Elementary versus overall reactions

If a reaction is a one-step reaction, that is, if it occurs on the molecular level as it is written, then the reaction is called an elementary reaction. In an elementary reaction, either the particles collide to produce the product, or a single particle shakes itself to become something different. For example, Reactions 1-1 and 1-2 occur at the atomic scale as they are written. That is, a parent nuclide shakes itself to become a more stable daughter nuclide (or two daughter nuclides). 

If a reaction is not an elementary reaction, i.e., if the reaction does not occur at the molecular level as it is written, then it is called an overall reaction. An overall reaction may be accomplished by two or more steps or paths and/or with participation of intermediate species. For example, nuclear hydrogen burning Reaction 1-3, 4 H He, is an overall reaction, not an elementary reaction. There are several paths to accomplish the reaction, with every path still an overall reaction accomplished by three or more steps. One path is called aPP I chain and involves the following steps  

Each of the above three reactions is an elementary reaction. During the first step, two nuclides collide to form one (in the process, one proton plus one electron become a neutron). In the second step, one collides with to form 

The net result by adding up all the above reactions is four reacting to form a 

Reaction 1-5, 203(gas) 302(gas), is an overall reaction. Both Reactions l-6f and l-6b, 2CO(gas) + 02(gas) 2C02(gas), are also overall reactions. Both Reactions l-9f and l-9b are elementary reactions. Whether a reaction is an elementary reaction or an overall reaction can only be determined experimentally, and cannot be determined by simply looking at the reaction. Many simple gas-phase reactions in the atmosphere involve intermediate radicals and, hence, are complicated overall reactions. 

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Elementary reactions versus overall hydrocarbon cracking... 

In both cases, the overall reaction is simply A —D, with B and C as intermediate species. Each individual step in a mechanism has a rate law based on the stoichiometry of the individual elementary process. Lotka showed that if [A] is assumed to be constant (that is, it is present in a large excess), the differential equations that relate the concentrations of A, B, C, and D have mathematical solutions that predict oscillations in the concentrations of intermediates B and C if the rate constants have the appropriate values. In mechanism 1, [B] and [C] follow damped oscillations, and for mechanism 2 the intermediate concentrations oscillate more evenly. Figure 20.21 shows the concentration behaviors of the intermediates versus time. 

Few reactions have been studied over the enormous range indicated in Figure 5.1. Even so, they will often show curvature in an Arrhenius plot of n k) versus T. The usual reason for curvature is that the reaction is complex with several elementary steps and with different values of E for each step. The overall temperature behavior may be quite different from the simple Arrhenius behavior expected for an elementary reaction. However, a linear Arrhenius plot is neither necessary nor sufficient to prove that a reaction is elementary. It is not sufficient because complex reactions may have one dominant activation energy or several steps with similar activation energies that lead to an overall temperature dependence of the Arrhenius sort. Arrhenius behavior is not necessary since some low-pressure, gas phase, bimolecular reactions exhibit distinctly non-Arrhenius behavior even though the reactions are believed to... 

Global versus Elementary Reaction An important distinction between reaction steps, which is needed to understand the material presented in the rest of the book, is the concept of a global and elementary reaction. Consider the overall fuel cell reaction ... 

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