Sykes / Robinson / Staveley Kinetics of Inorganic Reactions

The Commonwealth and International Library: Chemistry Division
1. Auflage 2013
ISBN: 978-1-4831-8567-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 318 Seiten

Kinetics of Inorganic Reactions provides a comprehensive account of the mechanisms of inorganic reaction. The book is comprised of 15 chapters that deal with the two main fields of inorganic reaction, the homogeneous gas-phase reactions and solution reactions. The first chapter of the text provides an introduction to some of the basic concepts in inorganic reaction, which include the mechanisms of a reaction, reactions in different phases, and the feasibilities of a reaction. Next, the book details the experimental techniques and treatment of data. The next series of chapters talks about gas-phase reactions. The book also dedicates a chapter in covering various types of reactions, including isotopic reaction and redox reaction. Chapters 12 to 14 deal with substitution reactions, while Chapter 15 talks about acid-base reactions. The text will be most useful to chemists and chemical engineers, particularly those who deal with inorganic chemistry.

Autoren/Hrsg.

Sykes, A. G.

Robinson, Robert

Staveley, L. D. K.

Leseproben

CHAPTER 1

INTRODUCTION

Publisher Summary

Chemistry is concerned not only with the properties of elements in their combined and uncombined states, that is with substances at equilibrium, but with the nature of transitions between equilibrium states. Kinetics is the study of the speed and manner of all such transitions. The first aim in kinetics is to identify the basic elementary reactions, that is, those reactions which cannot be resolved into a series of simpler steps. Very often, the overall stoicheiometric equation gives no information as to the reaction sequence or mechanism. In the vast majority of reaction sequences, that is mechanisms, the elementary reactions are bimolecular processes. The simplest reactions to consider, are those in the gas phase, since they are not complicated by solvation effects. A few reactions have been studied in both the gas phase and in solution, but these rarely show much agreement. For reactions in the gas phase, radicals are formed much more readily than ions, and ionic processes are practically non-existent below 800°C.

CHEMISTRY is concerned not only with the properties of elements in their combined and uncombined states, that is with substances at equilibrium, but with the nature of transitions between equilibrium states. Kinetics is the study of the speed and manner of all such transitions.

The mechanism of a reaction

The first aim in kinetics is to identify the basic elementary reactions, i.e. those reactions which cannot be resolved into a series of simpler steps. Very often, the overall stoicheiometric equation gives no information as to the reaction sequence or mechanism. Thus the fast reaction

cannot proceed by the simultaneous collision of fourteen reactant ions (which is highly improbable), but must proceed in a stepwise manner with the intermediate formation of different oxidation states of manganese. Just which oxidation states are involved in such a complex system as this is difficult to establish however. Much simpler overall equations,

are not always, themselves, representative of the elementary reactions and in this particular case the mechanism has been shown to be

Rate constants

In the vast majority of reaction sequences, i.e. mechanisms, the elementary reactions are bimolecular processes. The rate constant for a bimolecular reaction

can be defined by a rate equation

In other words, it is the rate with unit concentrations of the two reactants. If the time is in seconds, and reactant concentrations are in mole 1-1, the dimensions of are 1 mole-1 sec-1. For a unimolecular reaction

the dimensions of are sec-1, and for a termolecular process

the dimensions of are 12 mole-2 sec-1.

It is hoped, ultimately, that it will be possible to calculate rate constants by considering fundamental properties of individual atoms and molecules. Collision theory and transition-state theory, which are considered in later sections of this chapter, represent attempts, so far made, to relate measured rate constants with more fundamental processes.

Reactions in different phases

The simplest reactions to consider, are those in the gas phase, since they are not complicated by solvation effects. A few reactions have been studied in both the gas phase and in solution, but these rarely show much agreement. The decomposition of dinitrogen pentoxide

is something of an exception, in that rates measured in eight different solvents agree to within a factor of two with those for the gas-phase reaction. Contrast the reaction of oxalyl chloride with water

which is a complex chain reaction in the gas phase, but, in carbon tetrachloride, shows excellent second-order behaviour.

