5. Reactivity

2016 ◽  
pp. 90-109
Author(s):  
William H. Brock
Keyword(s):  
19Th Century ◽  
Chemical Reactions ◽  
Chemical Change ◽  
Inert Gases ◽  
Chemical Equilibria ◽  
Atoms And Molecules ◽  
Ideal Gases ◽  
Boyle’S Law

By the mid-19th century many different kinds of chemical change had been recognized and systematized in textbooks. It became the task of physical chemists to explain these different transformations in terms of exchanges between atoms and molecules powered by energy changes and the shifts in equilibrium that underlay all reactivity. Physical chemists found ways of expressing chemical change in mathematical terms and so brought generalization and systematization to chemical practice. ‘Reactivity’ considers the conditions for chemical equilibria and the mechanisms involved in chemical reactions by discussing concepts such as thermodynamics, periodicity, spectroscopy, ideal gases, Boyle’s law, electrolysis, ionic theory, kinetics, and inert gases.

Chemical Reactions In Condensed Phases

2006 ◽  
Author(s):  
Abraham Nitzan
Keyword(s):  
Chemical Reactions ◽  
Condensed Phase ◽  
Chemical Change ◽  
Classical Mechanics ◽  
Chemical System ◽  
Condensed Phases ◽  
Molecular Configuration ◽  
Middle Stage ◽  
Condensed Phase Reactions ◽  
Energy Surfaces

Understanding chemical reactions in condensed phases is essentially the understanding of solvent effects on chemical processes. Such effects appear in many ways. Some stem from equilibrium properties, for example, solvation energies and free energy surfaces. Others result from dynamical phenomena: solvent effect on diffusion of reactants toward each other, dynamical cage effects, solvent-induced energy accumulation and relaxation, and suppression of dynamical change in molecular configuration by solvent induced friction. In attempting to sort out these different effects it is useful to note that a chemical reaction proceeds by two principal dynamical processes that appear in three stages. In the first and last stages the reactants are brought together and products are separated from each other. In the middle stage the assembled chemical system undergoes the structural/chemical change. In a condensed phase the first and last stages involve diffusion, sometimes (e.g. when the species involved are charged) in a force field. The middle stage often involves the crossing of a potential barrier. When the barrier is high the latter process is rate-determining. In unimolecular reactions the species that undergoes the chemical change is already assembled and only the barrier crossing process is relevant. On the other hand, in bi-molecular reactions with low barrier (of order kBT or less), the rate may be dominated by the diffusion process that brings the reactants together. It is therefore meaningful to discuss these two ingredients of chemical rate processes separately. Most of the discussion in this chapter is based on a classical mechanics description of chemical reactions. Such classical pictures are relevant to many condensed phase reactions at and above room temperature and, as we shall see, can be generalized when needed to take into account the discrete nature of molecular states. In some situations quantum effects dominate and need to be treated explicitly. This is the case, for example, when tunneling is a rate determining process. Another important class is nonadiabatic reactions, where the rate determining process is hopping (curve crossing) between two electronic states. Such reactions are discussed in Chapter 16.

scholarly journals Positron Scattering from Atoms and Molecules

Atoms ◽  
2020 ◽  
Vol 8 (2) ◽  
pp. 29 ◽  
Author(s):  
Sultana N. Nahar ◽  
Bobby Antony
Keyword(s):  
Cross Section ◽  
Electron Scattering ◽  
Cross Sections ◽  
Optical Potential ◽  
Inert Gases ◽  
Positron Impact ◽  
Positron Scattering ◽  
Atoms And Molecules ◽  
Section Data ◽  
Theoretical Approaches

