Final answer:
Concentrations of a mixture at equilibrium are constant because the rates of the forward and reverse reactions are equal, leading to no net change in reactants and products over time. The equilibrium constant (K) and the reaction quotient (Qc) both become stable values that characterize the system's equilibrium. Any disturbance to this equilibrium initiates a shift to readjust to a new equilibrium position according to Le Chatelier's Principle.
Step-by-step explanation:
The concentrations of a mixture at equilibrium are constant as a function of time because the rates of the forward and reverse reactions are equal. When the system reaches equilibrium, it means that the amounts of reactants and products are stable, and there is no net change in their concentrations over time. This can be observed in a chemical reaction such as the dimerization of NO₂ to N₂O₄, where NO₂ is being formed at the same rate that N₂O₄ is breaking down at a constant temperature, achieving a static, balanced state represented by a constant equilibrium constant (K).
For example, the equilibrium of the reaction CO₂(g) + C(s) → 2 CO(g) used in pottery glazes has concentrations that will not change after reaching equilibrium because the forward and backward reaction rates are equivalent. Additionally, the reaction quotient (Qc) becomes constant at equilibrium, meaning that the product of the concentrations of the products raised to the power of their coefficients divided by the product of the concentrations of the reactants raised to the power of their coefficients does not change anymore.
If the equilibrium is disturbed by altering the concentration of any reactant or product, the system will adjust to maintain equilibrium according to Le Chatelier's Principle, which means the reaction will shift in a direction that either consumes or releases substances to reestablish the equilibrium condition.