The ionic character of a substance is often emphasized in polar solvents. Thus, in aqueous solutions, hydrogen iodide behaves as a strong acid

or, more precisely,

and could hardly be expected to decompose, as in the gas phase

For reactions in the gas phase, radicals are formed much more readily than ions, and ionic processes are practically nonexistent below 800°C. Highly reactive ions can, however, be produced by electron bombardment in the ionization chamber of a mass spectrometer, and in a number of simple cases their reactions have been studied,

For the many reactions in solution involving inorganic ions, water is by far the most convenient solvent. Acetic acid, methanol and ethanol have also been used, and liquid ammonia would no doubt be more popular were it easier to handle. Ethylene glycol-water, and other mixtures, have been used to study the effect of a variation in bulk dielectric constant.

In solution, ions interact with the solvent and other molecules present. Interactions are particularly strong for transition-metal ions which form a wide range of complexes. In aqueous solutions, hexaquo ions are generally formed; thus Cr3+ exists as the hexaquo ion The affinity for water molecules is by no means exhausted by those in the inner-coordination sphere and there are further interactions with molecules at not very much greater distances. With non-transition metal ions, e.g. Na+, electrostatic interactions with solvent dipoles are much weaker and the number of nearest neighbours is often difficult to establish. For a detailed discussion of the properties of metal ions in solution, the reader is referred to Hunt’s (1)

Stereochemistry of transition metal ions(2)

The number of groups in the inner-coordination sphere of a transition metal ion is generally either six (octahedral complexes) or four (tetrahedral and square-planar complexes). Most ions show variable coordination numbers, thus in aqueous solution the cobalt ion Co2+ has six water molecules as nearest neighbours, but at high chloride ion concentrations, tetrahedral is formed.

Although X-ray crystallographic techniques can be used to determine a coordination number in the solid phase, it cannot always be assumed that a complex has the same configuration in solution. Absorption spectra generally change appreciably with coordination number and provide one of the best methods of checking a particular configuration. Even so, it is not always easy to predict what the spectrum of an alternative configuration might be, or to say whether an observed shift is at all relevant. A good example to consider is that of the nickel(II) ion. While it is true that in the crystalline state Ni2+ often exists as how do we know that a tetrahedral aquo-ion is not formed in aqueous solution? To answer this with any certainty, we must know what the spectrum of Ni2+ in a tetrahedral H2O environment looks like. This in itself is difficult, but the spectrum of Ni2+ in a tetrahedral oxide environment can be obtained by dissolving small amounts of NiO in a ZnO lattice. Similarly, by dissolving NiO in MgO, the spectrum of Ni2+ in an octahedral oxide environment can be obtained. Since the spectrum of NiO in MgO resembles that of in a crystal lattice and Ni2+ in aqueous solutions, it can be concluded that hexaquo ions are formed in solution.

The spectrum of Fe(ClO4)3 in perchloric acid solutions is very similar to that of in an alum. If Fe3+ adopted a tetrahedral configuration in water, appreciable changes would be expected. The monochloro complex is also octahedral, Fe(H2O)5Cl2+, but at higher chloride ion concentrations, tetrahedral is formed and the spectrum resembles that of KFeCl4. The stage at which the configuration actually changes can often be inferred from stepwise formation constants, where for successive reactions,

stepwise formation constants may be defined by

Thus for the formation of HgCl42-. such constants are found to be 1 = 107·15, 2 = 106·9, 3 = 101·0 and 4 = 100·7, and since 1 and 2 are very much bigger than 3 and 4, it can be inferred that the mono- and di-chloro complexes are linear, while the tri- and tetra-chloro complexes have tetrahedral configurations.

From a comparison of the spectra of VO2+ and VO(acac)2 (the structure of which is known with certainty) the VO2+ aquo ion is believed to have a distorted octahedral structure in which four of the water molecules lie in a plane somewhat below that of the vanadium atom and on the side remote from the oxygen. Evidence that actinide ions such as the uranyl ion are linear oxo-cations in solution as well as in the solid phase has been obtained by comparing visible infrared and Raman spectra. Although such ions form a variety of complexes with other ions and donor molecules, few structures have as yet been determined. It is thought that four, five and six ligand groups can lie in the equatorial plane of, for example, the O—U—O2+ ion.

Crystal-field theory: High-spin and low-spin complexes

Since in the chapters on solution reactions we shall be principally concerned with the reactions of octahedral metal ions (in particular the reactions of hexaquo metal ions), the discussion in this section will be limited to a...