How can you tell if a system is not at equilibrium?

To determine if a system is not at equilibrium, one key indicator is the comparison of the calculated equilibrium constant (Kc) with the provided Kc value. If these two values differ, it suggests that the ratio of the concentrations of products to reactants does not reflect the conditions of equilibrium for that reaction at the given temperature. Hence, the system is not at equilibrium.

For a system at equilibrium, the forward and reverse reactions occur at the same rate, and the concentration ratio represented by Kc remains constant. A discrepancy in the calculated Kc indicates that the system may still be shifting towards equilibrium, or that it has already moved away from the equilibrium conditions.

In contrast, temperature changes and the mixing of reactants do not directly inform you about the equilibrium status; they may influence the position of equilibrium but do not provide definitive evidence of whether a system has reached that state. Also, solids do not affect the equilibrium expression, as their concentrations remain constant and do not factor into the Kc expression. Thus, the reaction can still be in equilibrium despite the presence of solids.