Chemical equilibrium in a reaction is reached when the concentrations of the reactants and products remains constant. A chemical equilibrium is dynamic, implying that the forward and reverse reactions are occurring at the same rate. Since the reverse reaction occurs simultaneously with the forward reaction, a reaction at equilibrium may have reactants remaining.
It is often helpful to have an expression describing the amount of the reactants that will form products. Such an expression can be derived from the ratio of equilibrium product concentration to equilibrium reactant concentration, each raised to the power equal to its stochiometric coefficient in the chemical equation for the reaction
This ratio is the equilibrium constant for the reaction and is a constant value at a given temperature.
