A reaction mechanism is a series of simple molecular processes, such as the Zeldovich mechanism, that lead to the formation of the product. As with the empirical rate law, the reaction mechanism must be determined experimentally. The process of assembling individual molecular steps to describe complex reactions has probably enjoyed its greatest success for gas phase reactions in the atmosphere. In the condensed phase, molecules spend a substantial fraction of the time in association with other molecules and it has proved difficult to characterize these associations. Once the mechanism is known, however, the rate law can be determined directly from the chemical equations for the individual molecular steps. Several examples are given below.
Three basic types of fundamental processes are recognized: unimolecular, bimolecular and termolecular. Unimolecular processes are reactions involving only one reactant molecule. Radioactive decay is an example of a unimolecular process:
The rates of this process depend only on the concentration of reactant and the rate constant, which is generally temperature dependent.
The unit of the rate constant for a unimolecular process is 1 /s.
Photolytic reactions such as the decomposition of ozone by light are also unimolecular processes:
03 + hv 02 + O The rate of this photolytic reaction is given by d_ d
molecule to another and one bond is formed as another is broken. The rate of a bimolecular process depends on the product of concentrations of the two reactants. In this case d[NO] _ d[N02]
Rate constants for photolytic reactions are commonly represented by the symbol / (unit 1/s). The first-order photolytic rate constant can be calculated using the formula:
The empirically determined absorption cross-section, ff(/,T) in units of cm2/molecule, is a measure of the ability of a molecule to absorb light of a particular wavelength at a given temperature. The photon flux, 1(a) in units of photons/cm s nm), represents the number of photons of a certain wavelength range arriving at a 1 cm2 area per second. Therefore, /(/.) and hence / increase with altitude, vary with time of day, and decrease to zero at night. The dimen-sionless quantum yield, </>(/,T), describes the fraction of absorbed photons that results in the photolysis pathway of interest. For instance, 03 can photolyze along several pathways including
0(3P) and O('D) represent different electronic states of the oxygen atom. The quantum yield for 0('D) production, 0onD,(/.,T), is the fraction of photons of wavelength / absorbed by ozone at temperature T that result in the formation of the excited O('D) atoms. Since photolysis can occur over a range of wavelengths, / is calculated over the integral from the shortest, to the longest, /2, wavelength at which the photolytic reaction occurs.
Bimolecular processes are reactions in which two reactant molecules collide to form two or more product molecules. In most cases the reaction involves a rather simple rearrangement of bonds in the two molecules:
NO + 03 -> N02 + 02 Often, a single atom is transferred from one
The units for the rate constant, k, for a bimolecular reaction are cm3/molecule s.
Termolecular processes are common when two reactant molecules combine to form a single small molecule.
Such reactions are often exothermic and the role of the third body is to carry away some of the energy released and thus stabilize the product molecule. In the absence of a collision with a third body, the highly vibrationally excited product molecule would usually decompose to its reactant molecules in the timescale of one vibrational period. Almost any molecule can act as a third body, although the rate constant may depend on the nature of the third body. In the Earth's atmosphere the most important third-body molecules are N2 and 02.
The rate of the reaction depends on the product of reactant concentrations, including the third body:
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