Reversibility, of chemical reactions

Evidence for a glycosyl-enzyme intermediate of finite lifetime with inverting a-D-glycosidases, and details of its reaction, came from studies with 2,6-anhydro-l-deoxyhept-l-enitols and glycosyl fluorides. - Analysis of hydration and hydrolysis products on the one hand, and of glycosyla-tion products on the other, indicated an intermediate that could be approached by water from the yff-face only of the ring, and by other glycosyl acceptors only from the a-face (see Schemes 4 and 5 This can be considered a proof of the principle of microscopic reversibility of chemical reactions. 

What are the conditions of irreversibility and reversibility of chemical reactions ... 

The flow diagram technique can also be used to illustrate clearly the rather limited effects on autocatalytic systems which arise from the reversibility of chemical reactions. For this we replace step (6.8) by its reversible counterpart... 

The Reversibility of Chemical Reactions. The iron oxide FcgO can be reduced to metallic iron by heating it in.4 stream of hydrogen ... 

It follows from the principles of thermodynamics that all chemical reactions are reversible this fact has so far been omitted. In some cases the effects associated with reversibility of chemical reactions have to be taken into account. For example, the reaction reverse to (4.9c) ... 

In introductory chemistry lessons, chemical reactions are usually associated with observable phenomena (e.g., change of colour, evolution and absorption of heat, precipitation of a solid, evolution of a gas) and chemical reactions are presented as proceeding to completion, taking place in one direction (Andersson, 1990). The introduction of chemical equilibrium at a later stage, however, demonstrates the reversibility of chemical reactions and the possibility that chemical reactions do not proceed to completion. Moreover, the dynamic nature of chemical equilibrium requires students to assume that two opposite chemical reactions are taking place, in spite of the fact that this cannot be deduced from observation. As a consequence, the introduction of chemical equilibrium requires students to revise their initial conception of chemical reactions. This is illustrated in Table 1. 

First of all, these authors suggest an address to the reversibility of chemical reactions. Simple chemical experiments may demonstrate that the direction of a chemical conversion may be reversed by an apparently minor intervention, e.g. adding one of the products to a reaction mixture. This has proved to be a powerful method of challenging students conceptions. Many students appeared to be puzzled by the fact that the addition of a substance, which was already present in the mixture of reagents, resulted in an observable change. Moreover, they wondered why the products created by the addition did not immediately react in the opposite direction, resulting in yet another observable change (Van Driel et al., 1998, pp. 384-387). 

Microscopic reversibility and detailed balance are important kinetic concepts. The former provides a mechanical argument for the reversibility of chemical reactions, while the latter shows a connection between kinetics and thermodynamics, and provides an important principle that must be applied in writing mechanisms. 

The introduction of chemical equilibrium in the latter part of a two semester ehemistry course exposes the student to the possibility of incompleteness and reversibility of chemical reactions. The students are confronted with the idea of two opposing chemical reactions occurring at the same time but for which no visible evidence is available. These concepts are at odds with well established conceptions that students have about chemical reactions. 

To introduce the reversibility of chemical reactions, the system of the tetrachlorocuprate(n) and the tetraamminecopper(II) complexes in water was used. 

The processes of catalytic methanation and steam reforming illustrate the reversibility of chemical reactions. Starting with CO and H2 and using the right conditions, you can form predominantly CH4 and H2O. Starting with CH4 and H2O and using different conditions, you can obtain a reaction mixture that is predominantly CO and H2. An important question is. What conditions favor the production of CH4 and H2O, and what conditions favor the production of CO and H2 ... 

In cases where use of ferrocene is compromised due to poor electrochemical reversibility of chemical reactions with EL functionahties, then some researchers have proposed use of derivatised ferrocenes. A list of either the standard potentials or midpoint potentials for a series of derivatised ferrocenes is given in Table 7.7. In many cases the derivatised ferrocenes show more stable electrochemical response and less reaction with EL components. Consequently, they have been used to calibrate potentials in ELs either as a standard of choice or as a replacement for ferrocene. The methodology in use of these complexes is the same as for ferrocenes. After dissolution of the complexes in the EL, the potential is measured (usually but not limited to voltametric techniques) and from this the Et determined and hence calibrate potentials. If the ferrocene or derivatised ferrocene redox processes are in the same potential range as those of the electroactive species, alternative redox couples should be used. 

Abstract In this chapter we present a brief introduction to chemical kinetics. Key concepts like reversibility of chemical reactions, reaction rate, reaction rate constant, and chemical equilibrium, are introduced and discussed. The most important of the results here derived is the so-called law of mass action which we discuss from the perspective of chemical kinetics. In this chapter we follow a heuristic rather than a formal approach. We start by analyzing a few simple chemical reactions to gain insight into the chemical kinetics basic concepts. After that, we heuristically derive and discuss the corresponding results for the most general case. The interested reader can consult any of the many available books on the subject. We particularly recommend the book by Houston (Chemical kinetics and reaction dynamics. McGraw-Hill, New York, 2001). 

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