When calculating the equilibrium constant K, what is written in the numerator?

When calculating the equilibrium constant (K) for a chemical reaction, the numerator consists of the concentrations (or partial pressures) of the products. This is based on the law of mass action, which states that at equilibrium, the ratio of the product concentrations to the reactant concentrations remains constant at a given temperature. Specifically, for a generic reaction represented as:

aA + bB ⇌ cC + dD,

the equilibrium constant (K) is expressed as:

K = [C]^c [D]^d / [A]^a [B]^b,

where [C] and [D] are the molar concentrations of the products, and [A] and [B] are the molar concentrations of the reactants. The products are raised to the power corresponding to their coefficients in the balanced equation, culminating in the product concentrations forming the numerator of the K expression.

By focusing on the products in the numerator, it reflects the principle that equilibrium lies towards the formation of products once the reaction system has stabilized. Understanding this balance is crucial for predicting how changes in concentration or conditions will affect the system at equilibrium.