When a reversible reaction at equilibrium has additional product added, in which direction does the equilibrium shift to counteract the change?

• A. No shift
• B. It speeds up equally in both directions
• C. Shift to the left
• D. Shift to the right

Answer: (C)

When a reversible reaction is at equilibrium, the forward and reverse rates balance. If extra product is added, the product concentration rises and the system is momentarily pushed away from equilibrium. To counteract this change, the rates adjust so the reverse reaction becomes more favorable, converting some of the added product back into reactants. The overall effect is a shift toward the reactants, i.e., to the left. Le Châtelier’s principle describes this as the system responding to the disturbance by re-establishing equilibrium with the side that consumes the added substance.