Why is equilibrium only achieved in closed systems?

Equilibrium is only achieved in closed systems because they allow no exchange of matter or energy with the surroundings.

In more detail, a closed system is one in which neither matter nor energy can enter or leave. This is 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. In the context of chemical reactions, achieving equilibrium means that the rates of the forward and reverse reactions are equal, so the concentrations of the reactants and products remain constant over time.

In an open system, the addition or removal of reactants, products, or energy can shift the position of the equilibrium, according to Le Chatelier's principle. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. For example, if more reactant is added to a system at equilibrium, the system will respond by producing more product to restore equilibrium. Similarly, if energy is added or removed (in the form of heat, light, etc.), the system will respond by shifting the equilibrium to either absorb the added energy or replace the lost energy.

In a closed system, however, these external influences are not possible. The system can only change due to internal processes, such as the forward and reverse reactions themselves. Therefore, once equilibrium is achieved, it will remain as long as the system stays closed. The concentrations of the reactants and products will remain constant, and the system will be in a state of dynamic equilibrium.

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