Laidler / Robinson / Irving Reaction Kinetics

Reactions in Solution
1. Auflage 2013
ISBN: 978-1-4831-5626-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 146 Seiten

Reaction Kinetics, Volume II: Reactions in Solution deals with the kinetics of reactions in solution and discusses the basic principles and theories of kinetics, including a brief description of homogeneous gas reactions. This book is divided into two chapters. The first chapter focuses on the general principles of reactions in solution that includes reactions between ions and involving dipoles; influence of pressure on rates in solution; substituent effects; and homogeneous catalysis in solution. Chapter 2 primarily deals with general features of reactions in solution, emphasizing the relationship between the results of a kinetic investigation and actual reaction mechanism. This volume is intended for undergraduate students of chemistry who have not previously studied chemical kinetics. This book is also useful to more advanced students in other fields, such as biology and physics, who wish to have a general knowledge of the subject.

Autoren/Hrsg.

Laidler, Keith J.

Robinson, Robert

Irving, H. M. N. H.

Staveley, L. A. K.

Leseproben

CHAPTER 1

Reactions in Solution: General Principles

Publisher Summary

Liquids neither have a completely random nor a completely regular structure, and their theoretical treatment is consequently much more complicated. It is, therefore, necessary to proceed in a less fundamental and more empirical fashion than is possible in the case of gas reactions or of reactions in solids or on solid surfaces in dealing with reactions in solution. Despite that, much valuable knowledge has accumulated regarding reactions in solution, particularly for certain classes of reactions. Reactions in solution are of a variety of types. There are certain reactions for which the solvent plays a relatively subsidiary role; it seems to act as a mere space-filler and has only a minor influence on the rate of reaction. Such reactions are little affected by a change in solvent and occur in the gas phase at much the same rate as in solution. An example of such a reaction is the thermal decomposition of nitrogen pentoxide.

The liquid state is not understood in anything like the same detail as the gaseous and solid states. In the gaseous state the interactions between individual molecules are usually relatively un-important; the molecules therefore behave largely in a random manner and can be treated in terms of the kinetic theory, which deals with such randomness. Solids, having a regular structure, can also be treated in a satisfactory manner. Liquids, on the other hand, have neither a completely random nor a completely regular structure, and their theoretical treatment is consequently very much more complicated. It is therefore necessary, in dealing with reactions in solution, to proceed in a less fundamental and more empirical fashion than is possible in the case of gas reactions or of reactions in solids or on solid surfaces. In spite of this, much valuable knowledge has accumulated regarding reactions in solution, particularly for certain classes of reactions.

Reactions in solution are of a variety of types. There are certain reactions for which the solvent plays a relatively subsidiary role; it seems to act as a mere space-filler and has only a minor influence on the rate of reaction. Such reactions are little affected by a change in solvent, and occur in the gas phase at much the same rate as in solution. An example of such a reaction is the thermal decomposition of nitrogen pentoxide, some data(1) for which are given in Table 1. The rate constants, frequency factors and activation energies are seen to be very much the same in the solvents mentioned and in the gas phase. In nitric acid solution, on the other hand, the rate constant is significantly lower (0·147 × 10-5 at 25°C) and the activation energy higher (28·3 kcal per mole), indicating that this solvent plays a more active role in the reaction.

TABLE 1

The Decomposition of Nitrogen Pentoxide

Those solvents that have no effect on rates, frequency factors and activation energies probably do not interact very much with the reactant molecules or the activated complexes. An important question that arises in such cases is the frequency of collisions between solute molecules, as compared with the frequency in the gas phase. This matter has been treated theoretically both from the point of view of the kinetic theory of collisions and of the absolute rate theory. A collision theory approach was employed by Rabinowitch(1), who based his treatment on a theoretical study made in 1930 by Debye and Menke of the structure of liquid mercury. Mercury is a very simple liquid, the particles being atoms, and the arrangement of the atoms in the liquid is comparatively regular. Using a distribution function for mercury given by Debye and Menke, Rabinowitch calculated the frequency of collisions between a given pair of mercury atoms, and compared this frequency with the frequency in the gas phase. His conclusion was that in the liquid the frequency of collisions is approximately two to three times greater than that in the gas phase.

