In stoichiometry calculations, we assume that reactions run to completion. However, when a chemical reaction is carried out in a closed vessel, the system achieves equilibrium . Equilibrium occurs when there is a constant ratio between the concentration of the reactants and the products.
Different reactions have different equilibriums.
Some may appear to be completely products, however, all reactions have some reactants present. A reverse reaction is when the written reaction goes from right to left instead of the forward reaction which proceeds from left to right. It is possible to write an equilibrium expression for a reaction. This can be expressed by concentrations of the products divided by the concentration of the reactants with the coefficients of each equation acting as exponents. It is important to remember that only species in either the gas or aqueous phases are included in this expression because the concentrations for liquids and solids cannot change.