Define reaction quotient Q and how it differs from equilibrium constant Kc.

• A. Q uses current concentrations; Kc uses concentrations at equilibrium; both have the same expression but at different times, and Kc is constant at a given temperature while Q can change
• B. Q equals Kc always
• C. Q measures pressure, Kc measures concentration
• D. Q is always greater than Kc at equilibrium

Answer: (A)

Understanding how reaction quotient Q compares to the equilibrium constant Kc. Q is calculated from the current concentrations of reactants and products using the same expression as Kc, but it reflects whatever moment you measure in the reaction. The important difference is timing: Kc uses the concentrations at equilibrium and stays constant at a given temperature, while Q can change as the system proceeds toward equilibrium. At equilibrium, Q equals Kc. If the current Q is larger than Kc, the system will shift toward the reactants; if Q is smaller, it will shift toward the products.