What is the expression for the equilibrium constant for the reaction A + B ⇌ C + D?

• A. [A][B] / ([C][D])
• B. [C][D] / ([A][B])
• C. ([A][B]) / ([C][D])
• D. [C] + [D] / ([A] + [B])

Answer: (B)

Equilibrium constants quantify the balance between products and reactants at equilibrium by taking the ratio of product activities (or concentrations) to reactant activities, each raised to its stoichiometric power. For A + B ⇌ C + D, all coefficients are 1, so the expression is K = [C][D]/([A][B]).

This means the more C and D relative to A and B, the larger the constant, indicating the reaction favors products under those conditions. The net change in moles is zero in this case, so the constant is typically dimensionless when using activities, and often unitless in ideal terms when using concentrations.

The other forms misplace terms or use sums, which do not define the equilibrium ratio. Putting reactants in the numerator would describe the inverse relationship, and adding concentrations rather than multiplying them does not reflect how the equilibrium constant is defined.