Trimolecular steps

When the perturbation is small, the reaction system is always close to equilibrium. Therefore, the relaxation follows generalized first-order kinetics, even if bi- or trimolecular steps are involved (see chapter A3.41. Take, for example, the reversible bimolecular step... 

In this simplified version of the Brusselator model, the trimolecular autocatalytic step, which is a necessary condition for the existence of instabilities, is, of course, retained. However, the linear source-sink reaction steps A—>X—>E are suppressed. A continuous flow of X inside the system may still be ensured through the values maintained at the boundaries. The price of this simplification is that (36) can never lead to a homogeneous time-periodic solution. The homogeneous steady states are... 

The existence of trimolecular reactions is sometimes suggested. For example, H + OH + M —> H2O+M, where M is a third body. However, the reaction probably proceeds by a two-step mechanism, i.e., (1) H + OH — H2O, and (2) H2O + M — H2O + M, where H2O is an energy-rich water molecule with an energy that exceeds the dissociation limit, and the function of M is to take away the energy. That is, the reaction actually proceeds via bimolecular collisions. 

The net reaction is A I B -> E I F. This reaction scheme has been developed by the Brussels School of Thermodynamics, and consists of a trimolecular collision and an autocatalytic step. This reaction may take place in a well-stirred medium leading to oscillations, or the diffusions of the components A and B may be considered. In the latter case, the system may produce Turing structures. 

Here, the initial and final concentrations of S, B, D, and P are maintained constant so that only the concentrations of X and Y are the independent variables. The autocatalytic step 2X + Y = 3X involves a trimolecular reaction. The overall reaction in Eq. (a) is... 

Elementary reactions are individual reaction steps that are caused by collisions of molecules. The collision can occur in a more or less homogeneous reaction medium or at the reaction sites on a catalyst surface. Only three elementary kinetic processes exist mono-, bi-, and trimolecular processes. Of these, trimolecular processes are rarely found, because the chance of three molecules colliding at the same time is very small. Each elementary reaction consists of an activation of the reactants, followed by a transition state and decomposition of the latter into reaction products ... 

The molecularity of a step indicates how many reactant molecules participate in the step. For example, a step A— P is unimolecular, steps 2A— P and A + B — P are bimolecular. Trimolecular steps are rare, and quadrimolecular steps are unheard-of. 

A trimolecular step requires three molecules of the same or different reactants to collide simultaneously to form one or several products. If molecules were ideal billiard balls, the time of contact of two colliding partners would be infinitesimal, and so would be the probability of a third partner making contact at exactly the same time. However, molecules are somewhat soft and deformable. Moreover, bonds and bond angles are distorted in the force field of a near-by other molecule. In a collision, this happens before actual contact is made and lasts for a time after contact has been broken. Two colliding molecules thus are in states of contact and distortion for a finite time, during which a third may join in. However,... 

The dissociation of I2 in the first step is very fast, the trimolecular second step is slow. The quasi-equilibrium condition for the first step is... 

Molecularities. An equally elementary criterion is the fact that a great majority of reaction steps are uni- or bimolecular trimolecular steps are rare and slow, and steps of still higher molecularities are unheard of (see Section 2.1). A trimolecular forward or reverse step in a postulated mechanism calls for an explanation why its reactants are not consumed by bimolecular steps before they have a chance to undergo the trimolecular one (e.g., see the Example 7.8 farther below). No mechanism involving a forward or reverse step of even higher molecularity should ever be considered. 

Trimolecular steps are rare because of the large decrease in entropy associated with three molecules simultaneously assuming the proper orientation for reaction. 

Hint 2.10 Avoid formulating mechanisms involving trimolecular steps. Instead, try to break a... 

Example 2.11. Breaking a trimolecular step into several bimolecular steps. 

In addition, the consecutive oxidation of phenol to benzoquinone was regarded as a two step reaction with hydroquinone as intermediate, thus avoiding the the need to postulate formulation of a trimolecular collision. It was further assumed that hydroquinone was... 

If the catalyst used in the process is active in promoting both reaction (5) and the water gas reaction, it is perfectly possible that the proportion of reaction going according to (4) and (5) will be governed by the water-gas equilibrium. Since reaction (5) is a trimolecular reaction as written it is also quite possible that it occurs in steps represented by reactions (4) and (6) so that it is essential to employ a catalyst active in promoting the water gas reaction in order to realize the reaction at all quantitatively. 

Although the catalytic reaction nitrile — amide almost certainly proceeds in more than one step (which would be trimolecular), concentration histories in good agreement with observation were obtained with this trial network [5],... 

The C H activation processes were proposed to proceed via a trimolecular pathway, involving a linear four-centered concerted breaking of the C—H bond between two Rh(II) metalloradicals [see Fig. 59 (a)]. In principle, this reaction could also proceed via two subsequent separate steps hydrogen-atom abstraction from the hydrocarbon R3C H bond, thus generating a Rh H species and a carbon-centered radical wCRs, which is rapidly followed by capture of the thus... 

Elementary reactions are characterized by their molecularity, to be clearly distinguished from the reaction order. We distinguish uni- (or mono-), bi-, and trimolecular reactions depending on the number of particles involved in the essential step of the reaction. There is some looseness in what is to be considered essential , but in gas kinetics the definitions usually are clearcut through the number of particles involved in a reactive collision plus, perhaps, an additional convention as is customary in unimolecular reactions. 

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