Upon increasing the concentration of a reactant in a reaction at equilibrium, in which direction will the system shift?

• A. Toward products
• B. Toward reactants
• C. No shift
• D. Temperature change only

Answer: (A)

Adding more of a reactant to a system at equilibrium causes the system to shift toward the products. This follows Le Châtelier’s principle: the system counteracts the disturbance by using the extra reactant to form more products. When you raise the reactant’s concentration, the forward rate increases, and as products form, their concentrations rise while the reactant’s concentration falls, until the equilibrium condition is restored at the same temperature. Since the reactant was added, the reaction quotient drops below the equilibrium constant, prompting a shift in the forward direction to reestablish balance. Hence the shift is toward the products.