Reaction order terminology

Before we see how reaction orders are determined from initial rate data, let s discuss the meaning of reaction order and some important terminology. We speak of a reaction as having an individual order with respect to or in each reactant as well as an overall order, which is simply the sum of the individual orders. 

In the simplest case, a reaction with a single reactant A, the reaction is first order overall if the rate is directly proportional to [A]  

And it is zero order overall if the rate is not dependent on [A] at all, a common situation in metal-catalyzed and biochemical processes, as you ll see later  

Here are some real examples. For the reaction between nitrogen monoxide and ozone, 

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Reaction Order Terminology Determining Reaction Orders Experimentally Determining the Rate Constant... 

We return here to the issues of molecularity and reaction order, mentioned previously, because they deserve further consideration now that the basic terminology has been introduced. The need for careful usage can best be demonstrated by way of some examples. 

Overall reaction order is the sum of the orders for each reactant, For a discussion of how this term can be misleading, see John C. Reeve, "Some Provocative Opinions on the Terminology of Chemical Kinetics,"... 

The first step, which is rate determining, is an ionization to a carbocation (carbonium ion in earlier terminology) intermediate, which reacts with the nucleophile in the second step. Because the transition state for the rate-determining step includes R-X but not Y , the reaction is unimolecular and is labeled S l. First-order kinetics are involved, with the rate being independent of the nucleophile identity and concentration. 

From this expression, it is obvious that the rate is proportional to the concentration of A, and k is the proportionality constant, or rate constant, k has the units of (time) usually sec is a function of [A] to the first power, or, in the terminology of kinetics, v is first-order with respect to A. For an elementary reaction, the order for any reactant is given by its exponent in the rate equation. The number of molecules that must simultaneously interact is defined as the molecularity of the reaction. Thus, the simple elementary reaction of A P is a first-order reaction. Figure 14.4 portrays the course of a first-order reaction as a function of time. The rate of decay of a radioactive isotope, like or is a first-order reaction, as is an intramolecular rearrangement, such as A P. Both are unimolecular reactions (the molecularity equals 1). 

Until now, we have learned the basic terminology that you will need in order to predict products of addition reactions. To summarize, there are three pieces of information that you must have in order to predict products ... 

We will present mechanistic aspects of the Diels-Alder reaction, its selectivity and reactivity in order to explain solvent effects on the one hand, and the effects of Lewis acids on the other. Other catalytic systems like micelles will also be addressed. Some of the explanations may seem trivial or are well-known but, as we will use these in later sections, a clear terminology is desirable. 

The initiator is present in the water phase, and this is where the initiating radicals are produced. The rate of radical production if, is typically of the order of 1013 radicals L-1 s-1. (The symbol p is often used instead of Rj in emulsion polymerization terminology.) The locus of polymerization is now of prime concern. The site of polymerization is not the monomer droplets since the initiators employed are insoluble in the organic monomer. Such initiators are referred to as oil-insoluble initiators. This situation distinguishes emulsion polymerization from suspension polymerization. Oil-soluble initiators are used in suspension polymerization and reaction occurs in the monomer droplets. The absence of polymerization in the monomer droplets in emulsion polymerization has been experimentally verified. If one halts an emulsion polymerization at an appropriate point before complete conversion is achieved, the monomer droplets can be separated and analyzed. An insignificant amount (approximately <0.1%) of polymer is found in the monomer droplets in such experiments. Polymerization takes place almost exclusively in the micelles. Monomer droplets do not compete effectively with micelles in capturing radicals produced in solution because of the much smaller total surface area of the droplets. 

In Fig. 1 (top right) we show a sloped conical intersection in the terminology of Ruedenberg et al (29). Here the cone is tilted due to the fact that the force (gradient) vectors on both the upper and lower surfaces point in the same direction. The first-order topology (sloped vs. peaked) controls the nature of the photochemical reaction dynamics, and whether reactants are regenerated or photoproducts are formed (23,24). 

A carbocation is formed as shown in Eq. 12-11, which represents just one-half of the overall displacement reaction. In the common terminology of physical organic chemistry this is an SN1 reaction rather than an Sn2 reaction of the kind shown in Eqs. 12-3 and 12-5. This terminology is not quite appropriate for enzymes because the breakdown of ES complexes to product is usually a zero-order process and the numbers 1 and 2... 

Experimental data from nucleophilic substitution reactions on substrates that have optical activity (the ability to rotate plane-polarized light) shows that two general mechanisms exist for these types of reactions. The first type is called an S 2 mechanism. This mechanism follows second-order kinetics (the reaction rate depends on the concentrations of two reactants), and its intermediate contains both the substrate and the nucleophile and is therefore bimolecular. The terminology S 2 stands for substitution nucleophilic bimolecular. ... 

On the basis of such considerations, they have classified stereo-differentiating reactions into six types. While the terminology used is readily understandable, some comments on prochiral distinctions are in order. The thinking enzyme discussed earlier can use a sequence of binary congruence tests to examine the environmental relationships of materially identical groups within a molecule (topic analysis). The flow chart resembles that shown earlier for isomeric distinctions (see Scheme 2). 

The probability distribution in Figure 11.6 indicates that there are two stable states for the chemical reaction system of Equation (11.25). Since the system is open to species A, B, and C, these states are non-equilibrium steady states (NESS). A more careful discussion of the terminology is in order here. The concept of an NESS has different meanings depending on whether we are considering a macroscopic or a microscopic view. This difference is best understood in comparison to the term chemical equilibrium. From a macroscopic standpoint, an equilibrium simply means that the concentrations of all the chemical species are constant, and all the reactions have no net flux. However, from a microscopic standpoint, all the concentrations are fluctuating. 

As we note in our Science Milestone, opposite, van t Hoff classified reactions as unimolecular, bimolecular, etc., according to the relationship between their rates and the concentration of reactants. It did not escape van t Hoff that the power to which a concentration was raised in his equations did not necessarily correspond to the number of molecules involved in a particular reaction. For example, when we consider the formation of esters from acids and alcohols, we will find that if there is no added catalyst the rate of the reaction is proportional to the product of the concentration of alcohol groups and the square of the concentration of add groups (Figure 4-2). We now talk about the order of a reaction, of course, a clarification in terminology that was introduced by Ostwald in 1887. A first-order reaction is what van t Hoff would have referred to as unimolecular, and depends on the concentration of a single reactant. 

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