Chemical Equilibrium #3: Equilibrium Constants

In a closed chemical system at constant temperature, there's a constant (k_eq) that defines the relationships between the concentrations of products and reactants.

K_eq= (concentration of each product raised to a power of the coefficient of the corresponding product)÷ (concentration of each reactant raised to the coefficient of the corresponding reactant).

To make it more clear,

(where the lower case letters are the coefficients)

What k_eq tells:
Since k_eq is products over reactants, the larger it is, we know that the more products there are in the equilibrium (and vice versa). So a large k_eq (e.g., 1000000) means that the reaction is close to completion, and a very small k_eq (e.g., 0.00000001) means that the reaction barely occurs.