The theory of absolute reaction rates was applied to this problem by M. G. Evans and M. Polanyi(1), and also by R. P. Bell(2). Since it is not possible to write down satisfactory partition functions for molecules in the liquid phase (because of the complicated nature of their translational, rotational and vibrational motions) it is more convenient to apply the theory of absolute reaction rates in terms of entropies of activation rather than of partition functions. In Bell’s treatment empirical values for entropies of non-polar molecules in solution were employed, and from them were estimated the entropies of activation for reactions involving such molecules. In agreement with collision theory, his conclusion was that the frequency factor for a reaction in solution should be approximately three times as great as in the gas phase. On the assumption that the energies of activation are the same in solution and in the gas phase, both the collision theory and the theory of absolute reaction rates would therefore indicate that the rates in solution should be approximately three times as great as in the gas phase.

Another problem of some interest is that of the distribution of collisions in time when reactions are occurring in solution. This problem was studied experimentally by Rabinowitch and Wood(3), who employed a tray on which spheres were allowed to roll. Agitation of the tray caused the spheres to move around and by an electrical method the number of collisions between a given pair of spheres was determined. The behaviour in the gas phase is represented by the behaviour when very few spheres are present, while that in the liquid phase is represented by the situation in which the spheres are comparatively closely packed. The result was that the frequency of collisions between a given pair of spheres was roughly independent of the total number of spheres present, but that the distribution was quite different when many spheres were present. It was found, in fact, that collisions occurred in sets when the spheres were fairly closely packed, but not when only a few spheres were present. The reason for this is that after an initial collision has occurred, in the case of the closely packed spheres the surrounding spheres form a “cage” which holds the colliding spheres together and causes them to collide a number of times before they finally separate. This tendency for collisions to occur in sets does not make any difference for ordinary reactions, involving an activation energy, since reaction may occur at any collision within the set. In the case of reactions that do not involve an activation energy, such as free radical combinations, this tendency of collisions to occur in sets will make a difference to the frequency factors since reaction is bound to occur at the first collision in any set, with the result that the remaining collisions do not contribute to the rate. For such reactions the frequency factor is therefore related to the reciprocal of the average time elapsing between successive sets of collisions.

This , also known as the (1), has other important consequences. In the case of photochemical reactions in solution, for example, a pair of free radicals produced initially may, owing to their being caged in by the surrounding solvent molecules, be caused to recombine before they can separate from one another. This phenomenon is known as , as opposed to which occurs after the free radicals have separated from one another.

The preceding comments are strictly only valid for reactions in an inert solvent that has little effect on the kinetic behaviour. There are many reactions that do not occur at all in the gas phase; an example is the formation of the quaternary ammonium salt from ethyl iodide and triethylamine,

For this reaction the solvent is presumably necessary in order to stabilize the activated complex; the properties of this will be somewhat like those of the products, and an ionizing solvent is found to favour the reaction. Some data(1) for this reaction are given in Table 2, which shows that there are very wide variations in kinetic behaviour as one changes from non-polar solvents such as hexane and toluene to polar solvents like nitrobenzene. The frequency factors in all of the solvents are seen to be of the order of 104 to 105 litre mole-1 sec-1, much smaller than expected on the basis of simple collision theory (~1011 litre mole-1 sec-1). The discrepancies are much too large to be explained in terms of structural effects; there is little rotational freedom in the liquid state so that there can be no large loss of rotational entropy when the activated complex is formed. The low frequency factors must therefore be explained in terms of specific solvent influences, the nature of which will be discussed later.

TABLE 2

The Reaction between Ethyl Iodide and Triethylamine

Important factors that must be taken into account in this reaction, and in many other reactions for which the solvent has an important influence, are the electrostatic forces between solute and solvent molecules. In some reactions, such as those between ions, these forces exert a predominant effect on the kinetic behaviour, which can be treated fairly satisfactorily by considering only these electrostatic forces. In other cases, such as reactions between highly...