When Q is not equal to K, the reaction is not at equilibrium.

When Q is not equal to K, the reaction is not at equilibrium.

• A. Not at equilibrium
• B. At equilibrium
• C. Rate is maximum
• D. All reactants have been consumed

Answer: (A)

Understanding how Q and K relate helps explain why the system isn’t at equilibrium when Q ≠ K. Q is the reaction quotient calculated from the actual concentrations of reactants and products at any moment, while K is the fixed equilibrium constant for the same reaction at a given temperature. When Q differs from K, the system isn’t at equilibrium and the reaction will proceed in the direction that moves Q toward K. If Q is smaller than K, the forward reaction is favored until concentrations shift enough to raise Q to K; if Q is larger than K, the reverse reaction is favored until Q drops to K. At equilibrium, the forward and reverse rates are equal, so there’s no net change, but that doesn’t mean the rate is maximum. Also, achieving equilibrium doesn’t require all reactants to be consumed—both reactants and products can be present in steady amounts at equilibrium.

Understanding how Q and K relate helps explain why the system isn’t at equilibrium when Q ≠ K. Q is the reaction quotient calculated from the actual concentrations of reactants and products at any moment, while K is the fixed equilibrium constant for the same reaction at a given temperature. When Q differs from K, the system isn’t at equilibrium and the reaction will proceed in the direction that moves Q toward K. If Q is smaller than K, the forward reaction is favored until concentrations shift enough to raise Q to K; if Q is larger than K, the reverse reaction is favored until Q drops to K. At equilibrium, the forward and reverse rates are equal, so there’s no net change, but that doesn’t mean the rate is maximum. Also, achieving equilibrium doesn’t require all reactants to be consumed—both reactants and products can be present in steady amounts at equilibrium.