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

When calculating the equilibrium constant ( K ), the expression is formulated based on the concentrations or partial pressures of the products and reactants involved in the equilibrium reaction. The general formula for the equilibrium constant is given by:

[

K = \frac{[\text{Products}]}{[\text{Reactants}]}

]

In this expression, the concentrations of the products are placed in the numerator, while the concentrations of the reactants are placed in the denominator. This reflects the ratio of the products formed to the reactants present at equilibrium, allowing for a quantitative measure of the extent of the reaction at equilibrium.

Thus, the reactants are indeed represented in the denominator of the equilibrium constant expression, which is why choosing the reactants in this context is appropriate. It is essential to remember that only gaseous and aqueous species are included in this expression, while solids and pure liquids are typically omitted, as their activities are defined as one in the equilibrium constant expression.