A review on the positron scattering from atoms and molecules is presented in this article. The focus on positron scattering studies is on the rise due to their presence in various fields and application of cross section data in such environments. Positron scattering is usually investigated using theoretical approaches that are similar to those for electron scattering, being its anti-particle. However, most experimental or theoretical studies are limited to the investigation of electron and positron scattering from inert gases, single electron systems and simple or symmetric molecules. Optical potential and polarized orbital approaches are the widely used methods for investigating positron scattering from atoms. Close coupling approach has also been used for scattering from atoms, but for lighter targets with low energy projectiles. The theoretical approaches have been quite successful in predicting cross sections and agree reasonably well with experimental measurements. The comparison is generally good for electrons for both elastic and inelastic scatterings cross sections, while spin polarization has been critical due to its sensitive perturbing interaction. Positron scattering cross sections show relatively less features than that of electron scattering. The features of positron impact elastic scattering have been consistent with experiment, while total cross section requires significant improvement. For scattering from molecules, utilization of both spherical complex optical potential and R-matrix methods have proved to be efficient in predicting cross sections in their respective energy ranges. The results obtained shows reasonable comparison with most of the existing data, wherever available. In the present article we illustrate these findings with a list of comprehensive references to data sources, albeit not exhaustive.

Uniqueness of chemical equilibria in ideal mixtures of ideal gases

2008 ◽  
Vol 76 (9) ◽  
pp. 848-855 ◽  
Author(s):  
Joseph M. Powers ◽  
Samuel Paolucci
Keyword(s):  
Chemical Equilibria ◽  
Ideal Gases

scholarly journals Application of computational chemistry in chemical reactivity: a review

2021 ◽  
Author(s):  
C. W. Chidiebere ◽  
C. E. Duru ◽  
J. P. C. Mbagwu
Keyword(s):  
Computational Chemistry ◽  
Molecular Orbital ◽  
Chemical Reactions ◽  
Quantum Physics ◽  
Chemical Reactivity ◽  
Chemical Change ◽  
Molecular Orbitals ◽  
Automatic Data ◽  
Highest Occupied Molecular Orbital ◽  
Lowest Unoccupied Molecular Orbital

Molecular orbitals are vital to giving reasons several chemical reactions occur. Although, Fukui and coworkers were able to propose a postulate which shows that highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is incredibly important in predicting chemical reactions. It should be kept in mind that this postulate could be a rigorous one therefore it requires an awfully serious attention in order to be understood. However, there has been an excellent breakthrough since the introduction of computational chemistry which is mostly used when a mathematical method is fully well built that it is automated for effectuation and intrinsically can predict chemical reactivity. At the cause of this review, we’ve reported on how HOMO and LUMO molecular orbitals may be employed in predicting a chemical change by the utilization of an automatic data processing (ADP) system through the utilization of quantum physics approximations.

Chemical Reactions during Vulcanization. III

1942 ◽  
Vol 15 (3) ◽  
pp. 560-571
Author(s):  
E. A. Hauser ◽  
M. C. Sze
Keyword(s):  
Zinc Oxide ◽  
Double Bond ◽  
Chemical Reactions ◽  
Eleostearic Acid ◽  
Chemical Change ◽  
Membered Ring ◽  
Double Bonds ◽  
Tung Oil ◽  
Rate Of Reaction ◽  
Preceding Discussion

Abstract From the preceding discussion the following conclusions may be drawn: A. Vulcanization with Sulfur in General 1. Vulcanization with sulfur is fundamentally a chemical change involving activated sulfur and organic double bonds. 2. Sulfur must decompose from S8 (in the form of an eight-membered ring) to smaller molecular units, perhaps to S2 and S1 before it can be activated and react. 3. The ratio of atoms of sulfur combined to the number of double bonds lost is not restricted to one atom of sulfur for each double bond, but may have other values, which depend on the conditions and the reactions involved. B. Factice Formation 1. The rate of reaction of sulfur with tung and linseed oils is greatly influenced by small proportions of such agents as zinc oxide and mercaptobenzothiazole. 2. Combination of sulfur with the conjugated double bonds of the eleostearic acid in tung oil takes place at the terminal carbon atoms of the conjugated system, giving a typical 1,6-addition. 3. The following possible chemical reactions explain adequately the process of factice formation from tung and lineseed oil. Under different conditions different reactions predominate.

