What is the primary effect of adding a catalyst to a reaction mixture at equilibrium?

• A. Shifts the equilibrium toward products
• B. Speeds up attainment of equilibrium but does not change the position
• C. Changes the rate of the equilibrium constant
• D. Decreases the rate of the reverse reaction

Answer: (B)

When a catalyst is added to a reaction at equilibrium, it lowers the activation energy barrier for both the forward and reverse reactions. This speeds up both directions equally, so the system reaches equilibrium faster. However, the position of equilibrium—the ratio of products to reactants at that temperature—remains the same because the equilibrium constant is determined by thermodynamics and is not affected by a catalyst. Temperature is the factor that would change K. So the catalyst’s main effect is to speed up attainment of equilibrium without changing the final composition. That means it does not shift toward products, it does not change the equilibrium constant, and it does not decrease the reverse rate (it increases both forward and reverse rates).