Which statement best describes the expression for the equilibrium constant Keq for a general reaction?

Which statement best describes the expression for the equilibrium constant Keq for a general reaction?

• A. Keq is the difference in energy between products and reactants.
• B. Keq is the ratio of product activities to reactant activities, each raised to their stoichiometric coefficients.
• C. Keq is the sum of product concentrations.
• D. Keq is the ratio of reactant concentrations to product concentrations.

Answer: (B)

At equilibrium, the quantities that define the system are the activities of the species, and the equilibrium constant is given by the ratio of product activities to reactant activities, each raised to its stoichiometric coefficient. For a general reaction aA + bB ⇌ cC + dD, the expression is Keq = (a_C^c a_D^d) / (a_A^a a_B^b). In solutions where activity is well approximated by concentration, this becomes Keq ≈ [C]^c [D]^d / [A]^a [B]^b. Pure solids and liquids have activity ≈ 1 and do not appear in the expression. Keq is constant at a given temperature (though it changes with temperature). The other ideas don’t describe the equilibrium constant properly: an energy difference relates to ΔG° and thermodynamics rather than the ratio of activities; summing product concentrations isn’t how Keq is defined; and reversing the ratio would change the value of Keq, which shows the need for products in the numerator and reactants in the denominator with the appropriate exponents.

At equilibrium, the quantities that define the system are the activities of the species, and the equilibrium constant is given by the ratio of product activities to reactant activities, each raised to its stoichiometric coefficient. For a general reaction aA + bB ⇌ cC + dD, the expression is Keq = (a_C^c a_D^d) / (a_A^a a_B^b). In solutions where activity is well approximated by concentration, this becomes Keq ≈ [C]^c [D]^d / [A]^a [B]^b. Pure solids and liquids have activity ≈ 1 and do not appear in the expression. Keq is constant at a given temperature (though it changes with temperature).

The other ideas don’t describe the equilibrium constant properly: an energy difference relates to ΔG° and thermodynamics rather than the ratio of activities; summing product concentrations isn’t how Keq is defined; and reversing the ratio would change the value of Keq, which shows the need for products in the numerator and reactants in the denominator with the appropriate exponents.