Processing Video Images of Chemical Reactions

1990 ◽  
Vol 48 (1) ◽  
pp. 538-539
Author(s):  
Eyring LeRoy ◽  
W.J. de Ruijter
Keyword(s):  
Electron Microscope ◽  
Chemical Reactions ◽  
Chemical Reactivity ◽  
Scientific Information ◽  
Chemical Change ◽  
Atomic Level ◽  
Storage Medium ◽  
Video Images ◽  
Transmission Electron ◽  
Pertinent Information

The high-resolution transmission electron microscope is unique in its capacity to provide direct images of bulk matter at the atomic level. This suggests the electron microscope as a nanochemical laboratory in which elementary reaction mechanisms can be observed and deciphered. The nature, disposition, and evolution of defects, the agents of chemical change, must be studied intimately if the fundamentals of chemical reactivity in solids are to be understood fully. Chemical change during microscopical observation is inevitable since alterations in the electronic associations of the atoms are produced by the electron beam. For a chemist this presents an extraordinary opportunity for investigating the atomic-level mechanism of a chemical reaction.Chemical reactions observed at near atomic resolution can be recorded on videotape at the rate of 30 images per second. Since the onset, extent and rapidity of a reaction varies over many orders of magnitude and is not predictable it is sometimes necessary to record for hours. The videotape provides a convenient storage medium for this immense quantity of information. The tapes can then be edited for those stretches bearing pertinent information and can be processed as needed. The purpose may simply be clear communication of scientific information to peers or more extensive processing may be desired for pedagogical use.

scholarly journals The free energy of glycogen-lactic acid breakdown in muscle

1929 ◽  
Vol 104 (729) ◽  
pp. 153-170 ◽  
Keyword(s):  
Molecular Structure ◽  
Free Energy ◽  
Lactic Acid ◽  
Chemical Reactions ◽  
Thermodynamic Data ◽  
Chemical Change ◽  
Mechanical Work ◽  
The Other ◽  
Heat Of Reaction ◽  
Negative Difference

It is the purpose of the present discussion to show, upon the basis of thermodynamic data obtained within the last four of five years, that the free energy of glycogen-lactic acid breakdown in muscle is considerably greater than the heat of reaction, about one and one-half to two times. It is the intention to outline merely the orders of magnitude of the various quantities involved in the evaluation of this difference. This evaluation, as will be shown, need not depend upon a knowledge of the actual heat of reaction, which is still in dispute, varying between Meyerhof's value of —180 cal. and Slater's value of —235 cal. It will depend, rather, upon the specific heat differences, or ultimately, molecular structure differences, obtaining between glycogen and lactic acid. Stated briefly, the existence of this large negative difference, designated hereafter as (ΔF — ΔH), implies that the theoretical maximum mechanical work which a muscle can perform as a consequence of this breakdown is considerably greater than the corresponding heat of reaction. The notations of Lewis and Randall (1) will be used throughout. ΔH, the heat of reaction, and ΔF, the free energy of reaction, will be negative when heat and free energy respectively are liberated. Before presenting the thermodynamic data and calculations, it will be it historical interest to point out that in 1912 A. V. Hill (2) suggested the possibility of such a difference, when he first made the observation that during anaerobic lactic acid formation in muscle the heat evolved amounted to at least three times as much as would have been predicted if the precursor were a hexose carbohydrate. He suggested, "the breakdown from this body to lactic acid may be one of those somewhat rare but by no means unknown chemical reactions which can do more mechanical work than is equivalent to their total loss of energy; by virtue of their completeness they possess the power of absorbing heat from their surroundings to do this excess of work." Meyerhof (3, 1922) reconsidered the question, and while alive to the possibility of a considerable difference, offered an opinion, based upon the Nernst heat theorem, that probably no difference did exist. In general, however, little attention has been paid to A. V. Hill's original surmise, especially since further investigation of the other hydrolysis, neutralization, and deionization reactions occurring simultaneously with the formation of lactic acid, has shifted the attention to explaining the other more immediate problem, namely, the discrepancy between the observed chemical change and the required evolution of heat.

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