How is the equilibrium constant (K_eq) calculated?

• A. K_eq = [reactants]^[coefficients] / [products]^[coefficients]
• B. K_eq = [products]^[coefficients] / [reactants]^[coefficients]
• C. K_eq = [products] + [reactants]
• D. K_eq = [products] - [reactants]

Answer: (B)

The equilibrium constant, denoted as K_eq, is calculated based on the concentrations of the products and reactants involved in a chemical reaction at equilibrium. Specifically, it is defined as the ratio of the concentrations of the products, each raised to the power of their respective coefficients in the balanced chemical equation, to the concentrations of the reactants, also raised to their respective coefficients.

In other words, the correct formula for K_eq reflects the principle that at equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. Thus, K_eq quantitatively expresses the relationship between products and reactants.

When the reaction is written in the form:

aA + bB ⇌ cC + dD,

K_eq is calculated as:

K_eq = [C]^c * [D]^d / [A]^a * [B]^b.

This correct approach illustrates that the equilibrium constant provides a measure of the tendency of a reaction to favor products over reactants at equilibrium. The other choices do not reflect the proper mathematical relationship of the concentrations according to the balanced equation, either by misplacing reactants and products or by suggesting addition or subtraction, which is not applicable in the context of equilibrium constants.