Chemical reaction induction period

The dependence of reaction rates on pH and on the relative and absolute concentrations of reacting species, coupled with the possibility of autocatalysis and induction periods, has led to the discovery of some spectacular kinetic effects such as H. Landolt s chemical clock (1885) an acidified solution of Na2S03 is reacted with an excess of iodic acid solution in the presence of starch indicator — the induction period before the appearance of the deep-blue starch-iodine colour can be increased systematically from seconds to minutes by appropriate dilution of the solutions before mixing. With an excess of sulfite, free iodine may appear and then disappear as a single pulse due to the following sequence of reactions ... 

Organic metal salts have frequently failed to produce an appreciable chemical stabilization effect, either during dehydrochlorination induction periods or in later decomposition stages. While this does not rule out the occurrence of Frye and Horst substitution reactions, it does suggest that these reactions may not be responsible for the observed retardation of color developments [126-128]. 

Recently there has been an increasing interest in self-oscillatory phenomena and also in formation of spatio-temporal structure, accompanied by the rapid development of theory concerning dynamics of such systems under nonlinear, nonequilibrium conditions. The discovery of model chemical reactions to produce self-oscillations and spatio-temporal structures has accelerated the studies on nonlinear dynamics in chemistry. The Belousov-Zhabotinskii(B-Z) reaction is the most famous among such types of oscillatory chemical reactions, and has been studied most frequently during the past couple of decades [1,2]. The B-Z reaction has attracted much interest from scientists with various discipline, because in this reaction, the rhythmic change between oxidation and reduction states can be easily observed in a test tube. As the reproducibility of the amplitude, period and some other experimental measures is rather high under a found condition, the mechanism of the B-Z reaction has been almost fully understood until now. The most important step in the induction of oscillations is the existence of auto-catalytic process in the reaction network. 

In the absence of anything to prevent it, a chemical reaction will begin when the components and any necessary energy of activation are present in the reaction system. If an inhibitor (negative catalyst or chain-breaker) is present in the system, it will prevent the onset of normal reaction until the concentration of the inhibitor has been reduced by decomposition or side reactions to a sufficiently low level for reaction to begin. This delay in onset of reaction is termed the induction period. 

It is often useful to keep some of the reactants or the products in separate phases (principle of chemical protection by phase separation [53]). For instance, when the reaction is inhibited by its own substrate having the latter in an other phase than the one in which the catalyst is dissolved helps to eliminate long induction periods or complete stop of the reaction. An example is the biphasic hydrogenation of aldehydes with the water-soluble... 

Under conditions leading to a porous shell of magnetite, the kinetic curve displayed an induction period corresponding to formation of nuclei and the subsequent reaction followed the cube root law. Diffusion of the reducing gas to the reactant/ product interface took place readily with a porous product. Whether chemical or diffusion control predominated depended on reaction conditions. With small crystals... 

They also showed that true additivity of induction periods is only a special case of the memory effect in general Refs 1) N.N. Semenov, "Chemical Kinetics and Chain Reactions , Chap 17, Cla-rendenPress, Oxford (1935) 2) N.N. 

Influence of Interpolymer Properties. As stated earlier, the physical and chemical properties of interpolymers markedly influence the reaction rate after the induction period. If the monomer present yields a polymer comparable in viscosity with the initial mixture the rate of scission will not accelebrate. For example, the polymerization rate of chloroprene on mastication with natural rubber does not increase as markedly with conversion (69), see Fig. 19, as with methyl methacrylate and styrene. The reason is the chloroprene-rubber system remained elastic and softer than the original rubber. 

A period of induction is characteristic of chemical reactions which take place in a series of intermediate stages. This is a necessary consequence of the law of mass action. The duration of the period depends on the relative magnitude of the velocity constants of the intermediate reactions. For example, with the reaction A=M=B, the rate of formation of the intermediate compound, M in the A=M reaction, will be quickest at the start, and the rate of formation of B by the destruction of M in the M=B reaction will be slowest at first, and increase with time as the amount of M accumulates in the system. At first, during the period of acceleration, the speed of the A=M reaction exerts a preponderating influence and M accumulates in the system but the increasing speed of the M=B reaction gradually neutralizes the effect of the first reaction, until the rate of formation of M by the A=M reaction is equal to the rate of its destruction by the M=B reaction, and finally, the M is consumed faster than it is formed. The amount of M in the system at any moment thus determines the rate of formation of B, so that the curves showing (i) the rate of formation of B, and (ii) the amount of M in the system at different moments, are similar in shape. This is illustrated by the dotted line in Fig. 6. The duration of the period of induction naturally depends upon the relative speeds of the two reactions. If the rate of formation of the intermediate compound is immeasurably fast, there will be no appreciable period of induction. 

The induction period, followed by a sharp increase in rate is, however, the most characteristic feature of autocatalysis in closed vessels. One manifestation of this behaviour is the clock reaction . An experimental system which is a typical chemical clock and which also exhibits cubic autocataiysis is the iodate-arsenite reaction. In the presence of excess iodate, the system which is initially colourless eventually undergoes a sudden colour change to brown (or blue in the presence of starch). The potential of an iodide-sensitive electrode shows a barely perceptible change during most of the induction period, but then rises rapidly, reaching a peak at the point of colour change. 

Using the same conceptions of discontinuous reaction as Jost [31], it is quite possible to construct a strictly steady regime—a shock wave with subsequent additional compression to compensate for losses and a front of rapid chemical reaction at a constant distance (the product of the detonation velocity and the induction period) from the shock wave front. It is not clear whether the step-wise propagation described by Jost might not lead to just such a regime. 

Let us introduce a finite chemical reaction rate. One might think that the type of the chemical kinetics (autocatalysis or a classical reaction of some order) is in a certain sense not in itself essential autocatalysis changes the absolute value of the induction period and places it in dependence on small admixtures in the original mixture, but the form of the kinetic curve itself hardly changes since, in a classical reaction as well, with any significant activation heat one observes significant self-acceleration related to the increase in temperature. 

Regardless of how the hot spot in an expl is created, its chemical reaction will occur at a much higher rate than that in the body of the expl, where it may be virtually zero. However, during most of the induction period before expln... 

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