General points on process kinetics

Eveiy charge or mass transfer we have mentioned previously lepiesenls one or many steps of a more or less complex process. 

Modeling, however, is the anticipation, interpretation, description and then simulation of the course of these processes over time. This is all carried out using the information we have about the system being studied information about the nature and concentration of the present species, the nature and location of the phases, parameters such as temperature and partial gas pressure. Generally, the model of a process can also be expressed by a number of equations that relate these parameters to each other over time. To obtain these relations, the entire process has to be clearly described. If the simirlation and experimental results are compatible, the model will be validated, allowing us to interpret the observed phenomena. 

Although thermodynamics makes it possible to relate the concentration of chemical species in the inflow to the concentration in the outflow, which corresponds to the initial and final states, it does not allow us to take into account the intermediate steps that form the links in the chain and constitute the process being studied. These intermediate states, which have to be considered when modeling a process, cannot be observed. They can, however, be imagined or guessed at using experimental results, then be validated with kinetic-type irtformatioa 

Each step of the process will have to be identified with an elementaiy step that consumes and/or produces one or multiple intermediate compounds. 

Consequently, the expression for the rate of each step generally obeys van t Hoffs law. This rate, as with adsorption, is the product of a kinetic constant k and the concentration of reactant species, the respective order of each species being equal to its stoichiometric coefficient In fact, the global rate V of the reaction 

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