Bimolecular and Termolecular Reactions

An elementary reaction may also involve three particles colliding in a termolecular reaction. Termolecular elementary steps are rare, because it is unlikely that three particles will collide all at once. Tbink of it tbis way. You bave probably bumped into someone accidentally, many times, on the street or in a crowded hallway. How many times, however, have you and two other people collided at exactly the same time Figure 6.17 models unimolecular, bimolecular, and termolecular reactions. 

Again, the molecularity of a reaction is always an integer and only applies to elementary reactions. Such is not always the case for the order of a reaction. The distinction between molecularity and order can also be stated as follows molecularity is the theoretical description of an elementary process reaction order refers to the entire empirically derived rate expression (which is a set of elementary reactions) for the complete reaction. Usually a bimolecular reaction is second order however, the converse need not always be true. Thus, unimolecular, bimolecular, and termolecular reactions refer to elementary reactions involving one, two, or three entities that combine to form an activated complex. 

In most practical situations, however, the concentration of at least one of the other reactants is not constant but changes with time due to reactions, fresh injections of pollutants, and so on. As a result, using half-lives (or lifetimes) of a pollutant with respect to second- or third-order reactions is an approximation that involves assumed constant concentrations of the other reactants. These half-lives for bimolecular and termolecular reactions are thus directly affected by the concentrations of the other reactant. 

The first studies of the kinetics of the NO-F2 reaction were reported by Johnston and Herschbach229 at the 1954 American Chemical Society (ACS) meeting. Rapp and Johnston355 examined the reaction by Polanyi s dilute diffusion flame technique. They found the free-radical mechanism, reactions (4)-(7), predominated assuming reaction (4) to be rate determining, they found logfc4 = 8.78 — 1.5/0. From semi-quantitative estimates of the emission intensity, they estimated 6//t7[M] to be 10-5 with [M] = [N2] = 10 4M. Using the method of Herschbach, Johnston, and Rapp,200 they calculated the preexponential factors for the bimolecular and termolecular reactions with activated complexes... 

This problem was resolved in 1922 when Lindemann and Christiansen proposed their hypothesis of time lags, and this mechanistic framework has been used in all the more sophisticated unimolecular theories. It is also common to the theoretical framework of bimolecular and termolecular reactions. The crucial argument is that molecules which are activated and have acquired the necessary critical minimum energy do not have to react immediately they receive this energy by collision. There is sufficient time after the final activating collision for the molecule to lose its critical energy by being deactivated in another collision, or to react in a unimolecular step. 

Because the general principles of chemical kinetics apply to enzyme-catalyzed reactions, a brief discussion of basic chemical kinetics is useful at this point. Chemical reactions may be classified on the basis of the number of molecules that react to form the products. Monomolecular, bimolecular, and termolecular reactions are reactions involving one, two, or three molecules, respectively. 

Johnston et al also calculated the pre-exponential factors for the bimolecular and termolecular reactions, assuming the transition states to be represented by... 

Theoretical analysis of models of transition states in bimolecular and termolecular reactions of NO with F2 indicates that the termolecular rate would be eight orders of magnitude lower than the observed rate even if the activation energy of the termolecular reaction were zero. The bimolecular model yields a pre-exponential factor which is in agreement with the experimental results. Therefore, the reaction between NO and F2 is bimolecular even at dry-ice temperatures. 

If the hydrogen-iodine and the nitric oxide oxidation reactions did indeed result simply from the collision of the molecular species named in the overall stoichiometric equations, they would be bimolecular and termolecular reactions, respectively. There is considerable doubt, though, about whether this actually occurs. 

Bader s paper dealt primarily with the modes of decomposition of activated complexes, a special kind of unimolecular reaction. Several early papers by Salem 9) and Pearson also dealt with unimolecular reactions. Recently several papers have discussed bimolecular and termolecular reactions (Pearson ii)). Before presenting the theory in a formal sense. 

The closeness of fit may be gauged from the experimental and theoretical rate vs. concentration curves for hydrolysis of p-nitrophenyl carboxylates catalysed by quaternary ammonium surfactant micelles (Figure 3). The shape of the curve is satisfactorily explained for unimolecular, bimolecular, and termolecular reactions. An alternative speculative model is effectively superseded by this work. Romsted s approach has been extended in a set of model calculations relating to salt and buffer effects on ion-binding, acid-dissociation equilibria, reactions of weakly basic nucleophiles, first-order reactions of ionic substrates in micelles, and second-order reactions of ionic nucleophiles with neutral substrates. In like manner the reaction between hydroxide ion and p-nitrophenyl acetate has been quantitatively analysed for unbuffered cetyltrimethylammonium bromide solutions. This permits the derivation of a mieellar rate constant km = 6-5 m s compared to the bulk rate constant of kaq =10.9m s . The equilibrium constant for ion-exchange at the surface of the micelle Xm(Br was estimated as 40 10. The... 

Chemical reactions are, in general, complex combinations of simple elementary reactions. The molecular number of reactants of the elementary reaction, which are given as the l.h.s. terms of a reaction formula is one, two and three, and these are called unimolecular, bimolecular and termolecular reactions, respectively. The order of reaction is one, two and three corresponding to each case. It is not always true that the reaction of order one is the unimolecular reaction, and this is not always true similarly for other cases of the reaction order. 

The self-reaction of HO2 is known to proceed both via bimolecular and termolecular reaction in parallel (Kircher and Sander 1984 Kurylo et al. 1986 Takacs and Howard 1986 Lightfoot et al. 1988),... 

Chemical reactions include unimolecular, bimolecular and termolecular reactions. Collisions involving four molecules are improbable and even ter-nuclear processes are rare. 

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