From a known set of reaction mechanisms and the reaction steps involved, the reaction rate of each participating specimen (molecule, free atom, ion or radical) can, by kinetic reaction rate theory, be expressed as a function of the concentrations and the rate constants of each reaction step. The reaction rate functions for all the species constitute a differential equation system, the solution of which gives the time evolution of the amounts of the species. The kinetic model, i.e. the set of reaction mechanism, used was developed by Konnov and de Ruyck for ammonia decomposition in shock waves. The model contained 51 reversible reactions and 11 species, 10 of which could directly participate in the decomposition of ammonia together with argon that could act as an inert third-body reactant, Table . The forward reaction rate was calculated from an Arrhenius like equation using the parameters specified in the model. The reverse reaction rate was calculated from thermodynamic balance using thermodynamic data obtained from the Chemkin thermodynamic database. All kinetic modelling was preformed using the Reaction engineering module from the COMSOL Multiphysics® simulation software. 22 is given by