The general procedure for modeling chemical reactions is as follows:
Create a new task of the appropriate type.
Define the species, as described in Defining Chemical Species.
Define the reactions, as described in Defining Chemical Reactions.
Define a sub-task to solve the transport equation for each species, as described in Defining the Species Transport Equations and Defining a Closure Equation.
See also Chemical Reactions and Evolution for information about using evolution for a chemical reaction calculation.