When Q = K, the reaction is at equilibrium.

When Q = K, the reaction is at equilibrium.

• A. The reaction is not at equilibrium
• B. The reaction is at equilibrium
• C. The concentrations of all species are equal
• D. The rate of forward equals rate of reverse

Answer: (B)

Q is the reaction quotient, built from the current concentrations of products and reactants raised to their stoichiometric powers. K is the equilibrium constant for the reaction at a given temperature. When Q equals K, the system has reached the exact product-to-reactant ratio that defines equilibrium, so there’s no net drive for the reaction in either direction. That’s why the reaction is at equilibrium. At this point the forward and reverse rates are equal, so concentrations stop changing. Remember, equality of concentrations for all species isn’t required at equilibrium—the ratio matters, not every concentration being the same.

Q is the reaction quotient, built from the current concentrations of products and reactants raised to their stoichiometric powers. K is the equilibrium constant for the reaction at a given temperature. When Q equals K, the system has reached the exact product-to-reactant ratio that defines equilibrium, so there’s no net drive for the reaction in either direction. That’s why the reaction is at equilibrium. At this point the forward and reverse rates are equal, so concentrations stop changing. Remember, equality of concentrations for all species isn’t required at equilibrium—the ratio matters, not every concentration being the same.