Termolecular rate constant

Figure 6. Plot providing data for termolecular rate constant of reaction Ar+ + 2Ar Ar2 + + Ar...

In the atmosphere, [AT] is usually assumed to be atmospheric pressure at the altitude of interest. Therefore, unlike unimolecular and bimolecular processes, termolecular processes are pressure dependent. The units for the termolecular rate constant are cm /molecule s. 

The rate constant (sometimes called the specific reaction rate) is commonly designated by k. The SI unit of time is the second (symbolized by s). Thus, unimolecular rate constants are typically expressed in s and unimolecular processes are by definition concentration-independent reactions. A slight difficulty arises regarding SI units and bi- and termolecular rate constants. Concentrations in the SI system would be mol per cubic meter, but in chemistry concentrations are expressed in mobdm (or more commonly mol-L or simply M ). Thus, a bimolecular rate constant typically has units of M s whereas a termolecular rate constant is expressed with units of... 

To obtain the termolecular rate constant AtjJ/, the effective bimolecular rate constant k = k "[M] is plotted in Fig. 5.2 as a function of total pressure (i.e., of [M]). As expected from the earlier discussion, k increases with [M] at low pressures but approaches a plateau at higher pressures. 

These workers obtained an apparent rate constant for reaction (52) of 7.2 x 102° M z sec-1 at 20°C. To explain the unusually large magnitude of this termolecular rate constant, they invoked a suggestion of Kunkel and Gardner,270 that the mechanism consists of the reaction sequence... 

Although it is evident that the reaction is almost entirely homogeneous, Stoddart403,404 presented evidence that the rate of reaction is influenced by the history of the reaction vessel it diminishes from experiment to experiment when starting with a new vessel it also diminishes with time during the course of a single experiment. However, the termolecular rate constant does not depend on the surface-to-volume ratio. He concluded that these effects are due to activated adsorption or chemisorption of nitrosyl chloride on the adsorbed moisture of the... 

The termolecular kinetics result from two equilibria starting with the amide and involving two hydroxide ions followed by a unimolecular rate-determining step, and the termolecular rate constant 3 is actually a product of the two equilibrium constants and a unimolecular rate constant k = kx Ki x K2. 

It just turns out that the termolecular rate constant ks is actually the product of an equilibrium constant Ki and a genuine bimolecular rate constant A2 such that /(3= KiX /f2. You saw a similar thing in the rate expressions for amide hydrolysis (Chapter 13) and ElcB elimination (Chapter 19, p. 000)... 

The detailed kinetics and energetics of the reactions in the rf-ion trap can be understood by considering that the total pressure inside the ion trap is on the order of 1 Pa, which means that the experiment is operating in the kinetic low-pressure regime. Therefore, a Lindemann-t3rpe mechanism has to be considered for each reaction step, and the reaction rates depend on the buffer gas pressure [187, 188]. As a consequence, the obtained pseudo first order rate constant k contains the termolecular rate constant as well as the concentrations of the helium buffer gas and of the reactants in the case of the adsorption reaction of the first CO molecule (1.1) ... 

As the experiment is operating in the kinetic low-pressure regime, the decomposition rate constant can consequently be considered to be much larger than the stabilization rate constant term k fcs[He]. This leads to a simplified expression for the termolecular rate constant, which can be applied to the experimental conditions present in the ion trap experiment... 

Two-body ion-ion recombinations have rate constants in the same general range, although few have been studied in detail, none with precise analysis of reactants and products. Some three-body ion-ion recombinations have recently been studied by Mahan and co-workers (4, 21) who found effective termolecular rate constants in the range 4 X 10"26 to 3 X 10"25 cm.6 molecule 2 sec. 1 for NO+ + N02" + M near 300°K. With an approximate T 5/2 dependence, and at a total pressure of 1 torr, such processes would have effective first-order rate constants of ion removal in the 10 to 100 sec."1 range, too slow to be of importance. 

Table A. 10 compares ranges of concentrations and bimolecular and termolecular rate constants pertinent to atmospheric chemistry for, these two units. For example, the hydroxyl (OH) radical has an average tropospheric concentration of about 8 x 105 molecule cm-3. This is equivalent to 1.3 x 10 12 mol m-3 (1.3 pmol m 3).

Termolecular rate constants ( ) limiting low-pressure rate constants) ... 

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