Molecularity of a reaction

The molecularity of an elementary reaction refers to the number of particles in the reactants (left-hand side). If the molecularity is 1, the elementary reaction is said 

For example, the molecularity is 1 for radioactive decay reactions (1-1) and (1-2). The molecularity of the forward reaction does not have to be the same as that of the backward reaction. 

Although elementary reactions and overall reactions can only be distinguished in the laboratory, a few simple guidelines can be used to guess. If the number of particles of the reaction is 4 or more, it is an overall reaction. If the number of particles is 3, then most likely the reaction is an overall reaction because there are only a limited number of trimolecular reactions. Almost all elementary reactions have molecularities of 1 or 2. However, the reverse is not true. For example. Reaction 1-5, 203(gas) 302(gas), has a molecularity of 2 but is not an elementary reaction. 

In thermod5mamics, a reaction can be multiplied by a constant factor without changing the meaning of the reaction. However, in kinetics, an elementary reaction is written according to how the reaction proceeds, and cannot be multiplied by a constant. For example, if Reaction 1-7, C02(aq) + H20(aq) H2C03(aq), is multiplied by 2, thermodynamic treatment stays the same, but kinetically the forward reaction would have a molecularity of 4, and is different from Reaction l-7f. 

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The second use of activation parameters is as criteria for mechanistic interpretation. In this application the activation parameters of a single reaction are, by themselves, of little use such quantities acquire meaning primarily by comparison with other values. Thus, the trend of activation parameters in a reaction series may be suggestive. For example, many linear correlations have been reported between AT/ and A5 within a reaction series such behavior is called an isokinetic relationship, and its significance is discussed in Chapter 7. In Section 5.3 we commented on the use of AS to determine the molecularity of a reaction. Carpenter has described examples of mechanistic deductions from activation parameters of organic reactions. 

Molecularity of a reaction the number of reacting partners in an elementary reaction uni-molecular (one), bimolecular (two), or termolecular (three) in the mechanism above, the first and third steps are unimolecular as written, and the remainder are bimolecular. Molecularity (a mechanistic concept) is to be distinguished from order (algebraic). 

We must appreciate the essential truth that the molecularity of a reaction and the stoichiometric equation are two separate things, and do not necessarily coincide. Luckily, we find that reactions are quite often simple (or elementary ), by which we mean that they involve a single reaction step. The molecularity and the reaction order are the same if the reaction... 

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. 

The molecularity of a reaction is always an integer and only applies to elementary reactions. That is not always so for the order of a reaction, thus emphasizing the difference between molecularity and order. Molecularity... 

Since the order refers to the empirically found rate expression, it can have a fractional value and need not be an integer. However, the molecularity of a reaction must be an integer because it refers to the mechanism of the reaction, and can only apply to an elementary reaction. 

This elementary reaction is an example of a bimolecular reaction because two reactant species come together to react. The molecularity of a reaction is the number of reactant molecules involved in a specified elementary reaction. The molecularity of a unimolecular reaction is 1 and that of a bimolecular reaction is 2. 

Molecularity of a reaction. The number of molecules involved in a specific reaction step. 

The molecularity of a reaction is simply the number of different molecules (or atoms or ions) that are involved in the rate-determining step. It equals 1 for a unimolecular reaction, 2 for a bimolecular reaction, and 3 for a termolecular reaction... 

Thus, often molecularity of a reaction coincides with its order, but the two need not be always identical. The molecularity must be an integral value, but the order of reaction may be zero, whole number or even fractional. Whereas molecularity can be given on the basis of some proposed theoretical mechanism so as to satisfy the experimental findings, the order of reaction can be obtained from experimental results. 

Reactions which are not unimolecular, but obey the first order rate expression are known as pseudo-unimolecular reactions. For example, hydrolysis of methyl acetate, inversion of cane sugar etc. are pseudo-unimolecular reactions. In general, when the order of reaction is generally less than the molecularity of a reaction, it is said to be a pseudo order reaction. 

The difference between the molecularity and the order of a reaction is important. The molecularity of a reaction, or a step within a reaction, describes what happens on the molecular level. The order of a reaction describes what happens on the macroscopic scale. We determine the order of a reaction by watching the products of a reaction appear or the reactants disappear. The molecularity of the reaction is something we deduce to explain these experimental results. 

