatelier’s principle. This principle asserts that if a dynamic equilibrium is disturbed by a change in conditions, the equilibrium position will shift in a way that counteracts the disturbance. For instance, if additional reactant is introduced to a system at equilibrium, the system will respond by producing more product in an effort to restore equilibrium. Likewise, if energy is either added or removed (in forms such as heat or light), the system will adjust by shifting the equilibrium to either absorb the added energy or compensate for the energy that has been lost.

Conversely, in a closed system, such external influences cannot occur. Changes can only arise from internal processes, such as the forward and reverse reactions themselves. Therefore, once equilibrium is established in a closed system, it will persist as long as the system remains closed. The concentrations of the reactants and products will stabilize, placing the system in a state of dynamic equilibrium.

In summary, equilibrium is achieved solely in closed systems since these systems do not allow for the exchange of matter or energy with their surroundings. This isolation prevents external disturbances that could shift the equilibrium position, enabling the system to reach a state of dynamic equilibrium where the rates of the forward and reverse reactions are equal.