Which of the following represents a system at equilibrium?

A system is considered to be at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products over time. The equilibrium constant, Kc, is a measure of the ratio of the concentrations of the products to the concentrations of the reactants at equilibrium.

When Kc equals the calculated Kc, it indicates that the system has reached a state where the concentrations of reactants and products remain constant, aligning with the definition of equilibrium. For a reaction at equilibrium, this calculated Kc value will not change unless the conditions (such as temperature) alter the system. Hence, this relationship is a clear indicator of a system at equilibrium.

In contrast, the other choices describe scenarios that do not necessarily represent an equilibrium state. For instance, Kc being greater than 1 indicates that products are favored over reactants at equilibrium but does not by itself confirm that the system is at equilibrium unless compared to the calculated Kc. Similarly, Kc being less than 1 suggests that reactants are favored, but, again, this doesn't confirm that the system is at equilibrium without the context of it matching a calculated value. Lastly, if Kc is not defined, it implies that