Single-step unidirectional reactions

The simplest reactions have the one-step unimolecular or bimolecular mechanisms illustrated in Table 4.1 along with their differential rate equations, i.e. the relationships between instantaneous reaction rates and concentrations of reactants. That simple unimolecular reactions are first order, and bimolecular ones second order, we take as self-evident. The integrated rate equations, which describe the concentration-time profiles for reactants, are also given in Table 4.1. In such simple reactions, the order of the reaction coincides with the molecularity and the stoichiometric coefficient. 

Reaction Molecularity rate equation rate equation 

Throughout, there is no requirement that the product be a single compound. 

Because simple rate laws follow necessarily from these simple mechanisms, a postulated simple unimolecular or bimolecular mechanism must have the corresponding rate law and this is easily tested (see Chapter 3) [9], However, observation of a first- or second-order rate law is not sufficient evidence that the mechanism is unimolecular or bimolecular, respectively. 

---

In SSITKA, it is assumed that the catalyst surface is a system made up of a number of interconnected compartments. Each compartment is a homogeneous or well-mixed system like a continuous stirred tank reactor (CSTR). A separate compartment is assumed to be present for each uniquely adsorbed reaction intermediate species. Based on this assumption, the response of a single unidirectional reaction step to a step change in the isotopic concentration of one of the reactants, which is made at time t = 0, is given as... 

---