Reversible Many-Stage Reactions

Reversible electrode potendal, 30 225-227 Reversible many-stage reactions, kinetics, 28 203-207... 

It is well known that most of the antioxidant enzymes and substrates can exhibit prooxidant activity under certain conditions, mainly because many stages of the reactions catalyzed by such enzymes are reversible. The question of possible prooxidant effects of SODs and the ability of SODs to react with the other substrates than superoxide have been studied for a long time. It is known that CuZnSOD is inactivated by the hydrogen peroxide formed. Hodgson and Fridovich [3] proposed that this inactivation depends on the reaction of hydrogen peroxide with the oxidized form Cu(II)ZnSOD yielding the bound hydroxyl radicals. 

Polymerization reactions involve the union of a number of similar molecules to form a single complex molecule. A polymer is any compound, each molecule of which is formed out of a number of molecules which are all alike, and which are called monomers. In many cases polsonerization can be reversed and the poisoner be resolved to the monomer. Many polymerization reactions which are of industrial importance involve in the initial stages condensations, that is, reactions in which elimination of water or other simple molecules takes place. Compounds which polymerize have some type of unsaturation in the molecule. Olefins, unsaturated halides, esters, aldehydes, dicarboxylic acids, anhydrides, amino acids and amides are among the important groups of compounds which are used in industrial polymerization reactions. The commercial products produced by polymerization reactions may be conveniently classified into (a) resinotds, or synthetic resins (b) elastomers, which possess rubber-like properties and (c) fibroids, used as textile fibers. Two types of resinoids are illustrated in this experiment Bakelite, formed from phenol and formaldehyde, and methacrylate resin formed from an unsaturated ester. 

Many biogeochemical reactions that occur in nature are reversible, and under most conditions reactions never reach 100% completion, thus reaching equilibrium. The stage at which reaction approaches steady state is usually expressed as equilibrium constant For a system at equilibrium, the rates of forward and backward reactions are equal. 

The study of ionization and dissociation equilibria in liquid sulfur dioxide is of concern to physical organic chemistry because these directly observable processes closely parallel analogous transformations assumed to occur in many heterolytic reactions which involve transient intermediates or activated complexes not susceptible to direct observation. Sulfur dioxide is usually inert even to carbonium ions as well as to less electrophilic carbon atoms. Thus irreversible solvolysis does not occur. Because ionophores associate in liquid sulfur dioxide, the reversible formation of ion pairs must also be considered as a possible stage in the ionization of a suitably substituted alkyl halide or similar compound. Equation (2) was first used in interpretation of ionization equilibria in sulfur dioxide by Ziegler and Wollschitt (19). If this... 

In many cases, addition or removal of water proceeds sufficiently slowly that some of the physical properties of unstable species (such as hydrated neutral quinazoline or anhydrous 2-hydroxypteridine) can be observed. In these cases, reaction kinetics can also be examined. Addition of water to pteridine is of special interest in relation to studies of the formation and hydrolysis of Schiflf bases. The reaction proceeds in two reversible stages, 3 4 5 ... 

There have been several kinetic studies of the calcination of dolomite [29], a reaction of considerable technological importance. As in many reversible reactions, the rate of carbon dioxide release is sensitive to the prevailing pressure of this product (.Pco2) in the vicinity of the reaction interfaces. At low pressures (PCo2 < 12 Torr), reaction proceeds to completion in a single stage between 900 and 950 K... 

The oxygen electrode has been the subject of intensive study for many years. The electrode reaction is complex and is greatly affected by the electronic conductor and electrolyte used. In basic solution, it may be considered as a two stage process only the first of these is reversible. The two steps may be written as... 

We assume—at present this is only a hypothesis—that our theoretical investigations relate also to the combustion of powders, and especially to smokeless powders. Classical internal ballistics does not consider the question of the intimate mechanism and stages of the chemical reactions which comprise the essence of powder combustion. However, many of Belyaev s arguments and our—Frank-Kamenetskii and the author—work remain applicable. The presently proposed hypothetical scheme of powder combustion differs only in that, instead of reversible evaporation, we must speak of a primary (irreversible) reaction transforming the powder into a gas, but not into the final combustion products, followed by the reaction of combustion proper, accompanied by the release of the primary part of the reaction heat. 

Many reactions of this type are assumed to take place in a single stage, with no side reactions, other than the possible occurrence of the reverse process (see below). Three well-studied examples are ... 

---