Which statement about the equilibrium constant K is correct?

Which statement about the equilibrium constant K is correct?

• A. Expresses the ratio of product to reactant concentrations at equilibrium.
• B. Represents the ratio of products to reactants at any time during the reaction.
• C. Equals the sum of product and reactant concentrations at equilibrium.
• D. Is always equal to 1 for any reaction.

Answer: (A)

At equilibrium the forward and reverse reactions occur at the same rate, so the system’s composition stops changing. The equilibrium constant captures that balance by forming a ratio of product concentrations to reactant concentrations (each raised to its stoichiometric power) evaluated at that temperature. For a general reaction aA + bB ⇌ cC + dD, this ratio is Kc = [C]^c [D]^d / [A]^a [B]^b. This is the quantity described by the statement: it expresses how much of the products there are relative to the reactants once equilibrium is reached. It isn’t the ratio during the reaction, because concentrations are continually changing as the reaction proceeds toward equilibrium. It isn’t a sum of concentrations, since K is a ratio, not a total. And it isn’t always 1—only for special cases where the system’s balance between products and reactants happens to be equal at that temperature. Remember that K is specific to a particular temperature, so changing the temperature changes K by altering the thermodynamics of the reaction.

At equilibrium the forward and reverse reactions occur at the same rate, so the system’s composition stops changing. The equilibrium constant captures that balance by forming a ratio of product concentrations to reactant concentrations (each raised to its stoichiometric power) evaluated at that temperature. For a general reaction aA + bB ⇌ cC + dD, this ratio is Kc = [C]^c [D]^d / [A]^a [B]^b. This is the quantity described by the statement: it expresses how much of the products there are relative to the reactants once equilibrium is reached.

It isn’t the ratio during the reaction, because concentrations are continually changing as the reaction proceeds toward equilibrium. It isn’t a sum of concentrations, since K is a ratio, not a total. And it isn’t always 1—only for special cases where the system’s balance between products and reactants happens to be equal at that temperature. Remember that K is specific to a particular temperature, so changing the temperature changes K by altering the thermodynamics of the reaction.