This paper deals with mathematical modeling of chemical reaction systems. For the sake
of simplicity the stirred tank assumption is made which allows us to remain in the
framework of ordinary differential systems. The principles of chemical kinetics are recalled
and the equations for the evolution of concentration of the chemical species involved
in the reactions are given. The equilibrium of reversible reactions is also characterized.
Then the case where low and fast reactions coexist is specifically considered by using
asymptotic techniques to obtain limit models. Numerical methods are proposed and the
whole methodology is applied to water quality models.