In the study of reaction orders and kinetic mechanisms, reference is sometimes made to the molecularity of a reaction. The molecularity is the number of atoms, ions, or molecules involved colliding) in the rate-limiting step of the reaction. The terms unimolecular, bimolecular, and tennoleailar refer to reactions involvings respectively, one, two, or three atoms (or molecules) interacting or colliding in any one reaction step. 

The third term to be considered in this section is molecularity. The molecularity of a reaction is the total number of molecules taking part in the slowest of the elementary reaction steps. In most chemical reactions, two molecules collide and react the molecularity is 2 and the reaction is said to be bimolecular. Reactions in which only one molecule is involved (unimolecular) are known, but usually occur in the gas phase. Reactions with a molecularity higher than 2 are very rare, since this would require three or more reactants all encountering each other at the same time. 

The molecularity of a reaction is the number of molecules reacting in an elementary step. These molecules may be of the same or different types. Each of the elementary steps discussed above is called a bimolecular reaction, an elementary step that involves two molecules. An example of a unimolecular reaction, an elementary step in which only one reacting molecule participates, is the conversion of cyclopropane to propene discussed in Example 13.3. Very few termolecular reactions, reactions that involve the participation of three molecules in one elementary step, are known, because the simultaneous encounter of three molecules is a far less likely event than a bimolecular collision. 

Molecular orbital. An orbital that results from the interaction of the atomic orbitals of the bonding atoms. (10.6) Molecularity of a reaction. The number of molecules reacting in an elementary step. (13.5)... 

An important concept in chemical kinetics is molecularity of a reaction or the number of particles (molecules, atoms, ions, radicals) participating in it. Most common are bimolecular reactions, unimolecular reactions being also encountered. In very rare cases termolecular reactions may be observed as well. Reactions of higher molecularity are unknown, which is due to a very low probability of a simultaneous interaction of a larger number of molecules. Consequently, our further considerations will be confined to the examination of uni- and bimolecular reactions. On the other hand, the reactions of a termolecular character, whose kinetic equations have a number of interesting properties, are sometimes considered. As will appear, a termolecular reaction may be approximately modelled by means of a few bimolecular reactions. For an elementary reaction its molecularity is by definition equal to the order whereas for a complex reaction the molecularity generally has no relation whatsoever to the reaction order or the stoichiometry. 

Rate extremes with systematic variation of substituents are often considered to be evidence of changes in the molecularity of a reaction, but for hydrolyses of arenesulfonyl chlorides, rate extremes are more reasonably ascribed to variations in the extents of S-O bond making and S-Cl bond breaking. Variations in k+/k support this hypothesis (Table VI), and as for hydrolyses of acid chlorides (Table V), bond making seems to be important but introduction of electron-donating groups increases the importance of bond breaking in the transition state. 

Explain the difference between the order and the molecularity of a reaction. 

Because an elementary reaction occurs on a molecular level exactly as it is written, its rate expression can be determined by inspection. A unimolecular reaction is a first-order process, bimolecular reactions are second-order, and termolecular processes are third-order. However, the converse statement is not true. Second-order rate expressions are not necessarily the result of an elementary bimolecular reaction. While a stoichiometric chemical equation remains valid when multiplied by an arbitrary factor, a mechanistic eqnation loses its meaning when multiplied by an arbitrary factor. Whereas stoichiometric coefficients and reaction orders may be integers or nonintegers, the molecularity of a reaction is always an integer. The following examples indicate the types of rate expressions associated with various molecularities. Unimolecular ... 

Finally, we will take a brief look at the relation between the order of a reaction and the molecularity mentioned above. Reaction orders are experimentally determined quantities while molecularity (of a reaction step) is a theoretical quantity essential for the elucidation of a reactitMi mechanism. In single-step reactions, molecularity and reaction order (as well as conversion number sum) agree with each other because all the particles react simultaneously with each other according to their appearance in the conversion formula. Conversely, it is not necessarily possible to infer the molecularity of an arbitrary reactimi from its order. This is because in complex reaction processes made up of several single-step reactions, simple rate laws might still be vaUd. 

Very often people confuse rate orders with what is called the Molecularity of a Reaction . This expression makes a statement of how many molecules interact in a reaction during the slowest (rate-... 

The following table gives a comparison between order and molecularity of a reaction ... 

Enzyme, p. 599 First-order reaction, p. 570 Half-life ( ), p. 575 Intermediate, p. 588 Molecularity of a reaction, p. 588... 

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